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1 GRAZING MANAGEMENT OF WARM SEASON GRASS ES IN SOUTH FLORIDA By ANDR DE STEFANI AGUIAR A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2013
2 2013 Andr De Stefani Aguiar
3 To my parents and sister for all the support
4 ACKNOWLEDGMENTS Firstly I would like to be grateful to God and Nossa Senhora de Aparecida for the protection and blessing; to my parents, Heliodoro Henrique Aguiar and Renata Maria De Stefani Aguiar, who have worked diligently toward my personal and profession al life. I also would like to thank my sister, Priscil a De Stefani Aguiar, and Robson Palermo for their support and friendship Gabriela Aguiar Palermo and Mateus Aguiar Palermo for be ing the inspiration for my life I love and admire you all with all of my heart. To my nonos and grandparents Dino De Stefani (deceased) and Brgida Amlia Ros lia Brodela De Stefani, Dirceu Aguiar (deceased) and Doraci De Mateo Aguiar, thank you for your encouragement and support. Also I would like to thank all my family that in way or the other have been supporting me all of th e se years i n school. I also would lik e to express gratitude to Dr. Ven dramini and Dr. Sollenberger for giving me the opportunity and guidance to accomplish my goals I would not be able to finish without your support and encouragement. I also thank my committee members, Dr. Hersom, Dr. Arthington and Dr. Bennett for readily accepting to participate in my committee and for their helpful suggestions and assistance. I also would like to thank Dr. DiLorenzo for assist me with laboratory analysis. I would like also to express thanks to Ric hard Fethiere for helping me during the forage analyses. Thanks to all Range Cattle Research and Education Center staff, Mr. Dennis Kalich, Austin Bateman, Mr. Terry Neels, Mr. Jeffery Steele, Clay N ew man, Ryan Nevling, Randy Crawfis h Tom Fussel, Joe Ald ana, Mr. Alvin English, Mrs. Ch r istina Markham, Mrs. Laurine Gause, Mrs. Kim Parks, Mrs. Andrea Dunlap, Ms. Carly Althoff, Ms. Cindy Holley and Mrs. Toni Wood for their assistance, and inestimable help during
5 my time in Ona with sample collections, labora tory analyses and the day to day activities at the station. Appreciation is expressed to all exchange students Leticia Custodio, Eveline Alves (Galega), Gislene Manarim, Jos Neto, Pedro Salvo, Jo o Sanchez, Andr Valente Daniel Abe, Wilton Ladeira, Gr eagory Caputti, Odisle i Cunha, Dr. Arlindo Saran Neto, Dr. Paulo Martins and Dr. Leandro Galzerano, for their valuable help I also would like to thank my fellow graduate students, Nick Kruger, Seth Byrd, Marcelo Wallau, Miguel Castil l o, Daniel Pereira, Kim Mullenix, Eduardo Alava, and Dr. Neha Rana for their help and friendship. Also thanks to all my friends that I made in Gainesville during my time there
6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 9 LIST OF FIGURES ................................ ................................ ................................ ........ 11 ABSTRACT ................................ ................................ ................................ ................... 12 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 14 2 LITERATURE RE VIEW ................................ ................................ .......................... 19 Warm season Grasses ................................ ................................ ........................... 19 Limpograss General Description ................................ ................................ ...... 21 Limpograss Forage Management ................................ ................................ ..... 23 Stockpiling ................................ ................................ ................................ ........ 27 Supplementation on Forage Based Systems ................................ ................... 30 Protein and Energy Supplementation ................................ ................................ ..... 31 Protein Supplementa tion ................................ ................................ .................. 31 Rumen Degradable Protein ................................ ................................ ....... 32 Non Protein Nitrogen ................................ ................................ ................. 34 Rumen Undegradable Protein ................................ ................................ ... 36 Energy Supplementation ................................ ................................ .................. 38 Creep Feeding ................................ ................................ ................................ .. 43 Bermudagrass ................................ ................................ ................................ ........ 48 Grazing Management and Animal Responses ................................ ................. 50 Stocking Rate ................................ ................................ ................................ ... 52 Animal Performance and Forage Responses ................................ ................... 52 3 THE EFFECTS OF DIFFE RENT SOURCES OF RUME N DEGRADABLE PROTEIN (RDP) SUPPLE MENTATION ON PERFORM ANCE OF COWS AND CALVES GRAZING STOCKPILED LIMPOGRAS S PASTURES IN FLORID A DURING THE WINTER ................................ ................................ .......................... 56 O verview of the Research Problem ................................ ................................ ........ 56 Material and Methods ................................ ................................ ............................. 58 Grazi ng Study ................................ ................................ ................................ ... 58 Pasture Sampling ................................ ................................ ............................. 59 Animal Response Variables ................................ ................................ ............. 61 Statistical Analysis ................................ ................................ ............................ 61 Drylot Study ................................ ................................ ................................ ...... 62 Statistical Analysis ................................ ................................ ............................ 63
7 Metabolic Study ................................ ................................ ................................ 63 Statistical Analysis ................................ ................................ ............................ 64 Results and Discussion ................................ ................................ ........................... 65 Grazing Study ................................ ................................ ................................ ... 65 Drylot Study ................................ ................................ ................................ ...... 74 Metabolic Study ................................ ................................ ................................ 76 Important Findings and Implications ................................ ................................ ....... 79 4 EFFECTS OF LIMIT CREEP FEEDING SUPPLEMENT O N PERFORMANCE OF COWS AND CALVES G RAZING LIMPOGRASS PA STURES IN SOUTH FLORIDA ................................ ................................ ................................ ................ 81 O verview of the Researc h Problem ................................ ................................ ........ 81 Material and Methods ................................ ................................ ............................. 82 Herbage Measurements ................................ ................................ ................... 83 Animal Responses ................................ ................................ ........................... 85 Economics ................................ ................................ ................................ ........ 85 Res ults and Discussion ................................ ................................ ........................... 86 Herbage Responses ................................ ................................ ......................... 86 Animal Responses ................................ ................................ ........................... 90 Economics Analysis ................................ ................................ ......................... 93 Important Findings and Implications ................................ ................................ ....... 94 5 EFFECT OF STOCKING R ATE ON HERBAGE RESPO NSES AND ANIMAL PERFORMANCE OF BEEF PASTURES ................................ ................................ ................................ ............ 96 O verview of the Research Problem ................................ ................................ ........ 96 Material and Methods ................................ ................................ ............................. 98 Herbage Measurements ................................ ................................ ................... 99 Animal Measurements ................................ ................................ .................... 101 Statistical Analyses ................................ ................................ ........................ 102 Results and Discussion ................................ ................................ ......................... 102 Herbage Responses ................................ ................................ ....................... 102 Nutritive Value and Botanical Composition ................................ ..................... 107 Animal Responses ................................ ................................ ......................... 108 Important Findings and Implications ................................ ................................ ..... 110 6 SUMMARY AND CONCL USIONS ................................ ................................ ........ 111 Summary ................................ ................................ ................................ .............. 111 Stockpiled Limpograss Studies ................................ ................................ ...... 113 Grazing study ................................ ................................ ........................... 113 Drylot study ................................ ................................ .............................. 114 Metabolic study ................................ ................................ ........................ 114 C reep feeding Study ................................ ................................ ...................... 115 Jiggs Bermudagrass Grazing Study ................................ ............................... 116
8 Conclusions ................................ ................................ ................................ .......... 117 LIST OF REFERENCES ................................ ................................ ............................. 120 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 138
9 LIST OF TABLES Table page 3 1 Ingredient composition and nutrient profile of treatments fed to animals during Experiments 1, 2, and 3. ................................ ................................ .......... 60 3 2 Month effects on herbage mass, in vitro digestible organic matter concentrations and herbage allowance of stockpiled limpograss pastures grazed by cow calf pairs supplemented with molasses and urea or cotton seed meal. ................................ ................................ ................................ .......... 66 3 3 Year effect on crude protein and in vitro digestible organic matter concentrations of stockpiled limpograss pastures ................................ .............. 66 3 4 Average monthly precipitation from 1942 to 2012 and during the experimental period from 2010 to 2012 at the Range Cattle Research and Education Center Ona, FL. ................................ ................................ ................ 66 3 5 Average monthly temperature from 1942 to 2012 and during the experimental period from 2010 to 2012 at the Range Cattle Research and Education Center, Ona, FL. ................................ ................................ ................ 67 3 6 Year effect on animal response variables of stockpiled limpograss pastures. .... 70 3 7 Year x treatment x month interaction on average daily gain (kg d 1 ) of cows grazing stockpiled limpograss pastures supplemented with molasses based supplement plus urea or cotton seed meal (CSM). ................................ ............. 71 3 8 Year x treatment x month interaction on blood urea nitrogen (mg dL 1 ) of cows grazing stockpiled limpograss pastures supplemented with molasses based supplement plus urea or cotton seed meal (CSM). ................................ .. 73 3 9 Year x treatment x month interaction on average daily gain (kg d 1 ) of calves grazing stockpiled limpograss pastures supplemented with molasses based supplement plus urea or cotton seed meal (CSM). ................................ ............. 75 3 10 T reatment effects on rumen and blood parameters of fistulated steers fed with limpograss hay and supplemented with molasses based supplement and urea or cottonseed meal (CSM). ................................ ................................ .. 77 4 1 Herbage and animal responses of cow calf pairs grazing limpograss pastures supplemented on creep feeding with 200 g d 1 of soybean meal (200) or control (no supplement) in 2011 (Experiment 1). ................................ ................ 88
10 4 2 Month effects on herbage and animal responses of cow calf pairs grazing limpograss pastures and supplemented o n creep feeding with 200 g d 1 of soybean meal (200) or control (no supplement) in 2011 (Experiment 1). ........... 88 4 3 Herbage and animal responses of cow calf pairs grazing limpograss pastures and supplemented by creep feeding with 400 g d 1 of soybean meal (400), 200 g d 1 of soybean meal (200), or control (no supplement) in 2012 (Experiment 2). ................................ ................................ ................................ ... 89 4 4 Month effects on herbage and animal responses of cow calf pairs grazing limpograss pastures and supplemented by creep feeding with 400 g d 1 of soybean meal (400), 200 g d 1 of soybean meal (200), or control (no supplement) in 2012 (Experiment 2). ................................ ................................ .. 91 4 5 Average dail y gain, added gain, added BW, amount of feed, cost of feed, cost of added gain, and efficiency of added gain responses of suckling calves grazing limpograss pastures and supplemented on creep feeding 400 g d 1 of soybean meal, 200 g d 1 of soybean meal, or control in 2012 (Experiment 2). ... 94 4 6 Economic analysis of cow calf pairs grazing limpograss pastures and supplemented o n creep feeding with 400 g d 1 of soybean meal, 200 g d 1 of soybean meal, or control in 2012 (Experiment 2). ................................ .............. 94 5 1 Effect s of stocking rate on herbage responses of Jiggs bermudagrass pastures. ................................ ................................ ................................ ........... 103 5 2 Month effects on forage responses of Jiggs bermudagrass pastures. Data are means across three stocking rates. ................................ ............................ 103 5 3 Mont h effect on leaf, stem, and dead material proportion of Jiggs bermudagrass pastures. Data are moans across three stocking rates ............. 105 5 4 Correlations among herbage mass, light interception, and forage height of Jiggs bermudagrass pastures grazed at different stocking rates. ..................... 106 5 5 Botanical composition of Jiggs bermudagrass pastures. ................................ .. 108 5 6 Animal responses of heifers grazing Jiggs bermudagrass pastures. ................ 108 5 7 Month effects on animal responses of Jiggs bermuda grass pastures. Data are means across three stocking rates. ................................ ............................ 109
11 LIST OF FIGURES Figure page 3 1 Time effect on ruminal pH ( P < 0.01) and ammonia ( P < 0.01) on fistulated steers supplemented with two sources of rumen degradable protein, mean between treatments. ................................ ................................ ........................... 79 3 2 Time effect on blood urea nitrogen on fistulated steers supplemented with two sources of rumen degradable protein, mean between treatments ( P = 0.06). ................................ ................................ ................................ .................. 80 4 1 Average monthly precipitation from 1942 to 2012 and during the experimental period 2011 to 2012 at the Range Cattle Research and Education Center, Ona, FL. ................................ ................................ ................ 87 5 1 Average monthly precipitation from 1942 to 2012 and during the experimental period 2011 to 2012 at the Range Cattle Research and Education Center, Ona, FL. ................................ ................................ .............. 104
12 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillm ent of the Requirements for the Degree of Doctor of Philosophy GRAZING MANAGEMENT OF WARM SEASON GRASSES IN SOUTH FLORIDA By Andr De Stefani Aguiar December 2013 Chair: Jo o M. B. Vendramini Cochair: Lynn E. Sollenberger Major: Agronomy Limpograss [ Hema r t h ria altissima ( Poir ) Stapf and C.E. Hubb ] is a widely used forage for cow calf production in South Florida because it produces greater herbage mass and contains greater total digestible nutrients concentration than other warm season grasses at late maturity during the autumn and winter. However, crude protein concentrations may reach levels below the animal requirements and supplementation may be necessary to maintain animal production. Research was conducted at Ona, FL to investigate the effects of urea or cottonseed meal as sources of rumen degradable protein supplementation on performance of cow calf pairs receiving stockpiled limpograss forage. There were no differences in animal performance, pasture characteristics, forage and total dry matter in take, and rumen metabolites between treatments. In addition, a grazing study was conducted to evaluate the effects of creep feeding calves with increasing levels of soybean meal on performance of cow calf pairs grazing limpograss pastures. Treatments were 0 or 200 g d 1 and 0, 200, or 400 g d 1 of soybean meal in 2011 and 2012, respectively. There were no differences in forage characteristics and animal performance in 2011; however, there was a linear increase in average daily gain as levels of soybean meal increased in 2012. Results of th e se
13 experiments indicate that urea can be the main source of rumen degradable protein for cow calf pairs grazing stockpiled limpograss pastures. In addition, creep feeding calves with soybean meal may be an effective manage ment practice to increase calf body weight and weaning weights. [ Cynodon dactylon ( L.) Pers.] for grazing systems in Florida; however, there is limited information about grazing management of Jiggs Research was conducted to test the effect of stocking rates [SR; 4, 9, and 13 animal units (450 kg liveweight) ha 1 ] on animal performance and herbage characteristics of Jiggs pastures. There was a linear decrease in herbage mass, forage height, and herb age allowance with increasing stocking rate Animal performance and ground cover decreased with increasing stocking rate Jiggs is a useful warm season perennial grass to be grazed in South Florida; however, grazing at shorter stubble heights must be avoid ed due to negative effects on animal performance and forage persistence.
14 CHAPTER 1 INTRODUCTION The majority of cow calf operations in the Unite d States are located in the Southeast and Southern Plain regions. These regions are more suitable for forage production, require less supplemental feed, and have lower production costs compared to other regions in the United States (McBride and Matthews, 2 011). Florida is the 10 th state in number of beef cows and 16 th in total cattle number in the Unite d States with approximately one million animals (USDA, 2012). Florida is one of the most important calf producer regions in the Unite d States with 890,000 c alves born in 2011 (USDA, 2012). Cow calf operations in Florida rely heavily on warm season grass pastures with limited supplemental feed used during the winter period. Florida grasslands cover ~ 4.5 million ha and the predominant forages are C4 (warm sea son) grasses (Vendramini et al., 2008 a ). The climate in southern Florida varies during the year; with average temperatures ranging from 16C during the winter to 28C in the summer, and few events below 0 C occur during the winter. This temperature variati on during the year has significant effects on herbage yield and nutritive value of warm season grasses. Limited forage production and nutritive value during the winter months may limit livestock production in Florida (Vendramini et al., 2006). Therefore, a n efficient grazing management program, which includes conserved forage and supplementation, is crucial to improve the profitability of cow calf operations in Florida. Stockpiling forage for the winter months has some advantages compared to hay or haylage including less equipment, labor, fuel, and consequently at lower cost. Poore et al. (2000) conclude d that stockpiled tall fescue grass ( Festuca arundinacea Schreb.)
15 with moderate N fertilization levels (50 and 100 kg ha 1 ) was more economical than hay t o provide forage for cows during the winter. According to Lalman et al. (2000), stockpiled bermudagrass can be used to reduce cost of animal production and is more economical tha n making or purchasing hay However, nutritional strategies are needed during fall and winter to maintain animal production on stockpiled pastures (Stateler et al., 1995). Supplemental strategies can decrease weight loss in cows and optimize stocker gains during the winter (Rush and Totusek, 1976). Limpograss [ Hemarthria altissima ( Poir. ) Stapf & C.E. Hubb. ] is a warm season forage species widely planted in seasonally flooded areas of South Florida. Limpograss is tolerant to poorly drained soils and has significant winter growth when compared to other warm season grass spe cies. Additionally, limpograss tends to have greater digestibility than other warm season grasses at greater maturity and has been used successfully as stockpiled forage for beef cattle production in South Florida. However, limpograss has low crude protein (CP) concentrations at late maturity which may limit animal performance ( Sollenberger et al., 1988, Rusland et al., 1988, da Lima et al., 1999, Newman et al., 2002). Nitrogen fertilization, protein supplementation, and overseeding with legumes are some strategies used to overcome the limited CP concentrations of limpograss. S upplemental feed such as hay, haylage, and concentra ted feed may represent approximately 60% of the total production cost of cow calf operations ( Quanbeck and Johson, 2009) S upplementation strategies such as different rates and ingredients need to be investigated to improve efficiency and decrease cost o f supplemental feed.
16 Supplementing grazing animals with rumen degradable protein is a strategy that can be used to improve performance of cattle grazing warm season grass. Creep feeding is a nother approach for offer ing supplemental nutrients to nursing cal ves and increas ing weaning weight s (St r icker et al., 1979; Tarr et al. 1994; Faulkner et al. 1994). In addition, creep feeding concentrate to nursing calves may increase total tract organic matter digestibility (Soto Navarro, 2004) and decrease forage intake (Cremin et al. 1991 ; Falkner et al., 1994). Furthermore, creep feeding has potential to increase body weight of heifer calves and decrease age of puberty, increasing pregnancy rates at earlier ages. However feed conversion (supplemented feed/gain) of creep fed calves is often low ; approximately 8 kg of concentrate feed per k g of added gain and may not be economically feasible (Sticker et al., 1979; Reed et al. 2006). A n alternative management practice is to limit creep feed protein supplements to nursing calves (Lusby et al., 1985). Lusby et al. (1985) reported an improvement i n calf performance (0.13 kg d 1 of added gain) and feed efficiency (2.5 k g of concentrate feed per k g of extra gain) when calves were supplemented with 0.37 kg d 1 of cotton seed ( Gossypium spp.) meal for 63 d. Thus, limit creep feeding rumen degradable protein to nursing calves grazing limpograss may be an efficient management practice to increase weaning weights an d profitability of cow calf operations in South Florida Scre ening and testing new forage germplasm under grazing is another management practice to improve the efficiency of cow calf operations in South Florida (Mislevy et al., 2008) According to Staples (2003), bermudagrass [ Cynodon dactylon (L.) Pers.] is the mos t planted warm season perennial forage in the southeastern USA because of high yields, resistance to dry periods and acid soils, long term persistence
17 when well managed and tolerance to frequent defoliation. In addition, bermudagrass bermudagrass, the first bermudagrass hybrid, was a landmark in bermudagrass breeding and forage production in t he southeastern USA. Other bermudagrass hybrids ( Cynodon spp.) (Burton et al. 1993). Although Coastal and Tifton 85 are the most planted cultivars of bermudagrass in the southern USA, they are not productive and persistent on poorly drained soils, which are common ly found bermudagrass hybrid released in Texas with perceived greater tolerance to poorly drained soils. Vendramini et al. (2010) observed tha t Jiggs harvested at 6 wk regrowth interval had greater herbage accumulation than Tifton 85 in poorly drained soils in South Florida. There is increasing interest in Jiggs for grazing systems in South Florida; however, there is limited informatio n in the l iterature on grazing management of Jiggs pastures available. The general objective of the studies listed in this dissertation is to increase the efficiency of forage and livestock production in Florida. The specific objectives are to evaluate the effects of rumen degradable protein supplementation on cow calf pairs grazing limpograss pastures and to determine the effects of stocking rates on forage characteristics and animal performance of beef heifers grazing Jiggs bermudagrass pastures. In order to addr ess those objectives three grazing studies were conducted The first study evaluate d the effects of different sources of rumen degradable protein supplementation on performance of cows and calves grazing stockpiled limpograss
18 pastures during the winter. Th e second experiment evaluate d the effect of creep feeding increasing levels of soybean meal supplements to nursing calves grazing limpograss pastures during the summer. Lastly, the third stud y evaluate d the effects of stocking rate on forage characteristic s and animal performance of beef heifers grazing Jiggs bermudagrass pastures
19 CHAPTER 2 LITERATURE REVIEW Warm season G rasses In 1972, Parson stated that warm season perennial grasses from Africa were introduced in the U nited States and had potential to improv e livestock production in the southern part of North America, northern part of South America, and Central America. Currently, Florida has ~ 4.5 million ha of grasslands and several species of warm season grasses, including bermudagra ss [ Cynodon dactylon (L.) Pers.] elephantgrass ( Pennisetum purpureum Schumach), limpograss [ Hemarthria altissima (Poir.) Stapf et C. E. Hubb.] stargrass ( Cynodon nlemfuensis Vanderyst var. nlemfuensis ) and brachiariagrass ( Brachiaria sp ) which are ori ginally from Africa. These grasses are being used in grazing systems and as conserved forage for livestock production. Warm season grasses have superior production in tropical and subtropical regions of the world, however, are generally low in nutritive va lue and may not meet animal requirements (Moore, 1992). Brown and Simmons (1979) reported that warm season grasses produce more forage than cool season grasses in tropical and subtropical climates, as a result of better water use and light conversion effi ciency. However, warm season grasses tend to have lesser nutritive value [crude protein ( CP ) and digestibility ] than cool season grasses, due in part to the parenchyma bundle sheath cells and a higher proportion of cell wall material (Akin and Burdick, 197 5). Although cell wall s are potentially digestible, chemical barriers and anatomical structures decrease microbial attachment, degradation rate, and fermentation (Akin and Burdick, 1975). The leaves of warm season grass es have lower degradability in the rumen compared to leaves of cool
20 season grasses (Van Soest, 1982) due to greater proportions of vascular tissues, bundle sheath, and sclerenchyma (Coleman et al., 2004) Warm season grasses differ from cool season grasses i n carbon fixation pathway. In C3 plants, the first stable product of CO 2 fixation is 3 phosphoglyceric acid (3 PGA). This reaction occurs at the carboxy lation site in the mesophyll cells where CO 2 and H 2 O are added to ribulose 1,5 bisphosphate by the enzyme ribulose bisphosphate carboxylase oxygenase (Rubisco). In C4 plants, the first product of CO 2 fixation is the four carbon compound oxaloacetate (OAA) that is converted to malic or aspartic acid. The C4 pathway begins in the mesophyll cells when CO 2 diffuses across the mesophyll cell membrane into the cytoplasm where it is converted to HCO 3 In the mesophyll, HCO 3 is incorporated by the enzyme phosphoe nolpyruvate carboxylase (PEP) into OAA. Oxaloacetate is then converted into malate or aspartate in the cytoplasm and transported to the bundle sheath cells. In the bundle sheath cells, malate or aspartate is decarboxylated and CO 2 is released. Phosphoenol pyruvate carboxylase has a high affinity for CO 2 which can efficiently fix CO 2 into the 4 C compound. The decarboxylation of this 4 C compound in the bundle sheath increases the CO 2 concentration around Rubisco, not allowing O 2 to compete for binding sites. Photorespiration does not occur in C4 plants due to the high efficiency of the PEP enzyme and the CO 2 shuttle mechanism of the 4 C compound that moves CO 2 from the mesophyll to the bundle sheath. The increase in herbage yiel d of warm season grasses is observed during the summer, while during the winter the production decreases significantly as a consequence of dry and cold weather ( Brown and Simmons, 1979 ). Warm season
21 grasses have their optimum growth when temperature is be tween 20 35C, while the range of growth temperatures is 15 45C (Bassam, 1998). Growth is minimal below 15C (Volenec and Nelson, 2003) and temperatures above the optimum can increase lignin and cell wall concentrations, and decrease soluble carbohydrates (Van Soest, 1994). According to MacAdam and Nelson (2003), temperatures below 0C have an effect on rate of maintenance respiration of the plants and cause damage to membranes, especially for C4 plants with low adaptation to those conditions. Limpograss General Description Limpograss is a perennial warm season grass from the Poaceae family originated from the Limpopo River region in South Africa. It is a stoloniferous grass, with decumbent branching stems simple long leaf, spikelike raceme inflorescence small membranous ligule no rhizomes and can grow up to 150 cm tall (Bodgan, 1977). The propagation of limpograss occurs primarily vegetatively from stolons (Wilms et al., 1970; Newman, 2001; Vendramini, 2008). The usage of limpograss has been increasin g i n the last decades and it is cultivated o n approximately 150,000 ha in the state of Florida (IFAS, 2007 ). Limpograss is well adapted to seasonal ly flooded soils, has superior herbage accumulation during the winter, and is resistant to most pests and dis eases (Wilms et al., 1970, Quesenberry et al. 1984). Four cultivars of limpograss were introduced in Florida in 1964 by Dr. Oakes, with three species being introduced vegetatively (Wilms et al., 1970). In a study comparing diploid, tetraploid and hexaploid limpograss cultivars, Wilms et al. (1970) reported that diploid plant s had more leaves, were more erect and had shorter internodes than polyploids. In addition, the authors indicated the possibility to cross those plants to obtain more productivi ty hybrids. Dr. Quesenberry started a limpograss
22 selection more accepted by cattle; howe ver it lacks persistence under grazing (Pitman et al. 1994 ; Quesenberry et al., 2004). Kretschmer and Snyder (1979) evaluated the production and quality of three limpograss cultivars digitgrass ( Digitaria decumbe n s Stent. ) and Coastcross 1 bermudagrass [ Cynodon dactylon ( L.) Pers.] from September to June 1969 to 1971. All grasses had the same fertilization level (84, 37, and 70 kg of N, P K ha 1 respectively) and three cutting intervals were tested (2, 3 and 4 weeks). The forage was harvested at 10 cm stubble height. Pangola had the least herbage accumulation compared to the other grasses. Herbage accumulation of Bigalta, Redalta and Greenalta were 15.2, 13.7 and 14. 8 Mg ha 1 respectively. Bigalta had the greatest IVDOM (633 g kg 1 ) compared to Greenalta (599 g kg 1 ) and Redalta (556 g kg 1 ) which had similar values to Transvala (627 g kg 1 ) and Pangola (622 g kg 1 ). Coastcross 1 had the least in vitro digestible org anic matter ( IVDOM ) concentrations (588 g kg 1 ). There were no differences in herbage accumulation between Bigalta, Coastcross and Transvala at the same cutting intervals; however, Bigalta produced less forage at the 2 wk regrowth interval treatments. The authors also concluded that Bigalta can be fertilized early in the autumn and would produce 4.0 to 5.0 Mg ha 1 as hay or as deferred forage. Subsequently, Quesenberry et tent than Bigalta under grazing. According to Christiansen et al. (1988) Floralta stem is highly digestible because it has higher percentage of parenchyma cells and starch and
23 less percentage of vascular bundles and structural carbohydrates. Currently, Fl oralta is the most planted l impograss cultivar in Florida, due to its persistence under grazing, high yield, long grazing season, reasonable digestibility, and cold tolerance (Sollenberger et al., 1989; Newman, 2001; Vendramini and Arthington, 2010). Limpograss Forage Management Sollenberger et al. (1988) evaluated animal performance, nutritive value and carrying capacity of continuously stocked bahiagrass ( Paspalum n otatum Flgge) and limpograss pastures. Pastures were stocked with variable stocking r ate to maintain stubble height of 15 and 30 cm for bahiagrass and limpograss respectively. Limpograss showed greater IVDOM when compared to bahiagrass, 539 vs 484 g kg 1 ; however CP was greater for bahiagrass than fo r limpograss, 93 vs 58 g kg 1 There was no difference in average daily gain (ADG) between the grasses 0.35 kg d 1 In spite of greater digestibility of limpograss, CP was deficient and led to poor animal performance. Similar conclusions were reached by Sollenberger et al. (1989) comparing F loralta yr study evaluating animal and forage performance. Pastures were rotationally stocked to 20 to 25 and 6 to 8 cm stubble height for limpograss and bahiagrass, respectively. On average, Pensacola had g reater CP concentration 116 vs. 83 g kg 1 In vitro digestible of organic matter was greater for limpograss, 613 vs. 581g kg 1 ; however, here was no difference in ADG between species (0.41 and 0.38 kg d 1 for limpograss and bahiagrass, respectively). Limpograss supported greater stocking rate than bahiagrass, 2150 vs. 1680 kg of live weight ha 1 d 1 and produced greater live weight gain 460 vs. 318 kg ha 1 Pitman et al. (1994) compared Bigalta and Flor alta and concluded that Bigalta had greater in vitro digestible organic matter (IVDOM) (520 vs. 480 g kg 1 ) and CP (61
24 vs. 51 g kg 1 ) when pasture s were stocked with low stocking rate (4 yearling steers ha 1 ), and resulted in greater ADG (0.5 vs. 0.3 kg d 1 ). In addition, green leaves had almost four tim es the CP values than stems, 7 to 8 vs. 2 g kg 1 in both cultivars. Although limpograss can maintain greater levels of IVDOM concentrations than other warm season grasses at late maturity, the C P levels are usually less than 70 g kg 1 ( Quesenberry and Ocumpaugh, 1980 ), which may limit animal performance. Several studies reported low CP values and animal performances with acceptable levels of IVDOM 550 g kg 1 (Sollenberger et al., 1987, Sollenberger et al., 1988, Rusland et al., 1988, Sollenberger et al., 1989, Holderbaum et al. 1991, Pitman et al., 1994, Lima et al., 1999, Newman et al. 2002). In order to address the potential protein deficiency in animals g razing limpograss pastures, Sollenberger et al. (1987) tested overseeding aeschynomene ( Aeschynomene americana L.) on limpograss pastures to increase forage CP concentration. Overseeding with aeschynomene was effective in increas ing CP concentration of th e consumed forage from 40 to 70 g kg 1 Rusland et al. (1988) measured animal performance on limpograss pastures overseeded with aeschynomene (LA) and fertilized with nitrogen (LN) during three years. Animals grazing LA had on average 80% greater ADG than animals grazing LN. Aeschynomene hand plucked sample s had 250 g kg 1 of CP, and 720 g kg 1 of IVDOM whereas limpograss hand plucked sample s from LN pastures had greater CP compared to limpograss samples from LA. Total diet consumed by animals had greater nutritive value for animals grazing LA leading to an increase in animal performance. In addition carrying capacity was greater for LN pastures than for LA (2200 vs. 1700 kg live weight ha 1 per day). The authors also
25 concluded that N fertilization can be u sed to increase nutritive value in limpograss pastures. Holderbaum et al. (1992) evaluated rotationally stocked limpograss pastures with 35 d of rest ing and 7 d of grazing period Pre grazed forage was harvest ed from 5 cm above the soil level and was divi ded similarly in two parts, upper and lower. The authors reported a greater leaf/stem ratio for the upper layer compared to the low er layer, values from three to six times greater but varying through the summer. Herbage mass was greater i n the lower layer compared to the upper layer with approximately 50% difference between layers. Newman et al. (2003) evaluated the effect of different canopy heights on bulk density and plant part proportion of continuously stocked limpograss pastures. Ca nopy heights were 20, 40 and 60 cm and plants were separated into three strata top 5 cm, upper 25% by height, and th e remaining 50% of the canopy height The authors reported a decrease in the bulk density, from 137 to 63 kg ha 1 cm 1 as stubble height i ncreased from 20 to 60 cm. The upper layer had the greatest percentage of leaves (mean = 19%) followed by the remain ing 50% layer (mean = 11%). Nutritive value varied among layers. Crude protein concentration o f leaf (from 129 to 123 g kg 1 ) and stem (from 50 to 40 g kg 1 ) was greater for upper than lower layers, respectively. There was a linear decrease in leaf IVDOM from approximately 650 to 600 g kg 1 as stubble height increased from 20 to 60 cm and the lower layer had inferior IVDOM compared to upper an d 5 cm layer, approximately 630 and 600 g kg 1 respectively. The variation i n CP was more prominent than in IVDOM for leaf and stem and the 40 cm stubble height
26 was recommended by the authors due to the greater opportunity for leaf selection and greater nutritive value. According to Moore et al. (1999), ruminants consuming forage with IVDOM :CP ratio greater than 7 may respond positively to protein supplementation. Limpograss plant parts vary widely in their nutritive value. Limpograss leaves usually have IVDOM :CP ratio below 7 ; however, the stems tend to have greater IVDOM :CP ratio (Pittman et al., 1994). Crude protein supplementation is typically required to overcome the CP deficiency of beef cattle grazing warm season grasses. Holderbaum et al. (1991) supplemented steers with two levels of corn ( Zea mays L.) urea supplement with low (21%) and high (50%) CP concentrations and compared animal performance to non supplemented (NS) steers grazing limpograss pastures overseeded with aeschynomen e (LA). A rotational stocking management of 7 d grazing and 35 d resting period was used. Pastures were grazed to a 20 cm stubble height using a variable stocking rate Blood urea nitrogen (BUN) was used to monitor the N status of the animals. The steers g razing overseeded pastures or receiving high and low CP supplements had similar BUN 11.0, 11.4, and 8.2 mg dL 1 respectively. Those values were greater than the NS treatment (6.0 mg dL 1 ). Steers receiving the supplement treatments or grazing overseeded pastures had greater ADG (0.60 kg 1 ) than non supplemented animals (0.30 kg d 1 ). In addition, the authors reported the relationship between digestible organic matter (DOM) and CP was 8.7 for limpograss pastures alone, suggesting the benefits of protein supplementation when the relationship is greater than 7 to 8 (Moore et al., 1999)
27 Lima et al. (1999) evaluated the effects of N fertilization levels (50 and 150 kg ha 1 ) and supplementat ion with 0.27 kg d 1 of rumen degradable protein (RDP) and two levels of rumen undegradable protein (RUP) 0.05 and 0.140 kg d 1 Heifers supplemented with a greater amount of RUP had superior gain compared to the RDP treatment (0.52 vs. 0.40 kg d 1 ) and b oth supplemented treatments had greater gains than unsupplemented (0.21 kg d 1 ). Supplementation had no effect on ADG when 150 kg ha 1 of N was applied and increasing N levels from 50 to 150 kg ha 1 increased ADG on unsupplemented animals from 0.06 to 0.36 kg d 1 Hand plucked samples from pastures fertilized with 150 kg N ha 1 showed greater CP concentration than 50 kg N ha 1 56 vs. 73 g kg 1 Limpograss pastures fertilized with greater N had DOM:CP of ~7.4, a level at which protein supplementation is unl ikely to be effective (Moore et al., 1999). Conversely, lower levels of N fertilization resulted in DOM:CP of 9.1 and protein supplementation increased animal performance. Newman et al. (2002) reported an increase in animal performance of supplemented anim als with corn plus urea over nonsupplemented animals when animals grazed limpograss to 20 and 60 cm stubble height primarily due to decreased RDP of the forage. Limpograss is an important forage for South Florida due to tolerance to poorly drained soils, acceptable digestibility after extended regrowth interval, and greater production than other warm season grasses during the winter. Despite of reasonable HM accumulation, CP protein may be limiting and protein supplementation may increase animal performanc e. Stockpiling Conserved or stockpiled forage are alternatives to supply forage for ruminants during the periods of forage shortage, usually late autumn and winter (Ruelke and
28 Quesenberry, 1983). According to Mays and Washko, (1960) stockpil ing is a pra ct ice that allows forage to grow for a certain period of time for fu ture utilization. Stockpiled forage can be used to maintain pregnancy and body condition score (BCS) in beef cows during the winter at low cost (Hitz and Russel, 1998). A distinct characteristic of limpograss is the slower decline in digestibility with advancing maturity during the growing season when compared with other warm season grass species (Schank et al. 1973). In addition, limpograss is a feasible option as wint er forage because it can produce approximately 35% of its annual herbage accumulation during the winter in Florida (Brown and Kalmbacher, 1998). In a 2 yr study in North Central Florida, Quesenberry and Ocumpaugh (1980) compared stockpiled Bigalta, Redalta and Greenalta. Pastures were staged at three different dates i n the first year, early and late July and late August and harvested from September to March. There was no difference i n dry matter yield between the two July stag ing dat es, which were higher t han staging i n late August. Redalta had the greatest herbage accumulation in both year s (11.5 and 6.8 Mg ha 1 for Y ear 1 and 2, respectively). In addition there was no difference in nutritive value (CP and digestibility) between stag ing date s. Bigalta had the lowest decline in digestibility ( from 70 0 to 45 0 g kg 1 ) from August to March, compared with a 50% decline for Greenalta and Redalta. Ruelke and Quesenberry (1983) evaluate d yield and nutritiv e value of stockpiled Floralta limpograss. Pastures were staged and fertilized with 75 kg N ha 1 i n August and harvest ed thr o u gh February a t 2 wk interval s Herbage mass reached ~8 Mg ha 1 i n late September and was similar from late September to December (~10 Mg ha 1 ). Crude protein was at a maximum in the middl e of September (110 g kg 1 ) and decreased to
29 below 50 g kg 1 in December. After the first frost, the CP concentrations of limpograss decreased to 40 g kg 1 Davis et al. (1987) imposed eight levels of fertilization (0 to 400 kg N ha 1 ) on Bigalta limpograss staged i n early October; monthly samples were taken from December to April and analyzed for nutritive value. There was no effect on neutral detergent fiber (NDF) and acid detergent fiber (ADF) between fertilization rates ; however C P and IVDMD increased when N fertilization was greater than 68 kg ha 1 In addition there was an increase in yield from 0.55 to 4.35 Mg ha 1 with N fertilization levels from 0 to 135 kg N ha 1 Kretschmer et al. (1996) evaluated different autumn applicati on dates (early and late autumn ) and N levels (0, 50 and 150 kg ha 1 ) on stockpiled Bigalta and reported no difference in dry matter yield, IVDOM and TDN between all combinations of dates and N fertilization levels However CP was greater when late fertil ization was used. Kalmbacher et al. (1998) evaluated the effects of fertilization levels on stockpiled limpograss. Pastures were fertilized with fixed 50 N ha 1 level and different doses of phosphorus and potassium. Fertilization occurred at different tim es of the year from June to November but not in July, and forage was sampled from September to December generating stockpil ing periods ranging from 30 to 105 d. There was no difference in limpograss yield among P and K fertilization treatments but the fer tilized treatments had greater yield ( 1 3.0 Mg ha 1 ) than the unfertilized control (9.9 Mg ha 1 ). In addition CP and IVDOM concentrations increased with N fertilization levels Vendramini and Arthington (2010) reported that heifers grazing stockpiled limpog rass had
30 increasing ADG (0.14, 0.44, and 0.64 kg d 1 ) when receiving increasing levels of cottonseed ( Gossypium spp.) meal (CSM) (0, 1.1, and 2.2 kg head 1 d 1 ), respectively. Producers rely on different management strategies to overcome the decreased production of warm season grasses during the winter. Stockpil ing forage may be an economically attractive management practice to supply forage to beef cattle during the winter in South Florida However, supplementation may be necessary to meet the animal requirements due to the usually limited nutritive value of stockpiled warm season grasses. Supplementation on Forage Based S ystems Most of the cow calf production in Florida occurs in grazing system using warm season grasses; therefore, seasonal herbage a ccumulation and decreased nutritive value may limit animal production during some months of the year (Moore et al. 1991). Well managed warm season grasses may meet the nutritional requirements of the cow herd during spring, summer, and early fall, howeve r a supplementation program is required to support the nutritional status of the herd during the periods of shortage of forage (Vendramini and Arthington, 2010). Moore et al. (1991) summarized the nutritive value of forages commonly used in Florida and rep orted that the majority of the samples have CP concentrations between 50 to 70 g kg 1 and total digestible nutrients (TDN) from 480 to 510 g kg 1 According to the NRC (1984) these values do not meet the requirement of a lactating beef cow (110 g CP kg 1 and 620 g TDN kg 1 ). When energy and protein requirements increase due to lactation, pregnancy, and growth, part of the forage component of the diet may need to be replaced by concentrates (Fontaneli, 1999) Concentrates generally are more digestible than forages and have greater fermentation rates. According to Stockdale et al. (1987)
31 several factors may affect the response to supplement ation including forage quantity and quality and the amount and composition of the concentrate fed. A database developed by Moo re et al. (1991) from a large number of publications involving CP supplementation of temperate and tropical grasses and crop residues revealed that cattle consuming forages with TDN:CP ratio of 7.0 likely have positive responses to CP supplementation. Prot ein and energy are the nutrients required in greater quantities by ruminants; therefore, protein and energy supplementation programs are the most explored during the periods of decreased forage supply in beef cattle production systems. Protein and Energy S upplementation Energy and protein are the nutrients required in greater amounts by grazing cattle, therefore, those are the main components supplemented to grazing livestock. There are usually interactions between supplement and forage, also known as associative effects which may decrease or increase forage digestibility and intake (Moore et al., 1999). Such interactions occur by changes in passage and digestion rate of the forage by the supplement (Ellis, 1978) which alters forage dry matter intake ( DMI ) (McCollum and Galyean, 1985). Protein S upplementation Protein is a compound that yields N in an amino form and contains one or more chains of aminoacids in addition to C H O, and N plus S In feedstuffs for livestock, CP is generally expressed as the N concentrati on multiplied by 6.25. In ruminant nutrition, CP can be fractionated in to three different components RDP which is converted into microbial protein, RUP that is degraded in the gastrointestinal tract and lastly undegradable protein which is excreted in feces (NRC, 2001 ). R umen degradable
32 protein is divided in non protein nitrogen and true protein which are the sources of nitrogen for rumen microbial population (Owens and Zinn, 1998). Rumen Degradable P rotein Rumen degradable protein is the protein frac tion that is degraded in the rumen by the microbial population and produces microbial protein and amino acids The proportion of the protein ingested by ruminants that can be degraded in the rumen ranges from 20 to 100% (Owens and Zinn, 1998). Rumen degrad able protein is used to improve microbial protein production in the rumen. Microbial protein is a high quality protein that is highly digestible in the small intestine. The usual amino acid profile expressed as a percent of total CP in the ruminal microbia l protein are : histidine (2.2%), isoleucine (7.3%), leucine (9.4%), phenylalanine (6.8%), threonine (6.4%), tryptophan (6.8%), valine (7.2%), methionine (2.6%) and lysine (11.3%), the last two aminoacids are known as the first limit ing amino acids in the r uminant diet (Van Soest, 1994). According to NRC (2001) feedstuffs have lower values of essential amino acids than microbial protein and amino acids from microbial protein ha ve greater use efficiency due to balance, consistence of the aminoacid profile, a nd extensive degradation (Owens and Zinn, 1988). A nimals grazing low nutritive value forage are usually deficient in RDP which is the first limiting factor for forage digestibility in the rumen (K ster et al., 1996) due to limited microbial activity. Ru men degradable protein increases forage intake and digestibility, improv ing microbial synthesis and improv ing performance o f animals grazing low quality forage (Guthrie and Wagner, 1988; McCollum and Horn, 1990; Mathis et al. 1999) .T here is a positive relationship between RDP and rumen ammonia (McCollum and Galyean, 1985; Mathis et al., 2000) to levels above the minimum
33 required 5 mg dL 1 (Satter and Slyter, 1974). K ster et al., (1996) reported that supplementation of 4 g of RDP kg 1 BW .75 increased the total tract digestibility of NDF, digestibility of organic matter, which lead ed to an increase in forage intake on cows consuming tallgrass prairie forage. Guthrie and Wagner (1988) worked with steers and heifers receiving low nutrit ive value prairie hay (52 g kg DM 1 CP), which consisted mainly of little bluestem ( Schizycharium scoparius (Michx.) Nash]), big bluestem ( Andropogon gerardii Vitman), i ndiangrass [ Sorgbastrum nutans (L.) Nash] and switch grass ( Panicum virgatum L.), and su pplemented with different levels of CP, low level (0.36 kg d 1 of 32 0 g kg 1 CP), high level (0.67 kg d 1 of 34 0 g kg 1 CP), grain based supplement (1.41 kg d 1 of 13 0 g kg 1 CP), or control (no supplement). The low CP level and grain supplements provided the same amount of CP, which was half of the high level of CP. There w ere greater dry matter intake digestibility and faster passage rate s for animals supplemented with high CP supplement level than the other treatments. Animals receiving low CP supplem entation lev els and grain based supplement had similar dry matter intake which was greater than the dry matter intake of animals that did not receive concentrate supplementation. Mathis et al. (1999) evaluated the performance of beef cows grazing low qua lity tall prair ie (53 g kg 1 CP, 490 g kg 1 RDP) forage and consuming increasing soybean [ Glycine max (L.) Merr] meal levels from 0.08 to 0.48 % body weight (BW) daily. Soybean meal supplementation with 0.30% BW d 1 improved performance of cows grazing low quality pastures. Wickersham et al. (2008) reported a linear increase in total organic matter and NDF digestibility increasing or ganic matter intake on crossbre d
34 steers receiving tallgrass prairie hay (big bluestem little bluestem and i ndiangrass) as RDP intake increased from 0 to 177 mg N kg 1 BW In general, RDP increases dry matter intake as a consequence of the increase in passage rate and forage digestion (McCollum and Galyean, 1985). In addition, Wickersham et al. (2008) stated that RDP supplementation increases N recycling in the rumen providing from 25 to 33% of N utilized by ruminal microbes and retained in the rumen which improve d forage utilization by providing N for microbial protein synthesis. Non Pro t ein Nitrogen The term non protein nitrogen (NPN) is used in ruminant nutrition to describe feedstuffs that are not protein but can be converted into microbial protein by ruminal microbes through incorporation of the ammonia released by the enzymatic bre akdown of NPN (Hammond, 1992). Usually animal performance increases when NPN is fed to low nutritive value forage based diet moreover animals can live and reproduce when NPN is the only source o f nitrogen in the diet ( Owens and Zinn, 1998) The most common source of NPN is urea. Urea has 450 g N kg 1 and it is 100% soluble with a digestion rate of 400%/hr (NRC, 2000). Urea has been utilized in ruminant diets for more than 100 years (Kertz, 2010). Urea is often used to decrease the cost of p rotein supplementation and can substitute up to 33% of the degradable protein intake in high protein supplement without affecting cow calf performance (Kster et al., 2002) ; however, it is not recommended to exceed 1% of the total dry matter intake in the concentrate, or 20% of the total dietary CP (Kertz, 2010). Urea is rapidly dissolved and hydrolyzed to ammonia in the rumen and utilized by microorganisms to convert ammonia into microbial protein. Excess ammonium in the rumen can increase rumen pH, increa se ammonia absorption by the rumen epithelial,
35 and may reach toxic levels in the blood (Essig et al., 1988). Urea is a feasible CP supplement that can be added to liquid molasses to supplement grazing animals (Tillman et al., 1951; Brown et al., 1987). Hol derbaum et al. (1991) reported an increase in ADG of steers grazing limpograss pastures fed corn urea supplement compared to non supplemented steers, 0.53 vs. 0. 29 kg d 1 Brown and Adjei (2001) tested the inclusion of urea and / or hydrolyzed poultry feath er meal in molasses supplement of yearling steers grazing limpograss pastures. Supplements were isonitrogenous and with the same amount of RDP and RUP T he authors reported improve d performance of supplemented steers in one out of three years of the experiment but there was no difference between supplement type s In several experiments, Kster et al. (2002) tested different levels of urea in the supplement on forage intake, digestion and animal performance o f cows consuming tallgrass prairie or fo rage sorghum ( Sorghum bicolor L.). Levels of urea we re 0, 20, 40, and 60% of RDP and 0, 15, 30 and 45 % of the supplemental RDP T here was no difference in total OM and forage intake and OM and NDF digestibility of tallgrass prairie hay (CP = 24.2 g kg 1 ). However, there was a linear increase in rumen ammonia as urea concentration increased. T here was no effect of urea concentrations on performance of cows and calves or on body condition score of the cows grazing dormant tallgrass prairie pastures. C ows f ed forage sorghum hay in the feedlot for two months and move d to tallgrass prairie pastures had no difference i n cumulative body weight change and body condition score and there was no difference in calf performance ; however, t here were linear decreases in body weight change as urea concentration increased. C ows gr azing dormant tallgrass prairie decreased body weight
36 and body condition score during calving and breeding season in all treatments but not during weaning season. In addition calf performance was not affect ed on any of the experiments. Currier et al. (2004) tested the effect of NPN source on performance of cows consuming low nutritive value hard fescue ( Festuca trachyphylla Hack. Krajina ) straw (CP = 43 g d 1 ) on drylot The NPN sources were urea and biuret to provide 90% of RDP requirement. Cows receiving protein supplementation had greater body weight change than the control treatment during the pre calving season (33 vs. 10 kg). Additionally, cows lost less weight during post c alving season ( 14 vs. 40 kg). Kim et al. (2007) compared two sources of protein, soybean meal and urea, mixed with a dried citrus pulp based supplement on performance and dry matter intake on steers consuming bahiagrass hay (CP = 72 g kg 1 ). The authors reported an increase in hay consumption for the urea treatment compared to the control while the greatest hay intake was for steers consuming soybean meal In addition ADG was greater for steers consuming soybean meal than for urea which wa s greater than control. Due to the usual limited CP concentration of warm season grasses, RDP is an important nutritional entity to provide N to the microbial population and potentially increase forage digestibility and intake When RDP requirements are me t, RUP supplementation may improve animal performance Rumen Undegradable P rotein Rumen undegradable protein is defined as the fraction of the protein that is not digested in the rumen by microorganisms but escapes rumen digestion and is digested in the g astrointestinal tract (GIT) starting in the abomasum and finish ing in the large intestine. The digestion is enzymatic until reaches large intestine (Owens and Zinn,
37 1988). According to Van Soest (1994) the sum of microbial protein plus the RUP that escap es rumen digestion is the amount of protein that is digested in the GIT. The author also stated that protein not digested in the rumen has better utilization efficiency if essential amino acids are present. The amount of protein that escapes ruminal digestion is affected by feed process ing dry matter intake rate of passage and digestion rate (Van Soest, 1994). Vendramini et al. (2008) observed that 700 g CP kg 1 of total CP of Tifton 85 ( Cynodon spp.) disappeared in the rumen. However, growing animals may not meet the metabolizable protein requirements from warm season grasses only and RUP supplementation may be needed (Klopfenstein et al., 1996). Supplementing animals with RUP has the obj ective of increas ing amino acid flow to gastrointestinal tract especially the small intestine (Legleiter et al., 2005). Anderson et al. (1988) fed yearling steers a diet based of smooth brome grass ( Bromus inermis Leyss ) (CP = 124 g kg 1 ) with three levels of RUP supplement, 0.11, 0.23, and 0.34 kg d 1 of a blood meal and corn gluten meal supplement. The authors observed linear and quadratic effect s of supplement levels on animal performance (from 0.91 to 1.01 kg d 1 ) as supplementation levels increased (from 0. 1 1 to 0.34 kg d 1 ). Lima et al. (1999) supplemented beef heifers grazing limpograss pastures with RDP and RUP plus RDP in combination with two pasture N fertilization levels (50 vs. 150 kg N ha 1 ). The authors repo rted an increase i n animal performance from 0.06 to 0.41 to 0.56 kg d 1 for control, RDP alone and RDP plus RUP respectively, when pastures were fertilized with 50 kg N ha 1 However the response was less when pastures were fertilized with 150 kg N ha 1 0.36, 0.39 and 0.47 for control RDP and RDP plus RUP
38 respectively. The forage CP increase d from 56 to 73 g kg 1 as fertilization increased from 50 to 150 kg N ha 1 Low levels of forage CP plus RDP and RUP supplement increased animal performance; however when forage CP was higher as with a greater rate of N fertilizer, there was no respon se to supplementation, suggesti n g that animal protein requirement was met by RUP supplementation or by increasing N fertilization. Wickersham et al. (2008) repor ted a decrease in forage digestibility but an increase in forage dry matter intake and N when stee rs were infused in the abomasum with increasing levels of casein (0, 62, 124 and 186 mg N kg BW 1 ). The authors also found an increase in N recycling as infu sion levels increased providing N for microbial forage digestion. Vendramini et al. (2012) tested the effect of increasing levels of RUP supplementation on performance of calves grazing stargrass pastures. The authors reported no difference i n performance (ADG = 0.56 kg d 1 ) of early weaned steers grazing stargrass (CP = 171 g kg 1 RDP = 550 g kg 1 and RUP = 450 g kg 1 of total CP) pastures and supplemented with three levels of RUP ( 0. 350, 0. 475 and 0. 600 kg d 1 ) in Flor ida. In addition, there was no e ff ect of supplementation on forage and total dry matter intake ; however, apparent forage dry matter digestibility was negatively affected by RUP supplementation. The authors stated that supplementation with RUP did not affect performance because CP of the forage was above animal requirement s Bandy k et al. (2001) stated that animal performance response s to RUP are expec ted after the RDP requirements are met Energy Supplementation During the seasonal periods of decreased forage production or nutritive value, forage only may not meet the energy requirement for maintenance, gestation and milk
39 production of grazing beef cows (NRC, 2001). Consequently supplemental feed is required to meet cow requirements and maintain animal production Differently from p rotein supplementation, energy supplements tend to reduce forage intake when supplemental feed was offered at greater rates than 0.7% of BW (Moore et al., 1999). The decrease in forage intake ha s been correlated with decrease d ruminal pH, which reduces act ivity of the cellulolytic enzymes (Martin et al., 2001) and fiber digestion (Mould et al., 1983). Moreover microbial protein is also affected by lowering the ruminal pH (NRC, 2001). In an energy supplementation review, Caton and Dhuyvetter (1997) reported that the decrease in forage intake is correlated with substitution of forage by energy supplement, however, when energy supplement is offered at low levels, forage dry matter intake and digestibility may be improved. According to Poppi and McLennan (1995) energy supplements for grazing animals are fiber, sugar and starch. The most common energy supplement in South of Florida is sugarcane ( Saccharum officinarum L.) molasses due to low cost, availability of the product, and convenience of less frequent fee ding One of the most used supplementation energy source used by beef cattle producers in Florida is molasses. Molasses Sugarcane molasses is any feed containing more than 43% sugar and a minimum of 79.5 Brix (Curtin, 1983). Molasses had been used in animal supplementation for more than 50 yr and the most used molasses product in Florida is the blackstrap molasses (Pate an d Kunkle, 1989). Molasses is a s ource of readily available carbohydrate for ruminants. Molasses may improve diet digestibility ; however, it may decrease forage and fiber digestibility. The amount of molasses and protein in
40 the final diet drives the magnitu de of the decrease (Pate, 1983). Molasses has low levels of protein thus adding protein sources to molasses is necessary to have a better energy:protein bal ance in the supplement. According to Pate and Kunkle (1989), Florida molasses has higher levels of protein compared to molasses from other regions In a review of molasses utilization in beef cattle nutrition, Pate (1983) summarized several studies comparing molasses with other sources of supplementation to beef cattle The authors stated that molasse s supplementation had similar outcomes to corn grain supplementation as an energy source when plant protein sources were added to the mixture. However, when urea was used as source of CP supplementation added to molasses, animal performance was negatively affect ed when compared to corn grain. In a 4 yr study, Pate et al. (1990) compared the inclusion of CP supplements (urea and cottonseed meal ) fed to cow calf pairs grazing dormant bahiagrass pastures receiving stargrass hay and molasses supplements during the winter in Florida. There were no difference s in cow ADG and body condition score although an increase in pregnan cy rate was found for cows that received molasses cottonseed meal urea supplement. In addition, no differen ces were found in performance of the calves. Mature cows (3 yr old) were heavier when supplemented with molasses cottonseed meal urea supplement than molasses. Kalmbacher et al. (1995) reported an increase in apparent OM digestibility of bluestem ( Schizachyr ium scoparium var. stoloniferum) fed to mature cows supplemented with molasses with or without urea or soybean meal. The molasses urea soybean meal treatment decreased apparent NDF, ADF, and hemicellulose digestibility. Moreover, molasses supplement plus u rea was as efficient as soybean meal in
41 maintain ing cow body condition score and calf weaning weight. Vendramini and Arthington (2010) studied the effect of soybean hulls added to molasses supplement on first cal f heifers grazing bahiagrass. Heifers receiv ed three levels of soybean hulls (0, 1.6, and 3.2 kg DM heifer 1 d 1 ) plus 1.6 kg of liquid molasses and 0.8 kg of cottonseed meal The authors reported a linear increase in ADG body condition score change, and calf ADG with increasing soybean hull supplem entation levels. In addition, i n a second experiment with heifers in a drylot, dry matter intake of stargrass hay decreased quadratically as the level of soybean hull supplementation increased. In addition to the composition of the supplement, f requency o f supplementation may be an important factor on efficiency and profitability of supplementation programs for grazing animals (Kunkle et al., 1999). Wettemann and Lusby (1994) tested two different supplementation frequencies (3 and 6 d wk 1 ) on performance of cows grazing dormant bermudagrass and receiving a 400 g kg 1 CP supplement. Authors reported no difference in weight loss, body condition score and pregnancy rate by cows consuming supplement 3 or 6 d wk 1 Huston et al. (1999) stated that no differen ce in body weight change and body condition score on cows grazing warm season grasses supplemented daily, 3x, or 1x wk 1 with a protein supplement (410 g kg 1 CP) In addition, there was no difference in forage intake between supplementation frequencies. In a review of supplementation studies, Kunkle et al (2002) reported that cattle receiving protein supplement fed infrequently had similar performance when compared to animals receiving daily supplementation. Bohnert et al. (2002) reported no difference in cow body weight change, pre calving (14 d of calving) and post calving (within 24 h of calving) body condition score
42 change, and calf birth date between animals fed RDP and RUP supplement daily, 2x, or 6x wk 1 Farmer et al. (2004) tested the effect of frequen cy of feeding and levels of protein supplementation on cow performance grazing tallgrass and reported similar performance for cows. The authors reported an improvement i n OM and NDF digestibility for animals fed daily compared to 3x wk 1 ; however no difference in forage, supplement and digestible OM intake were detected. Additionally no differences i n cow and calf ADG and cow body condition score were detected. Schauer et al. (2005) fed grazing cows either daily or 1x wk 1 with cottonseed meal (430 g kg 1 CP) and reported no difference in ADG and no e ffect on grazing behavior between supplementation frequencies Cows in b oth supplement treatments had greater performance than non supplemented animals. Drewnoski et al. (2012 ) supplemented steers daily, 2x, or 3x wk 1 with a blend of corn gluten and soybean hulls and tall fescue ( Festuca arundinacea Shreb.) hay. The authors reported a decrease i n hay and total dry matter intake as frequency of supplementation decreased. There w as a positive effect of supplementation on ADG compared to the control; nonetheless, no difference in animals performance was report ed between supplementation frequencies. Further, feed efficiency improved as frequency of supplementation decreased. Molasses is a supplement with self limiting intake and allow the convenience of infrequent feeding events, which is a favorable characteristic in supplementation of beef cattle grazing systems. Cooke et al. (2007) compared molasses based supplement fed 3x wk 1 with citrus pulp based supplement fed 3x wk 1 or daily to beef steers The authors reported
43 greater performance of steers fed with citrus pulp supplement. No differences i n animal performance between supplementation frequencies were observed. Cooke e t al. (2008) studied the effect of two supplementation frequencies (daily or 3x wk 1 ) of an energy supplement based on fibrous byproducts [wheat ( Triticum spp.) middlings and soybean hulls] and molasses on performance of heifers grazing bahiagrass pastures The authors reported greater ADG for heifers fed daily compared to infrequent supplementation (0.41 vs. 0.33 kg d 1 ) and heifers fed daily reached puberty earlier and had greater pregnancy rate. It is imperative to design a supplementation programs to a chieve desirable performance and increase the efficiency of cow calf p roduction systems. Molasses has been a feasible supplement to beef cows in Florida due to the convenience of infrequent feeding and availability of molasses base supplements in South Flo rida. In addition to provide supplements to the cows, supplementing the calves may be an option to improve weaning weights and increase the profitability of cow calf operations. Creep Feeding Creep feeding is a supplementation strategy used to provide addi tional nutrients to nursing calves. Creep feeding can be used to overcome limited herbage allowance, improve cal f uniformity, supply extra nutrients for calves, provide adaptation to concentrate diets before weaning, and increasing weaning weight (Bray, 19 34; Powell, 1936 ; Stricker et al. 1979; Martin et al., 1981; Lusby and Wettemann, 1986; Faulkner et al. 1994, Moriel and Arthington, 2013). Ad libitum creep feeding ha s been correlated with increase d calf performance; however the concentrate:gain ratio is usually inefficient, with 5 15 kg feed required per additional kg of body weight gain (Stricker et al ., 1979). Such low feed efficiency can be
44 related to decreased forage intake, likely due to decrease d ruminal and total tract NDF digestibility when ca lves are fed with grain in excess of 25% of their diet (Mould et al., 1983 ; Cremin et al., 1990). In addition, several authors have reported that calves can substitute forage and / or milk for the concentrate (Wyatt, 1977, Cremin et al., 1991, Tarr et al., 1994, Faulkner et al., 1994). Milk is a high quality feed that can potentially meet the amino acid requirement of calves, thus a decrease in milk intake would decrease intake of amino acids (Loy et al., 2002). Energy supplements, commonly used in creep fe eding diets may lead to more starch digestion, increasing volatile fatty acid ( VFA ) production and lowering ruminal pH (Tarr et al., 1994). Bray (1934) compare d non supplemented calves with two creep feeding supplementation periods, 70 and 133 d before w eaning. Creep feeding supplement was corn, rice bran ( Oryza sativa L.) and cottonseed meal mix. Average daily consumption of feed was 1.5 kg of both creep feeding treatments. Authors concluded that creep fe e d ing calves for 133 d resulted in a gain of 40% and for 70 d a gain of 28.4% more body weight than non supplemented calves. Moreover, there was a net increase in value for creep fed calves. Creep fed calves had 0.2 kg d 1 greater ADG than non creep fed calves (Bray, 1934). Powell (1935) tested three l evels of creep feeding supplement containing 150, 172, and 190 g kg 1 of CP for 187 d and compared these treatments to non supplemented calves. The average supplement intake was 0.8, 1.0, and 1.4 kg d 1 per calf, respectively for the protein levels. The a uthor reported an improvement i n ADG for 20% of creep fed calves ; however, no differences due to level of protein were found Powell (1935) compared the ADG of non supplemented calves with calves
45 supplemented with 126, 126 and 131 g kg 1 of CP supplement consuming 1.4, 0.88 and 1.22 kg d 1 during 165 d. Creep fed calves gained on average 15% more weight and were more uniform than non supplemented calves. Sticker et al. (1979) reported a 32 kg weaning weight increase in creep fed calves supplemented with 1.9 kg d 1 of an energy supplement compared to control calves; however, the feed efficiency was ~ 10 kg of supplement per extra kg of weight gain. In this study, creep feeding was not an economically viable practice due to low feed:gain efficiency. Martin et al. (1981) reported in a creep feeding literature review that creep fed calves with ad libitum energy supplement had superior performance (additional 15 kg BW) than calves that were no n supplemented. Prichard et al. (1989) compared two creep feeding pe riods (154 vs. 64 d) using a high energy supplement to a non supplemented group and reported greater ADG for creep fed groups compared to control. The feed:gain efficiency was 5.3 kg of supplement per extra kg of gain, and no difference between creep fed p eriods was reported. Similarly, Tarr et al. (1994) compared three creep feeding supplementation periods (28, 56 and 84 d prior to weaning) on calves grazing endophyte infected tall fescue. There was a linear increase i n calf ADG as creep feeding period increased, however, 56 d prior weaning showed the best feed efficiency. No effects of creep feeding periods were observed on cow performance. There is sufficient evidence in the literature that creep feeding calves using a grain ba sed energy supplement increases cal f ADG however, the economical feasibility is doubtful due to the decreased feed:gain efficiency. Warm season forages may have decreased CP and RDP concentrations (Minson, 1990), thus providing a limited amount of RDP ma y improve calf performance
46 with greater feed:gain efficiency than unlimited creep fed energy supplements. It is expected that protein supplementation would increase the dry matter intake and digestibility of warm season forages with limit ing CP concentrati on and increase the forage utilization by calves receiving protein supplementation (Wheeler et al., 2002). Lusby et al. (1985) proposed a limit fed protein supplementation (0.37 kg CSM d 1 ) to nursing calves for 63 d i n Y ear 1 and 76 d i n Y ear 2. Creep fe d calves had 0.13 kg greater ADG than control calves with a feed efficiency of 2.5 kg of supplement per extra kg of gain. There was no effect on cow performance. In addition, authors stated that in order to have the greater feed:gain efficiency, there was an increase in forage intake and/or improve ment in forage digestibility. Lusby and Wettemann (1986) supplemented calves (77 kg initial BW) with 0.45 kg soybean meal d 1 in creep feeding and compared to non supplemented calves for five months. There was gre ater ADG in the creep fed than control calves from December to March likely due to decline in milk production of the cows and increased forage intake of the calves. There was no difference i n cow performance between treatments. Cremin et al. (1990) compar ed limited intake of low and high protein supplements, or unlimited low protein supplement with a control group grazing cool season forages. There was no difference in forage, organic matter ( OM ) NDF, ADF, and milk intake between limited protein supplemen t treatments C alves receiving the unlimited low protein treatment consumed less forage and the total OM intake (forage plus supplement) was greater than the other treatments R uminal pH decreased as creep feeding levels increased, resulting in a decreased ruminal fiber digestibility. However, no differences were reported in NDF total tract digestibility between control
47 and limited creep fed treatments The authors attributed the similar NDF digestibility to a longer ruminal retention, which allowed the for age to be digested at a lower rate. Faulkner et al. (1994) compared calves that did not receive creep feeding supplementation (control) with creep fed calves supplemented with two different sources of energy, corn or soyhulls, with unlimited or limited intake of endophyte infected tall fescu e. There was a quadratic increase in calf ADG from control (0.66 kg d 1 ), limit ed (0.92 kg d 1 ), and unlimited forage intake treatments (1.04 kg d 1 ) and no difference between sources of supplement. However, calves consumed less soy bean hulls than corn (1 .53 vs. 1.77 kg d 1 ) on unlimited forage intake treatments. Feed efficiency (feed:gain) was similar between treatments. In addition, there was a negative correlation between supplement consumption and forage intake, and corn decreased forage digestibility at greater magnitude than soy bean hulls. The authors concluded that soy bean hulls can substitute for corn as a creep supplement without affecting performance and fiber digestion. Moriel and Arthington (2013) reported inconsistent benefits of limited fed protein creep feeding to calves 112 d prior to weaning. Calves were supplemented 3 x wk 1 at 0.23 kg d 1 of a loose meal protein supplement (CP = 210 g kg 1 TDN = 750 g kg 1 ).There was an increase in ADG from 0.88 to 0.95 kg for control and creep fed calves, respectively. In a second experiment, calves were fed a cubed supplement (CP = 190 g kg 1 and TDN 711 g kg 1 ) and there was no difference in calf performance between treatments. Although creep feeding is a management practice known for several years it is not widely adopted to the low gain:feed efficiency. However, l imited creep feeding is
48 an approach with potential to be incorporated into cow calf enterprises seeking to increase calf performance with greater gain:feed efficiency. Bermudagrass Hill et al. (2001) reported that bermudagrass is one of the most important grasses in southern USA. Bermudagrass covers around 15 million h a in the USA (Taliaferro et al., 2004). According to Hanna and Sollenberger (2007) bermudagrass is present in tropical and subtropical regions in all continents. Bermudagrass was introduced in the USA around the late 1600s, although breeding programs were not started until 1937 by Dr. Glenn Burton, United Stated Department of Agriculture Agriculture Research Service (USDA released in 1943, and was planted by several producers in the southeastern USA. Hill et al (2001) reported that bermudagrass supports the beef and dairy industry grazing programs from spring until autumn, and some dairy farmers use it as primary forage in the total mixed diets to feed dry cows and replacement heifers. Redfearn and Nelson (2003 ) stated that bermudagrass is used as a perennial forage and can be used for grazing, hay, or haylage in the southern USA. Bermudagrass is a warm season perennial grass, propagated by rhizomes and stolons. It forms a dense mat above the soil, tolerates a r ange of soils and soil conditions, but it does require high soil nutrient levels to produce and persist (Vendramini, 2005; Liu, 2009). It has a deep root system that allows growth during dry period s and its rhizomes provide protected bud sites for winter survival and spring regrowth (Pitman, 1991). In 1993, the hybrid Tifton 85 bermudagrass w as released and according to Bur ton et al. (1993), Tifton 85 produce d greater herbage accumulation is taller, has
49 broader leaves and is more digestible than Coastal yield was 26% great er and herbage was 11% more digestible than Coastal bermudagrass (Hill et al., 1993). Mislevy and Martin (1998) compared Tifton 85 with and Cynodon nlemfuensis Vanderyst var. nlemfuensis ) They reported a greater total DM yield during summer for Tifton 85 compared to the other grasses. Mandebvu et al. (1999) reported that Tifton 85 had greater dry matter (DM) yield (7.1%) and greater in vitro dry matter digestibility (IVDMD, 7.1%) than Coastal bermudagrass. Coastal had lower NDF and ADF levels than Tifton 85; however, Coastal had lesser IVDOM concentrations than Tifton 85 d ue to greater lignin and ether linked ferulic acid concentration. Although Tifton 85 is tolerant to drought and no stand losses occurred during dry periods (Hill et al., 2001) it is not tolerant of poorly drained soils (Newma n et al., 2011). Seasonal flo od events are common ly found in poorly drained soils in South Florida, where loss in Tifton 85 stand has been reported. Consequently, producers have cultivated Jiggs bermudagrass with the perception of greater tolerance to poorly drained soils. Jiggs is a bermudagrass variety that was released by J.C. Riggs in Texas (Ocumpaugh and Stichler, 2000). The parental material and date of release are unknown. According to Vendramini (2008), Jiggs tolerates poorly drained soils and has thinner stems than other ber mudagrass es which is a desirable characteristic for hay production. In addition, Jiggs establish es faster than other bermudagrass cultivars (Bade, 2000). Mislevy et al. (2008) compared Jiggs and Tifton 85 in a grazing study using the mob stocking techniqu e and reported greater DM yield for Jiggs than Tifton 85
50 (13.9 and 11.9 Mg ha 1 respectively). The same authors did not find a difference in CP concentration but Tifton 85 had greater IVDOM concentration than Jiggs (638 vs. 561 g kg 1 ). Similar results we re reported by Vendramini et al. (2010) working with different warm season grasses (elephantgrass, bahiagrass, stargrass, brachiariagrass, limpograss, and four cultivars of bermudagrass, Jiggs, Coastcross 2 Tifton 85, and Florakirk). Jiggs had lesser he rbage accumulation (4.6 Mg ha 1 ) than elephantgrass (13.0 Mg ha 1 ) but greater than the others grasses. However, no differences were found in the CP concentrations among the forage species and cultivars, (~ 100 g kg 1 ). The clipping studies published in t he literature has shown that Jiggs has favorable herbage accumulation and nutritive value when compared to other warm season grasses in South Florida; therefore, Jiggs may have potential to be used in grazing systems. Grazing Management and Animal R espons es Hill et al. (1993) compared the performance of steers grazing Tifton 78 and Tifton 85 for 3 yr The authors reported no difference in ADG (0.66 kg d 1 ); however, gain per area was greater for Tifton 85 than Tifton 78 (11 60 vs. 7 90 kg ha 1 ) and this was explained by greater number of steer grazing d ha 1 for Tifton 85 than Tifton 78 (182 0 vs. 13 20 d). In a 3 yr grazing study comparing DM production and animal performance on Tifton 85 and Florakirk bermudagrass, Pedreira et al. (1998) reported similar ADG (0.6 kg d 1 ) between Tifton 85 and Florakirk; however, Tifton 85 supported greater stocking rates (6.0 vs. 4.0 heifers ha 1 ) and gain per area (648 vs. 371 kg ha 1 ). Corriher et al. (2007) reported greater ADG (0.94 vs. 0.79 kg d 1 ) and weaning weights (25 3 vs. 240 kg) for calves grazing Tifton 85 than Coastal. Cows grazing Tifton 85 had greater milk protein than cows grazing Coastal. Burns and Fisher (2008) reported no difference
51 in forage mass between Tifton 44 and Coastal pastures grazed at similar can opy height s of 6, 10 and 13 cm (2.36, 4 08; and 5.25 Mg ha 1 ), although steers grazing Tifton 44 had greater ADG than steers grazing Coastal (0.58 vs. 0.51 kg d 1 ). Increasing canopy height resulted in a linear increase in herbage mass (from 2.3 to 5.2 Mg ha 1 ) and ADG in both cultivars (from 0.40 to 0.59 kg). Burns and Fisher (2010) reported ADG of 0.57 kg d 1 with stocking rate of 10 steers ha 1 on continuously stocked Coastal bermudagrass pastures. Liu et al. (2011) compared different post graz ing stubble heights (8, 16, and 24 cm) and grazing cycles (14, 21, and 28 d) on Tifton 85 in a 3 yr study in Central Florida. The authors concluded that the greatest herbage accumulation rate occurred with an 8 cm stubble height and 28 d grazing cycle or with a taller stubble height of 24 cm and shorter grazing cycle of 14 d. In addition, the greatest nutritive value was achieved when Tifton 85 was grazed at 8 cm stubble height. Radunz (2005) observed that thoroughbred pregnant mares ( Equus caballis ) preferre d grazing Jiggs than Tifton 68 and Tifton 85 due to the larger amount of leaves. The mares spent more time grazing Jiggs (26%) followed by Tifton 85 (18%) and Tifton 68 (13%). In addition, no difference was found in DM production between those three grasse s ( 3.8 Mg DM ha 1 ). Nutritive value was superior for Jiggs and Tifton 68 than Tifton 85 (180, 193, and 149 g kg 1 CP and 624, 595, and 667 g kg 1 NDF, for Jiggs, Tifton 68, and Tifton 85, respectively). Baker (2005) observed that Jiggs plots harvested a t 7.6 cm stubble height had an average herbage accumulation of 7.9 Mg DM ha 1 yr 1 and CP concentration of 132 g kg 1 from 1996 to 2 006 in Ardmore, OK. Dore (2006) compared three bermudagrass es
52 er herbage mass (1.2, 6.7, 8.6, 10.6, and 8.9 Mg DM ha 1 ) with different days of growth (14, 28, 42, 56, and 70 d) respectively, although CP (208, 114, 80, 58, and 58 g kg 1 ) and NDF (581, 638, 689, 679, and 706 g kg 1 ) were similar among cultivars Common bermudagrass had the least ADF (247, 284, 302, 29.6, and 294 g kg 1 ), while Jiggs and Russel had similar ADF values (274, 344, 359, 350, and 365 g kg 1 ). Stocking R ate Stocking rate is defined as the relationship between weight or number of animals per u nit of area in a certain period of time (Allen et al., 2011). Among of all the variables in the grazing management, stocking rate is the most important because it is related to herbage mass, animal performance and profitability of grazing systems (Hull et al., 1965, Hernndez Garay et al., 2004, Gunter et al., 2005). Pastures stocked at low stocking rate tend to increase animal selection for superior nutritive value material, due to a greater herbage mass and allowance which has potential to increase animal performance. Conversely, greater stocking rate tends to increase animal production per area (Mott, 1960; Adjei et al., 1980; Rouquette et al., 1983; Brasby et al., 1988, Gunter et al., 2005; Inyang et al., 2010). Moreover, there is an uppe r limit in stocking rate when maximum gain per area can be achieved (Riewe, 1961) and each forage type and grazing system would have its own relationship between stocking rate and animal performance (Brasby et al., 1988) Animal Performance and Forage Resp onses Hull et al. (1965) compared three stocking rate s (4.5, 9.0 and 13.5 steers ha 1 ) on performance of steers grazing orchardgrass ( Dactylis glomerata L.) and Ladino clover ( Trifolium repens L.) previously fed with two levels of energy intake. The autho rs
53 reported a decrease in animal performance as stocking rate increased for all steers. Average daily gain for steers previously receiving low energy supplementation w as 0.76, 0.57, and 0.38 kg d 1 for 4.5, 9.0, and 13.5 steers ha 1 respectively. Steers p reviously receiving medium energy supplementation gained 0.52, 0.38, 0.19 kg d 1 for 4.5, 9.0, and 13.5 steers ha 1 respectively. In addition, there was linear decrease in dry matter intake as stocking rate increase d from 8.2 to 5.7 kg DM d 1 Herbage acc umulation rate was greater for low and medium than for heavy stocking rate (80 vs. 68 DM ha 1 d 1 ). Adjei et al. (1980) tested three stocking rate s on three stargrass cultivars UF ( Cynodon nlemfuensis Vanderyst var. nlemfuensis Cynodon aethiopicus Clayton and Harlan). The stocking rates were 7.5 10, and 15 steers (240 kg LW ha 1 ) .There were different magnitude of responses to stocking rate s by different cultivars; however, there was a linear decrease in ADG with increasing stoc king rate for all cultivars (from 0.47 to 0.21 kg d 1 ) Rouquette et al. (1983) tested the effect of three stocking rate s (2.0, 3.4, and 6.7 AU ha 1 low, medium, and heavy, respectively) on performance of weanling calves rass ( Lolium multifolium clover ( Trifolium vesiculosum Savi). There was greater ADG for low and medium (average 1.11 kg d 1 ) compared to heavy stocking rate (0.77 kg d 1 ), and heavier calves were reported for low stocking rate ( 333 kg) at weaning compared to medium and heavy stocking rate (average 283 kg). In contrast, a linear increase in gain per area was reported (308, 478, 687 kg ha 1 for low, medium and heav y stocking rate respectively). Guerrero et al. (1984) concluded th at ADG was inversely related to stocking rate for five
54 different bermudagrass forages, and al though the amplitude of the difference varied among forage types, the linear decline was similar for all. In addition, there was a decline in herbage mass for all forage types as stocking rate increased Conversely digestibility increased as stocking rate increased. Aiken et al. (1991) tested the effect of three stocking rate s (2.0, 3.5, 5 steers ha 1 for Y ear 1 and 3.0, 5.3, 7.5 steers ha 1 for Y ear 2) on forage and animal responses. Steers (256 kg) grazed bahiagrass pastures overseeded with carpon desmodium [ Desmodium heterocaropon ( L. ) DC. ] aeschynomene, or phasey bean [ Macroptilium lathyroides ( L. ) Urb. ]. Results showed a linear decrease in he rbage allowance and ADG as stocking rate increased and a positive relationship between stocking rate and gain per unit area i n the first year of the study. Hern ndez Garay et al. (2004) studied the effect s of three stocking rate (2.5, 5.0, and 7.5 bulls ha 1 ) on stargrass pastures and cattle performance. Authors reported a linear and quadratic effect on herbage accumulation on the first year and no differences i n the second year as stocking rate increased. In addition there was a linear effect on herbage mass i n both years of the study as stocking rate increased. Nutritive value increased linearly as stocking rate increased moreover there was a linear and quadratic effect on herbage allowance on both years. Average daily gain decr eased quadratically as stocking rate increased i n Y ear 1 (0.70, 0.53, and 0.26 kg d 1 ) and Y ear 2 (0.65, 0.55, and 0.35 kg d 1 ). Gunter et al. (2005) tested the effects of N fertilization and stocking rate on performance of steer calves (231 kg) grazing dallisgrass ( Paspalum dilatatu m Poir.) and bermudagrass mix ed pastures. There was a linear decrease in ADG (0.63, 0.61, 0.51
55 and 0.34 kg d 1 for stocking rates of 3.7, 6.2, 8.6 and 11.1 steer ha 1 respecti vely) when pastures were fertilized with 112 kg N ha 1 ; however, ADG was not affected when the pastures were fertilized with 224 or 336 kg N ha 1 Inyang et al. (2010) tested the effect of three stocking rate s on herbage characteristics and animal response of beef heifers II Brachiaria sp.). Authors reported a linear decrease in herbage mass (5.9 to 3.2 Mg ha 1 ) and a quadratic increase in herbage accumulation rate (106 to 118 kg ha 1 d 1 ) as stocking rate increased from 4 to 12 heifers ha 1 A quadratic effect was also reported on herbage allowance (2.8, 1.2 and 0.6 kg DM kg 1 LW) and live weight gain (190, 353, and 218 kg ha 1 ) for 4, 8, and 12 heifers ha 1 On the other hand, there was a linear decrease in ADG from 0.28 to 0.01 kg d 1 Stocking rate had a greater effect on animal performance than forage species. There is limited information available in the literature about using Jiggs in grazing systems. Due to the importance of stocking rates on forage and animal producti on there is a necessity t o evaluate the effects of a range of socking rates on Jiggs forage characteristics and animal performance.
56 C HAPTER 3 THE EFFECTS OF DIFFE RENT SOURCES OF RUME N DEGRADABLE PROTEIN (RDP) SUPPLEMENTATIO N ON PERFORMANCE OF COWS AND C ALVES GRAZING STOCKPILED LIMPOGRASS PASTURES IN FLORIDA DURING THE WINTER O verview of the Research Problem Limpograss [ Hemarthria altissima (Poir.) Stapf et C. E. Hubb.] can produce approximately 35% of annual herbage accumulation during the winter in Flo rida (Brown and Kalmbacher, 1998) and generally contains greater total digestible nutrients (TDN) concentration than other tropical grasses at advanced maturity ( Sollenberger et al., 1988). Moore et al. (1981) utilized sheep ( Ovis aries ) in a feeding trial to estimate the digestibility and intake of bahiagrass ( Paspalum notatum Fl gge), bermudagra s s [ Cynodon dactylon ( L.) Pers.] stargrass ( Cynodon nlenfuensis Vanderyst), and limpograss at 4, 6, and 8 wk of regrowth interval. The digestibility of limpograss was the greatest at all regrowth intervals, resulting in greater forage intake. For these reasons, limpograss is a suitable warm season grass species to be stockpiled in the autumn for subsequent grazing during the winter in South Florida. Although limpo grass usually has greater digestibility than most other tropical grasses at mature regrowth intervals, crude protein (CP) concentration decreases significantly (Sollenberger et al., 1988), reaching levels below the minimum requirement for maintenance of a non lactating mature cow of ~ 80 g kg 1 (NRC, 1996). Therefore, CP supplementation is often required to maintain adequate nutritional status of the cow herd. In Florida, as well as in much of the Gulf Coast region, the use of molasses based supplements for beef cows is common ( Pate and Kunkle, 1989 ). due to decreased cost and convenience associated with infrequent feeding because of self
57 limiting intake characteristics. Arthington et al. (200 4 ) compared heifers receiving the same amounts of CP and TDN as dry feed or molasses and observed that heifers receiving dry feed consumed the supplement in less than 1 h, while the molasses wa s consumed in ~ 48 h. The authors concluded that heifers receiving liquid supplement had greater pregnancy rates than heifers receiving dry feed, primarily due to slower intake of the liquid supplement. The consumption of a relatively large amount of supp lement in a short period of time may result in alteration of the metabolic body rate and may negatively impact pregnancy rates (Arthington et al., 2004) Urea is commonly used as a non protein N source in molasses supplements, primarily due to the reduce d cost when compared to true protein supplements, such as cottonseed ( Gossypium spp.) meal. Holroyd et al. (1979a, 1979 b) observed that adding urea to molasses to lactating cows grazing native pastures improved pregnancy rate but not milk production or cal f weaning weight. However, in a subsequent study, Holroyd et al. (19 8 3) observed that lactating cows grazing native pasture and fed molasses urea supplement had pregnancy rates and calf weaning weights similar to cows not supplemented with urea. Pate et al. (1990) compared performance of cows receiving stargrass hay and supplemented with molasses plus urea or molasses plus cottonseed meal plus urea and observed that body condition score (BCS) and pregnancy rates we re similar between treatments According to Klopfenstein (1996), grazing cattle need approximately 130 g RDP kg 1 of digestible organic matter consumed, but microbial protein alone is likely sufficient to meet the needs of cattle at or near maintenance. Yo ung growing cattle and lactating
58 cows need RDP in addition to the microbial protein to meet the metabolizable protein needs (Klopfenstein, 1996). Therefore, it is necessary to identify whether the different increased performance of cow calf pairs receivin g molasses plus cottonseed meal versus molasses plus urea supplementation observed in the previous studies mentioned above, occurred due to the supply of amino acids to rumen microbes or to the additional RDP provided by the cottonseed meal. In addition, the effects of supplementing different sources of RDP to cow calf pairs grazing stockpiled limpograss are not known. The objective of this study was to evaluate the performance of cow and calves grazing stockpiled limpograss pastures in South Florida suppl e mented with different sources of RDP Material and Methods Grazing Study The study was conducted at the UF/IFAS Range Cattle Research and Education Center (RCREC), Ona, FL (27 o 26' N and 82 o 55' W) from January to March 2011 and 2012. The soil at the rese arch site is classified as Pomona fine sand (siliceous, hyperthermic, Ultic Alaquod). Before the initiation of the study, mean soil pH (in water) wa s 5.1, and Mehlich I (0.05 M HCl + 0.0125 M H2SO4) extractable P, K, Mg, and Ca concentrations in the Ap1 ho rizon (0 to 15 cm depth) we re 35, 7 5 155, and 1450 mg kg respectively. Eight limpograss pastures (experimental units, 1 ha per experimental unit) were established in 2010. Pastures were clipped at a 10 cm stubble height in early October 2011 and fertilized with 56 kg N ha The forage was stockpiled for approximately 90 d from October to January in 2011 /2012 and 2012 /2013
59 Twenty four crossbred cows and calves (Angus sired on crossbred cows) with initial body weight of 418 59 kg and 1 00 19 kg in 2011 and 413 46 kg and 78 12 kg in 2012 were randomly allocated to the eight pastures (three cow calf pairs per pasture). Pastures were stocked continuously using a fixed stocking rate. Treatments were two sources of RDP supplement, ur ea or cottonseed meal, replicated four times in a randomized complete block design. The composition of the supplement is described in Table 3 1. The treatments were isonitrogenous, with similar concentrations of RDP and rumen undegradable protein (RUP) and isocaloric. The supplement was fed 3x wk 1 therefore the amount of supplement fed in each of the three events was the daily amount multiplied by seven and divided by three. In a review of supplement studies Kunkle et al. (200 0 ) reported that cattle receiving protein supplement fed infrequently had similar performance when compared with animals receiving daily supplementation. Cows and calves had ad libitum access to complete salt based trace mineral mix (14% Ca, 9% P, 24% NaCl, 0.20% K, 0.30% Mg, 0.2 0% S, 0.005% Co, 0.15% Cu, 0.02% I, 0.05% Mn, 0.004% Se, 0.3% Zn, 0.08% F, and 82 IU/g of vitamin A). Pasture Sampling Pastures were sampled just prior to initiation of grazing and every 14 d during the grazing period. Herbage ma ss (HM), and nutritive value [ CP and in vitro digestible organic matter (IVDOM ) ] were measured. A direct measure was tak en to determine HM which involved hand clipping all herbage from soil level to the top of the canopy using an electric clipper inside a 0.25 m 2 ring. A t otal o f six measurements w ere tak en in each experimental unit. Clipp ed forage was dr ied for 72 h and weighed. Herbage allowance (HA) was calculated for each pasture as the average HM (mean across two sampling
60 dates within each 28 d period) divided by the average total cow calf live weight during that period (Sollenberger et al., 2005). Table 3 1: Ingredient composition and nutrient profile of treatments fed to animals during E xperiments 1, 2 and 3 Item CSM 1 Urea Ingredient, kg DM d 1 C otton seed meal 1.20 Urea 0.13 Feather Meal 0.28 Corn Meal 0.81 Molasses 1.80 1.80 Nutrient Intake, DM basis CP, kg d 1 0.75 0.76 RDP 2 kg d 1 0.48 0.49 RUP 3 kg d 1 0.28 0.28 TDN 4 kg d 1 2.57 2.66 1 Cotton seed meal 2 Rumen degradable protein 3 Rumen undegradable protein 4 Total digestible nutrients Herbage CP and IVDOM concentration were measured at the initiation of grazing and every 14 d thereafter. Hand plucked samples were taken from each pasture. Herbage was composite d across sites, dried a t 60C for 48 h in a forced air oven to constant weight and ground in a Wiley mill (Model 4, Thomas Wiley Laboratory Mill, Thomas Scientific, Swedesboro, NJ) to pass a 1 mm stainless steel screen. Analyses were performed at the University of Florida Forag e Evaluation Support Laboratory using the micro Kjeldahl technique for N (Gallaher et al., 1975) and CP was determined by multiplying N concentration by 6.25. T he two stage technique was used for IVDOM as described by Tilley and Terry (1963) and modified by Moore and Mott (1974).
61 Animal Response V ariables Cattle were weighed at initiation of the experiment and every 28 d thereafter. Cow body weight and calf body weight were recorded every 28 d. Cow b ody condition score w as visually estimated at the beginning and end of the study. Weights were taken at 0800 h following a 16 h feed and water fast. Average daily gain (ADG) was calculated each 28 d period through the experiment. Milk production was measured by the weigh suckle weigh technique at the beginning and end of the study. Calves were separated from their dams for 12 h, allowed to suckle for 30 minutes, and separated again for 8 h and the procedure repeated. Milk yield was calculated as the differenc e between pre and post suckling calf BW. Milk yield was adjusted to 24 h by dividing the observed differen ce in pre and post suckling calf BW by 20 hours and multiplying by 24 hours. The blood urea nitrogen (BUN) of cows was determined using a kit (Kit B 7551 120, Pointe Scientific, Inc., Detroit, MI) and read on a plate reader at 620 nm. Blood was collected from the jugular vein at each weighing. Samples were placed into 9 mL, Na heparinized syringes (Luer Monovette, LH, Sarstedt, Inc., Newton, NC) and placed on ice. Blood was centrifuged (2000 g relative centrifuge force for 30 min) and plasma was separated and frozen at 20C on the same day. Statistical Analysis Response variables were ADG, BCS, calf ADG, BUN, gain per hectare ( GHA ) HM, HA, CP, and IVDOM The data w ere analyzed using PROC MIXED of SAS (SAS Institute Inc., 2006) with treatment (main plot), year (subplot), and month as fixed effects. Month was a repeated measure. Replicates and their interactions were considered random effects. Treatm ents were considered different when P < 0.10. The
62 means reported were least squares means and were compared using PDIFF (SAS Institute Inc., 2006). Drylot Study Th is study was conducted at the UF/IFAS Range Cattle Research and Education Center (RCREC), Ona, FL (27 o 26' N and 82 o 55' W) in April 2011 and 2012. T reatments were the same as described for the grazing study and distributed in a completely randomized design w ith four replicates. The composition of the supplement is presented in Table 3 1. The supplement treatment was offered three times a week, with the daily amount been multiplied by seven and divided by three. Sixteen cow calf pairs (two pairs per pen) were allocated to one of eight drylot pens to evaluate the effect of the supplement treatment on voluntary forage dry matter intake. The cow calf pairs were selected from the grazing study and maintained in the same treatment. Cows and calves received ground li mpograss hay (63 g kg 1 CP and 520 g kg 1 TDN) with 10% refusals. The hay was processed through a hay chopper (Balebuster 2100, Haybuster Jamestown, ND) to an approximately 5 cm particle size and was fed four times a day. Cows and calves were fed hay separ ately and only the cows had access to the supplement. Cows and calves had ad libitum access to water and a complete salt based trace mineral mix as described previously for the grazing study. The experimental period included a 10 d adaptation period and a 7 d collection period Daily dry matter intake was determined for each pen. All feed refusal s were collected every morning at 0 800, weighed, and subsampled for determination of DM.
63 Statistical Analysis Response variables were total dry matter intake and forage dry matter intake. The data w ere analyzed using PROC MIXED of SAS (SAS Institute Inc., 2006) with treatment (main plot) and year (subplot). Day was analyzed as a repeated measure. Replicates and their interactions were considered random effects. Tr eatments were considered different when P < 0.10. The means reported were least squares means and w ere compared using PDIFF (SAS Institute Inc., 2006). Metabolic Study Th is experiment was also conducted at the UF/IFAS Range Cattle Research and Education Center from April to May 2011 and 2012. The treatments were the same as those described in the grazing study and were tested in a 2 x 2 Latin square design. Two rumen fistulated steers (500 kg of initial BW) were alloca ted in one of two metabolic cages and were fed the supplement treatments three times per week (Monday, Wednesday and Friday) with the daily amount be ing multiplied by seven and divided by three. Steers received ground limpograss hay (63 g kg 1 CP and 520 g kg 1 TDN) with 10% refusals during the experimental period. The hay was processed through a hay chopper (Balebuster 2100, Haybuster Jamestown, ND) to an approximately 5 cm particle size and was fed four times a day. The experimental period consisted of a 10 d adaptation period followed by 2 d of blood and rumen fluid collection. Blood and rumen fluid were collected in a two hour interval for the first 24 h and every four hours on the next 24 h after the supplement was offer ed The steers stayed in a drylot for 10 d between experimental periods with ad libitum access to water, hay, and a mineral supplement.
64 Blood was collected from the jugular vein into sodium heparin containing blood collection tubes (Vacuntainer, 10 mL, Becton Dickinson, Franklin Lakes, NJ ), placed on ice and then centrifuged (2000 g relative centrifuge force for 15 min), plasma was separated and frozen at 20C on the same day. The BUN was determined using a kit (Kit B 7551 120, Pointe Scientific, Inc., Detroit, MI) and read on a plate r eader at 620 nm. Rumen fluid was collected (50 ml) and filtered through four layers of cheesecloth into a 200 ml plastic container and pH was measured [ Orion pH meter ( Model 330) Perphect LOgR Orion Research, Boston, MA ]. Rumen fluid was then transferred into a plastic container and 0.5 ml of a 20% sulfuric acid solution was added. The container was placed in ice and froze n at 20C until further analysis. Before volatile fatty acids ( VFA ) and ammonia analysis rumen fluid was transferred to a plastic conta iner and centrifuged ( Beckman Coultier, Avanti JE rotor JA 20) for 15 min at 14000 rpm and 10 C and 3 ml of the solution was transferred to a container. Rumen fluid was analyzed Broderick and Kang, 1980) and quantified using a Beckman DU Spect rophotometer (Beckman Coulter, Palo Alto, CA) set at 620 nm and VFA using Agilent 7820A Gas Chromatograph (Agilent Technologies, Palo Alto, CA, 2.5 m x 0.32 mm x 0.45 mm glass column). Rumen ammonia was performed according to Broderick and Kang (1980) and quantifying using a spectrophotometer ( Beckman Coulter AD340 microplate reader, Beckman Coulter, Fullerton, CA) at 630 nm. Statistical Analysis The response variables were pH, BUN, VFA concentrations, and ruminal ammonia The data was analyzed using PROC MIXED of SAS (SAS Institute Inc., 2006) with treatment (main plot) and year (subplot). Day was analyzed as a repeated
65 measure. Replicates and their interactions were considered random effects. Treatments were considered different when P < 0.10. The means reported were least squares means and w ere compared using PDIFF (SAS Institute Inc., 2006). Results and Discussion Grazing Study There was no treatment effect on HM (mean = 3.3 Mg ha 1 P = 0.78, SE = 0.4) (Table 3 2) ; however, there was a year effect ( P = 0.02) with greater HM in 2012 than in 2011 (Table 3 3 ). There was less rainfall during the stockpiling phase (October to December) in 2010 than in 2011 (Table 3 4) resulting in lesser HM during the 2011 experimental period In addition there were five be low 0 C events in 2010 and none in 2011 (Table 3 5 ) contributing to the greater HM in 2012 (Table 3 3 ). The freezing events can affect growth by causing inhibiting photosynthesis (Quesenberry and Ocumpaugh, 1980) and limit ing the potential of limpograss to accumulate HM in the winter months. Ruelke and Quesenberry (1983) reported greater HM in December (10 Mg ha 1 ) when forage was harvested at 2 wk interval s f rom August to February for limpograss pastures fertilized with 75 kg N ha 1 in August. Herbage mass of stockpiled pastures wa s variable and highly depend ent on weather condition s from the time of stag ing to the time of use (Sollenberger et al., 2012). Rainfall, below 0 C temperatures, and N fertilization are the main factors affe cting HM of stockpiled limpograss.
66 Table 3 2: Month effects on herbage mass, in vitro digestible organic matter concentrations and herbage allowance of stockpiled limpograss pastures grazed by cow calf pairs supplemented with molasses and urea or cotto n seed meal. Month Response Variable January February March P v alue SE Herbage Mass (Mg ha 1 ) 4.1 a 3.1 b 2.6 b 0.08 0.4 In vitro digestible organic matter (g kg 1 ) 466 a 421 b 396 c < 0.01 22 Herbage allowance (kg DM kg 1 LW) 2.5 a 1.8 b 1.4 b 0.02 0.2 Means within rows are different if followed by different letters ( P Table 3 3: Year effect on crude protein and in vitro digestible organic matter concentrations of stockpiled limpograss pastures Year Response v ariables 2011 2012 P value SE Herbage mass (Mg ha 1 ) 2.9 b 3.6 a 0.02 0.3 Crude p rotein (g kg 1 ) 149 a 96 b <0.01 4 In vitro digestible organic matter (g kg 1 ) 467 a 388 b <0.01 21 Means within rows are different if followed by different letters ( P Table 3 4: Average monthly precipitation from 1942 to 2012 and during the experimental period from 2010 to 2012 at the Range Cattle Research and Education Center, Ona, FL. Month Rainfall (mm) 2010 2011 2012 3 yr a verage 69 yr a verage January 50 63 12 42 47 February 61 9 10 27 45 March 150 148 7 101 81 October 0.0 100 129 76 69 November 68 4 14 29 39 December 21 3 30 18 47
67 Table 3 5 : Average monthly temperature from 1942 to 2012 and during the experimental period from 2010 to 2012 at the Range Cattle Research and Education Center, Ona, FL. Month Temperature ( C) 2010 2011 2012 3 yr a verage 69 yr a verage January 12.5 14.9 15.3 14.2 12.0 February 12.6 17.8 19.0 16.5 13.3 March 15.4 19.1 20.8 18.4 15.5 October 22.7 21.9 23.3 22.6 20.6 November 19.2 19.7 17.3 18.7 16.4 December 10.9 18.1 17.8 15.6 13.4 There was a decrease in HM from January to March (Table 3 3 ). The decrease likely occurred due to the limited herbage accumulation and forage consumption by the animals. Vendramini and Arthington (2010) reported a decrease i n HM of stockpiled limpograss pastures grazed from February to April (2.7 to 1.5 Mg ha 1 ). During the grazing period (January to March), there was greater rainfall in 2011 than in 2012, especially in January and March (Table 3 4 ). Rainfall events tend to affect negatively the HM of sto ckpiled pastures because it lays the forage down in contact with the soil and decrease s access to the animals. There were three freeze events during the grazing period in 2012 and only one in 2011 (Table 3 5) There was no effect of treatment on CP (mean = 122 g kg 1 P = 0.61, SE = 4) and IVDOM (mean = 428 g kg 1 P = 0.69, SE = 22) concentration of stockpiled limpograss pastures. Arthington and Brown (2005) reported CP concentration lower than 50 g kg 1 on stockpiled limpograss for 10 wk; however, values were from whole plant. The average CP of the hand plucked samples reported in this study w as similar to values reported by Vendramini and Arthington (2010), 120 g kg 1 Crude protein
68 concentrations were greater in 2011 than 2012 (Table 3 3 ). Mislevy and Ma rtin (2007) reported no difference in CP concentration o f limpograss pastures 1, 2, and 4 wk after freezing averaging 8.8 g kg 1 The decreased CP observed in 2012 may be the result of a greater number of freezing events during the grazing period or altern atively to dilution associated with greater HM in 2012 than 2011 In addition, there was no month effect ( P = 0.44, SE = 4) on CP concentration. Vendramini and Arthington (2010) reported a decrease in CP concentration from February to March and no further decrease in April for the first year of the study; however, there was no change in CP from February to March and an increase was reported i n April. It was expected that CP concentration would decrease throughout the experimental period due to the decrease in proportion of leaves in the sward during grazing the negative effects of freezing events, and increasing maturity of the plants. Instead the appearance of new tissue with greater CP concentration during the experimental period may have compensated for the decrease in CP. Vendramini and Arthington (2010) stated that even though CP concentration was adequate, RDP may not meet animal require ment since a greater proportion of the CP of forages with long regrowth period is associated with the acid detergent fiber fraction which is not degradable in the rumen (Vendramini et al., 2008b). There was a month and year effect on IVDOM (Table 3 2 and 3 3 ). The IVDOM concentration was greater in 2011 compared to 2012. The decrease in IVDOM in 2012 was likely the result of greater number of freezing events in 2012. Mislevy and Martin (2007) reported a decrease in IV D OM of limpograss pastures after 1, 2, and 4 wk after freeze, from 627 to 539 g kg 1 Vendramini and Arthington (2010) reported no month
69 effect on IVDOM of stockpiled limpograss pastures averaging 500 g kg 1 but authors reported a year effect on IVDOM concentration. Similar results were repor ted by Arthington and Brown (2005) for the year effect on IVDOM concentration with values of 523 and 445 g kg 1 in 1999 and 2000, respectively Rainfall and temperature differ ed between years and impact ed HM and forage nutritive value G reater rainfall and temperatures during the stockpiling liked increase d HM which decrease d IVDOM concentrations. The IVDOM concentration decreased over time from January to March. The decrease in IVDOM concentrations is contrasting with the maintenance of the CP concentrati ons from January to March. Leaf aging, senescence, death and deterioration on stockpiled forages may reduce forage nutritive value (Burns and Chamblee, 2000). There may be a greater proportion of mature tissues with decreased IVDOM concentrations than youn g tissues, but still with greater CP concentrations. There was no effect of sources of RDP supplementation on HA (mean = 2.2 kg DM kg 1 LW, P = 0.98, SE = 0.3); however, there was a decrease in HA from January to March (Table 3 3). These results were expected because the treatments had similar stocking rates and HM, and there was a decrease in HM from January to March. Although the relationship of animal performa nce and HA in stockpiled pastures is not well documented in the literature, it is expected that HA greater than 1 kg DM kg 1 LW is sufficient for ad libitum forage consumption when forage is the only source of feed to cattle (Sollenberger and Moore, 1997). Therefore, it is expected that forage quantity was sufficient during the experiment, and variations in animal performance could be related to the RDP supplementation treatment or forage nutritive value. Herbage
70 allowance was greater in 2012 than 2011 beca use of the greater HM in 2012 (Table 3 6 ). Table 3 6 : Year effect on animal response variables of stockpiled limpograss pastures Year Response Variable 2011 2012 P value SE B ody condition score 4.9 a 4.4 b <0.01 0.1 Milk yield (kg d 1 ) 6.5 b 7.6 a 0.40 0.8 H erbage allowance (kg DM kg 1 LW) 1.8 b 2.1 a 0.09 0.2 Means are different if followed by different letters ( P There was a treatment x month x year interaction ( P < 0.01, SE = 0.04) on cow ADG (Ta b le 3 7 ). The interaction on cow ADG occurred because there was no difference in January and February 2011 among treatments, however, cows receiving cottonseed meal had greater ADG than urea in March 2011. In 2012, there was no difference between treatments in Ja nuary and March; however, cows receiving urea had greater ADG in February. Greater cow ADG in January 2011 and 2012 may be due to filling effects caused by greater HM and potentially greater forage intake in January 2011 and 2012. The variation in cow ADG throughout the experimental period may be intrinsic to the variation in mature animals with greater ruminal capacity when compared to young animals, since the variation in rumen content is the main source of error on animal weight (Bath et al., 1966). Pat e et al. (1990) reported a variation on cow weight change from 15 to 30% during the experimental period. Kalmbacher et al. (1995) observed no differences on cow weight change (mean = 21 kg) and calf final weight (mean = 194 kg) on cows supplemented with ur ea or cottonseed meal urea (1.6 kg d 1 ) on a molasses based supplement (CP = 300 g kg 1 ) grazing creeping bluestem [ Schizachyrium scoparium
71 (Michx.) ash var. stoloniferum (Nash) J. Wipff] (CP = 47 g kg 1 ) in South Florida. These authors also found a signif icant variation in cow body weight throughout the experimental period. Although it is reporte d that natural protein supplements are often more effective than urea (Clanton, 1978, Pate and Kunkle, 1989), the differences in cottonseed meal or urea as a sourc e of RDP in this study w ere not conclusive. Table 3 7 : Year x treatment x month interaction on average daily gain (kg d 1 ) of cows grazing stockpiled limpograss pastures supplemented with molasses based supplement plus urea or cotton seed meal (CSM). Month Year/Treatment January February March SE 2011 ----------kg d 1 ----------CSM 1.60 a 0.52 c 0.09 b 0.2 UREA 1.61 a 0.62 b 0.48 b 0.2 P value 0.97 0.69 <0.01 2012 CSM 0.63 a 0.31 c 0.23 b 0.2 UREA 0.36 a 0.15 a 0.14 a 0.2 P value 0.28 <0.01 0.69 SE 0.2 M eans followed by the same letter lowercase letter within rows are not different ( P > 0.10). P value for treatments effect within year and month. Similarly, Pate et al. (1990) reported no differences of cow performance when supplemented with molasses, molasses urea, and molasses cottonseed meal urea and receiving stargrass hay. In general, the ADG of cows was greater in 2011 than 2012, probably reflecting the greater forage nut ritive value in 2011. There was no effect of the treatments o n body condition score ; however, body condition score of the cows was greater in 2011 than 2012 (Table 3 5 ), likely because of the greater forage nutritive value in 2011. The diets were formulate d for the cows to maintain a body condition
72 score of 5; however, the lesser than expected IVDOM of the forage likely decreased body condition score There was a treatment x month x year interaction on cow BUN (Table 3 8 ). Although there was no difference between treatments, BUN concentrations increased from January to March in 2011. In 2012, BUN concentrations increased from January to February and were similar in February and March. Urea is more soluble than true source s of protein in the rumen, and if the N is not captured by the rumen microbes promptly, it may be absorbed by the rumen epithelial and increase levels of BUN concentrations. However, the levels of RDP provided by different sources in this study likely resu lt ed in similar amounts of N being absorbed in the rumen epithelial. Cross et al. (1974) observed no difference i n BUN concentration (mean = 12 mg dL 1 ) for steers fed urea or cottonseed meal supplement. Brown and Adjei (2001) reported similar BUN concentration s for heifers grazing limpograss and supplemented with urea, feather meal or urea plus feather meal (mean = 13 mg dL 1 ). Blood urea nitrogen concentrations were greater in 2012 than 2011 (Table 3 8 ). Changes in BUN concentration may be relat ed to energy intake at similar levels of protein intake (Hammond, 1997). Greater energy intake may lead to increase s in ruminal microbes and better utilization of the ruminal N. Therefore, the decreased limpograss IVDOM in 2012 may have led to decreased N use by the ruminal microbes and greater N amounts being absorbed by the rumen epithelial. According to Hammond et al. (1997), cattle BUN concentrations from 9 to 12 mg dL 1 represent a transition range below which responses to protein supplementation are generally positive. In the current study, cows had BUN concentrations above the optimal
73 range (9 and 12 mg dL 1 ) for all but the first month of the first year indicating that RDP was consumed in excess or the ruminal protein:energy ratio was inadequate to optimize ruminal fermentation and a greater amount of N was absorbed by the rumen epithelium. Absorption and excretion of excess ammonia wastes energy that otherwise could be used to improve animal performance (Van Vuuren et al., 1 993). Table 3 8 : Year x treatment x month interaction on blood urea nitrogen ( mg dL 1 ) of cows grazing stockpiled limpograss pastures supplemented with molasses based supplement plus urea or cotton seed meal (CSM). Month Year/Treatment January February March SE ------------mg dL 1 ------------2011 CSM 6.4 c 14.4 b 19.1 a 1.5 Urea 8.4 c 14.1 b 17.1 a 1.5 P value 0.25 0.83 0.24 2012 CSM 17.6 b 20.2 a 21.1 a 1.5 Urea 16.3 b 18.5 ab 20.5 a 1.5 P value 0.44 0.31 0.71 SE 1.5 M eans followed by the same letter lowercase letter within rows are not different ( P > 0.10). P value for treatment effect within year and month. There was no effect (mean = 7.3 kg d 1 P = 0.42, SE = 0.3) on milk production o f cows supplemented with different sources of RDP (Table 3 6) Similar values for milk production (6.6 kg milk d 1 ) of crossbred cows consuming low nutritive value forage were reported by Brown and Brown (2002). Alderton et al. (2000) reported no difference s i n milk production of 60, 90 and 120 d postpartum, approximately 9, 8 and 7 kg milk d 1 respectively, for cows receiving RUP RDP or a combination of RUP and RDP
74 There was a treatment x month x year interaction on calf ADG (Table 3 9 ). There was a no difference in calf ADG between treatments in 2011; however, calves from cows receiving urea had gr eater ADG than from the cottonseed meal treatment in February 2012. Conversely, calves from cows receiving cottonseed meal had greater ADG than urea treatments in March 2012. Pate et al. (1990) observed that calves from 3 yr old cows receiving molasses plus cottonseed meal plus urea had greater weaning weight than calves from cows receiving molasses but ADG was not different f ro m the cows receiving mol asses plus urea. Considering that the calves did not have access to the supplement, similar performance of the calves was expected because there was no difference in milk production between treatments. Drylot Study There were no differences i n hay dry matter intake (HDMI) (mean = 2.1% of BW, P = 0.16) and total dry matter intake (TDMI) (mean = 2.5% of BW, P = 0.11) between the two sources of RDP Similar to those results, Kster et al. (1997) substituted true protein (casein) for urea in level s from 0 to 100% on a supplement with 400 g kg 1 of CP and reported no difference i n tallgrass prairie forage (CP = 24 g kg 1 ) dry matter intake between treatments. Kster et al. (2002) reported no differences i n forage and total intake for steers consumin g dormant tallgrass prairie hay (CP = 24.2 g kg 1 ) when soybean meal was substituted for urea at 0, 20, or 40% of RDP The a uthors stated that differences between true protein versus urea based protein on forage dry matter intake are not expected.
75 Table 3 9 : Year x treatment x month interaction on average daily gain (kg d 1 ) of calves grazing stockpiled limpograss pastures supplemented with molasses based supplement plus urea or cotton seed meal (CSM). Month Year/Treatment January February March SE ----------kg d 1 ----------2011 CSM 1.37 a 0.53 c 0.91 b 0.1 UREA 1.28 a 0.57 b 0.86 b 0.1 P value 0.37 0.65 0.7 2012 CSM 0.70 a 0.22 b 0.72 a 0.1 UREA 0.71 a 0.47 b 0.52 b 0.1 P value 0.94 0.02 0.07 SE 0.1 Within rows, means followed by the same letter lowercase letter are not different ( P > 0.10). P value for treatments effect within year and month. Koeing and Beauchemin (2013) fed supplement to beef heifers with barley silage based diets (CP = 120 g kg 1 ) and did not find differences i n dry matter intake between different supplements, urea, urea+canola meal, urea+corn gluten meal, and urea+corn gluten meal+ xylose treated soybean meal. In addition, McGuire et al. (2013) reported no difference i n hard fescue [ Festuca trachyphylla (Hack.) Krajina] straw and total dry matter intake o f steers supplemented with urea or soybean meal daily or every other day with a CP intake of 0.10% of BW d 1 Authors showed no effect of infrequent supplementation on DM I and nutrient digestibility. Kalmbacher et al. (1995) fed steers with bluestem ( Schizachyrium scoparium var. stoloniferum) hay (CP = 47 g kg 1 ) and supplement ed with molasses plus urea or soybean meal and reported no effect on organic matter intake betwee n protein supplementation and control (no supplement). Different sources of RDP supplement are not likely to affect forage dry matter intake on cows consuming forage with decreased nutritive value.
76 Metabolic Study There was no effect of the supplements i n rumen fluid pH between treatments (mean = 6.5, P = 0.39) (Table 3 10 ). Similarly there were no differences i n total ruminal VFA (mean = 1 20 m M P = 0.35) and branched chain VFA (mean = 1.3 m M P = 0.24) between treatments (Table 3 10 ). In a review of several studies using molasses in beef nutrition, Pate (1983) concluded that molasses did not affect rumen pH and VFA concentration when fed at 15% of the diet. I n this study, molasses was 9.3% of the total dry matter intake and the pH an d VFA concentrations corroborated the findings by Pate (1983). Koeing and Beauchemin (2013) reported no differences i n total (mean = 132 m M ) and branched ruminal VFA concentrations and pH (mean = 6.24) when feeding animals different protein supplements in cluding urea, urea plus canola meal, urea plus corn gluten meal, and urea plus canola meal plus xylose treated soybean meal and consuming barley silage and concentrate (CP = 120 g kg 1 ). Kster et al. (1997) substitute d urea for true protein (casein) from 0 to 100% on a basal diet of dormant tallgrass prairie forage (CP = 24 g kg 1 ) and reported no difference on total VFA production (mean 82.4 m M ) and pH (mean = 6.5). Kster et al. (2002) fed dormant tallgrass hay to steers and showed no differences in tota l and branched VFA production when steers were supplemented with increasing levels of urea from 0 to 40% of the RDP in substitution of soybean meal. There were no differences i n propionic (mean = 25 mol 100 mol 1 P = 0.80), acetic (mean = 69.2 mol 100 mol 1 P = 0.92), butyric acids (mean = 4.5 mol 100 mol 1 P = 0.92), nor in branched chain VFA (mean 1.3 mol 100 mol 1 P = 0.24) proportions between sources of RDP (Table 3 10 ).
77 Table 3 10 : Treatment effects on rumen and blood parameters of fistulated steers fed with limpograss hay and supplemented with molasses based supplement and urea or cottonseed meal (CSM). Treatment Response v ariables CSM Urea P value SE Ruminal ammonia, mg dL 1 14. 5 14.5 0.99 2.6 Ruminal pH 6. 5 6.6 0.39 0.1 BUN, mg dL 1 7.9 7.8 0.91 0.8 Total VFA, m M 116. 2 123.7 0.35 5.5 Individual VFA, mol/100 ml Acetic acid 69.1 69.3 0.92 2.9 Propionic acid 25.2 24.7 0.8 0 2.2 Butyric acid 4.5 4.5 0.97 0.5 Branched chain acids 1.2 1. 5 0.24 0.3 Kster et al. (2002) supplemented steers with three levels of urea ( 0, 20, or 40 % ) and decreasing levels of soybean meal and reported no differences i n propionate, acetate and butyrate concentrations among levels of protein supplementation. Wickersham et al. (2008) evaluated steers fed low nutritive value hay and supplemented daily or every third day and reported no differences i n propionate, acetate and butyrate with levels of supplementation. Conversely, Kster et al. (1996) reported an increase i n acetate and a decrease i n butyrate proportions but no difference i n propionate for steers fed dormant tallgrass prairie hay. The a uthors stated that the difference i n butyrate proportion was affected by the increase i n acetate proportion. Similar concentrations of acetate were expected because there was similar forage dry matter intake between treatments in the metabolic and drylot study. Further there were no differe nces i n rumen fluid ammonia (mean = 14.5 mg dL 1 P = 0.99) and BUN (mean = 7.9 mg dL 1 P = 0.91) (Table 3 10 ). Animals were receiving molasses as a base supplement which limits intake (Kunkle et al., 1995, Arthington et
78 al., 2002). Urea has greater solu bility than cottonseed meal in the rumen (400 vs. 175 % h 1 ) (NRC, 2000) ; however the animals consumed the concentrate in 24 h, decreasing the rate of intake of the protein supplement. In dry feed supplements, as urea increased in the diet from 0 up to 40 % in substitution for soybean meal, there was an increase in rumen ammonia levels from 3.2 to 31.8 mg dL 1 (Kster et al., 2002). The slow rate of supplement consumption and soluble carbohydrates present in the molasses may have led to similar levels of BU N between treatments. There was a time effect on ruminal ammonia ( P < 0.01), pH ( P < 0.01) and BUN ( P = 0.05) (Figures 3 1 and 3 2 ). The pH values ranged from 6.2 to 6.7 (Figure 3 1 ) and according to Kster et al. (1997) the pH was adequate to maintain the activity of cellulolytic bacteria in the rumen. Ruminal ammonia reached the greatest concentration 2 h after feeding and decreased from 2 to 16 h. The concentration of ammonia was below 5 mg dL 1 at 40 h after feeding, which can affect r uminal microbial growth (Satter and Slyter, 1974). Farmer et al. (2001) reported an increase i n ruminal ammonia 2 h after feeding for steers receiving dormant tallgrass prairie hay and supplemented three or five times a week with protein supplement. The increase of rumen ammonia was likely due to supplement intake and a consequence of the infrequent protein supplementation (Wickersham et al., 2008). According to Hammond (1983) ruminal ammonia is highly correlated with BUN. Similar to rumi nal ammonia, BUN concentration increased from 0 to 1 2 h after feeding and decreased from 12 to 48 hours Hammond (1997) reported a maximum performance o f mature cows when BUN concentrations were between 7 and 8 mg dL 1 and t he levels observed in this study were in the referred range.
79 Important Findings and Implications There were no differences i n herbage responses and performance of cow calf pairs grazing stockpiled limpograss pastures and receiving molasses with cottonseed meal or urea as a source of RDP Cows supplemented with different sources of RDP had similar hay DMI and total DMI in a dry lot. In addition, ruminal parameters and BUN concentrations were also similar among steers receiving different sources of rumen degradable protein. The self limiti ng intake of molasses decreased the intake rate of urea and optimized the N use efficiency in the rumen. Urea can be as effective as cottonseed meal as a source of RDP to mature lactating beef cows grazing stockpiled limpograss pastures and the decision to use urea or cottonseed meal should be based on the cost of those protein supplements. Figure 3 1: Time effect on ruminal pH ( P < 0.01) and ammonia ( P < 0.01) on fistulated steers supplemented with two sources of rumen degradable protein me an between treatments 0 5 10 15 20 25 30 35 40 45 5.8 5.9 6 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 0 2 4 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 Ruminal ammonia (mg dL 1) Ruminal pH Time after feeding, h pH Ruminal Ammonia
80 Figure 3 2: Time effect on blood urea nitrogen on fistulated steers supplemented with two sources of rumen degradable protein mean between treatments ( P = 0.06) 0 2 4 6 8 10 12 0 2 4 6 8 10 12 14 16 18 20 22 24 28 32 36 40 44 48 Blood urea nitrogen (mg dL 1 ) Time after feeding, h
81 CHAPTER 4 EFFECTS OF LIMIT CRE EP FEEDING SUPPLEMENT ON PERFORMANCE OF COWS AND CALVES GRAZ ING LIMPOGRASS PASTU RES IN SOUTH FLORIDA O verview of the Research Problem Limpograss [ Hemarthria altissima (Poir.) Stapf et C. E. Hubb.] is a perennial warm season grass adapted to poorly drained soil, with reduced cru de protein (CP) concentrations ( Pitman et al. 1994). Thus, supplementation of animals grazing limpograss may be a feasible management practice to optimize animal production (Sollenberger et al., 1988 and Newman et al., 2002). In Florida, creep feeding hei fer calves with high protein diets may increase weaning weights, which could decrease the age of puberty and time of first conception ( Petterson et al., 1992 Yelich et al., 1995). Another benefit reported by creep feeding calves is an increase in cow per formance (Prichard et al., 19 8 9 ). However, the benefit of creep feeding on cow performance is not consistently reported in the literature (Lusby and Wettmann, 1986; Tarr et al., 1994; Vendramini et al., 201 2 ). A d libitum access to concentrate decrease s feed efficiency ( 5 to 15 kg feed per kg of additional body weight gain ) limit ing the economic feasibility of creep feeding beef calves. (Stricker et al., 1979; Cremin et al., 1989; Faulker et al., 1993). N evertheless creep feeding the most limiting nutrie nts for calf growth in smaller quantities may be an efficient management practice to improve calf performance and decrease feed cost. Limited creep feeding protein is generally used to provide N sources to rumen microbes to improve fiber digesti bility fo rage intake and improve calf performance ( Lusby and Wettemann, 1986 Cremin et al., 199 1 ). Lusby et al. (1985) proposed a limited creep fed protein supplement of 0.37 kg cotton seed ( Gossypium spp.) meal d 1 to cow calf pairs grazing for 63 d and reported an increase of 0.13 kg d 1 in ADG of
82 supplemented compared to control calves with feed efficiency of 2.5 kg of supplement per extra kg of gain. However, no difference i n cow performance and body condition sc ore was found. Moriel and Arthington (2013) reported inconsistent benefits of limit fed protein creep fed to calves for 112 d prior to weaning. In E xperiment 1, calves were supplemented 3 x wk 1 with 0.23 kg d 1 of a cubed protein supplement (CP = 210 g kg 1 ). There was an increase in ADG from 0.88 to 0.95 kg d 1 for control and creep fed calves, respectively. In E xperiment 2, calves were supplemented with a meal protein supplement (CP = 190 g kg 1 ) and there was no difference in calf performance between tr eatments. Soybean [ Glycine max (L.) Merr.] me al improves rumen degradability, enhance digestibility of low nutritive value forage forage intake and consequently improve animal performance and has concentrations of high quality aminoacids (). Soybean meal supplementation may (Mathis et al., 1999 and NRC, 2000 ). Although there is evidence in the literature of the benefits of limit creep feeding nursing calves grazing warm season grasses, the resul ts are not consistent. In addition, the effects of creep feeding nursing calves grazing limpograss pastures are not known. The objective of this study was to test the effect of limited creep feeding protein supplements to calves grazing limpograss pastures Material and Methods The research projects were conducted at the UF/IFAS Range Cattle Research and Education Center (RCREC), Ona, FL (27 o 26' N and 82 o 55' W) from June to September 2011 (Experiment 1) and from June to August 2012 (Experiment 2). The so il at the research site is classified as Pomona fine sand (siliceous, hyperthermic, Ultic Alaquod). Before initiation of the study, mean soil pH (in water) wa s 5.1, and Mehlich I
83 (0.05 M HCl + 0.0125 M H2SO4) extractable P, K, Mg, and Ca concentrations in the Ap1 horizon (0 to 15 cm depth) we re 35, 7 5 155, and 1450 mg kg respectively. Pastures were fertilized with 90 kg N ha 1 in April 2011 and 2012. The source of N fertilizer was ammonium nitrate. Limpograss pastures (1.0 ha per pasture, experimental units) were established in 2010 and grazed in 2011 and 2012. Twenty four cow and heifer calves ( Angus sired on crossbred cows) were randomly distributed in eight limpograss pastures with 3 cow calf pairs per pastures. Calves were approximately 6 mo of age at the initiation of the study. In E xperiment 1, t reatments were: 1) c alves receiving 200 g d 1 of soybean meal (480 g CP kg 1 ) by creep feeding, or 2) c alves not receiving supplement (Control). Treatments were distributed in a randomized complete block design with four replicates. In E xperiment 2, treatments were: 1) c alves receiving 200 g d 1 of soybean meal by creep feeding (2 00), 2) c alves receiving 400 g d 1 of soybean meal by creep feeding (400), or 3) c alves not receiving supplement (Control). The treatments were distributed in a randomized incomplete block design with three replicates for c ontrol and 200 treatments, and tw o replicates for the 400 treatment. Herbage Measurements In E xperiments 1 and 2, pastures were stocked continuously using a fixed stocking rate. Pastures were sampled just prior to initiation of grazing and every 14 d thereafter. Herbage mass and nutritive value [ CP and in vitro digestible organic matter ( IVDOM ) ] were measured. Herbage mass was determined by the double sampling technique. The indirect measure was the settling height of a 0.25 m 2 aluminum disk, whereas direct measure involved ha nd clipping all herbage at soil level to the top of the canopy using an electric clipper. One or two double samples were taken from each of
84 the eight experimental units for a total of 20 in a 28 d interval. Sites were chose n to represent the range of herba ge mass present on the pastures. At each site, the disk settling height was measure d and the forage was clip ped at ground level. Clipp ed forage was dr ied for 72 h and weighed. In order to ensure that all sections of the pasture were represented by the disk plate 20 sites were chosen by walking a fixed number of steps between each drop of the disk on the choose point for the disk measurement in a 14 d interval. The average disk height of the 20 sites was entered into the equation to predict actual HM. A cage technique was used to measure herbage accumulation since pastures were stocked continuously, by placing three 1 m 2 cages in the pasture at the initial samplin g date. Placement sites were ch ose n where the disk settling height was the same ( 1 cm) as t hat of the pasture average. Disk settling height was recorded at a specific site and the cage placed. After 28 d, the cage was removed and the new disk settling height recorded. Herbage allowance (HA) was calculated for each pasture as the average HM (me an across two sampling dates within each 28 d period) divided by the average total cow calf live weight during that period (Sollenberger et al., 2005). Hand plucked sampl ing technique was used to estimate herbage CP and IVDOM concentration at the initiatio n of grazing and at every 14 d thereafter. Herbage samples were composite d across sites, dried at 60C for 48 h in a forced air oven to constant weight and were ground in a Wiley mill (Model 4, Thomas Wiley Laboratory Mill, Thomas Scientific, Swedesboro NJ) to pass a 1 mm stainless steel screen. Analyses were perform ed at the University of Florida Forage Evaluation Support Laboratory using the micro Kjeldahl
85 technique for N (Gallaher et al., 1975) and the two stage technique for IVDOM (Moore and Mott, 1 974). Animal Responses The cow calf pairs were weighed at initiation of the experiment and every 28 d thereafter. Initial and final weights were taken at 0800 h with shrink period of 16 h and the animal s were un shrunk for the intermediate weights. The diff erence in BW was used to calculate ADG The body condition score of the cows w as evaluated on the same schedule. The gain per ha was determined based on the ADG of the calves multiplied by the number of calves within the pasture during that experimental pe riod and adjusted to a hectare basis. Economics The descriptive cost of added gain and efficiency of added gain were calculated. Cost of added gain was calculated by dividing the cost of feed by the added gain (BW gain of creep fed calves ADG of control calves) and efficiency of added gain calculated by dividing the amount of feed consumed for the entire experimental period by the added gain. The cost, income, and return on E xperiment 2 were calculated. The income was calculated based on the gain per are a multiplied by the calf price and the return was calculated by the added gain per area subtracted by the cost of feed. Statistical Analyses Response variables were cow ADG and BCS, calf ADG, gain per ha (GHA) HM, HA, CP, IVDOM income and return. The data w ere analyzed using PROC GLIMMIX of SAS (SAS Institute Inc., 2006) with creep feeding supplementation levels and month a s fixed effects. Replicate and their interactions were considered random effects. Months were analyzed as repeated measures. Years were analyzed separately because of
86 different creep feeding supplementation levels used in 2011 and 2012. In E xperiment 2, single degree of freedom orthogonal polynomial contrasts were used to test the treatment effects. Treatments were co nsidered different when P < 0.10. The means reported are least squares means and were compared using PDIFF (SAS Institute Inc., 2006). Results and Discussion Herbage Responses There was no difference in HM among treatments in E xperiment 1 and 2 (Table s 4 1 and 4 3). In E xperiment 1, HM was greater in June and decreased from June to September (Table 4 2). Pastures were fertilized in April and the extended regrowth period from April to June resulted in increased HM at the start of the experimental period. In E xperiment 2, HM decreased from June to July and subsequently increased in August (Table 4 4), probably due to favorable rainfall in August 2012 (Figure 4 1). Corroborating this finding, Vendramini et al. (201 1 ) did not report differences in bahiagrass ( Paspalum notatum Fl gge) HM of pastures with cow calf pairs receiving creep feeding or control. It was expected that the soybean meal supplementation would increase forage digestibility and forage intake by calves (Vendramini et al., 2013); however, the ma gnitude of the increased intake was not sufficient to cause differences in HM. Crude protein and IVDOM were not affected by the creep feeding treatments in E xperiments 1 and 2 (Tables 4 1 and 4 3). Reed et al. (2006) reported no difference in forage nutri tive value (mean CP = 124 and IVDOM = 503 g kg 1 ) of pastures grazed by creep fed and no creep fed nursing calves. There was a month effect on CP and IVDOM concentrations in E xperiment s 1 and 2. In E xperiment 1, CP increased from
87 June to July and subsequen tly decreased from July to August and September. In E xperiment 2, CP and IVDOM decreased from June to August. The decreased CP and IVDOM in June were likely because of the greater regrowth period from fertilization to starting the experimental period. The increased CP and IVDOM in E xperiment 1 were likely related to the lesser HM and greater appearance of new tissues with greater n utritive value. Figure 4 1: Average monthly precipitation from 1942 to 2012 and during the experimental period 2011 to 2012 at the Range Cattle Research and Education Center, Ona, FL. 0 50 100 150 200 250 May June July August Precipitation (mm) Month Average Preciptation 1942 2012 Average preciptation 2011 2012
88 Table 4 1: Herbage and animal responses of cow calf pairs grazing limpograss pastures supplemented on creep feeding with 200 g d 1 of soybean meal (200) or control (no supplement) in 2011 (Experiment 1). Treatment Response v ariables Control 200 P value SE Herbage r esponses Herbage mass (Mg ha 1 ) 4.4 4.7 0.54 0.8 Crude protein (g kg 1 ) 143 153 0.14 9 In vitro digestible organic matter (g kg 1 ) 624 626 0.92 14 Animal r esponses Herbage allowance (kg DM kg 1 LW) 2.2 2.4 0.68 0.3 Average daily gain (kg d 1 ) Calf 0.50 0.60 0.24 0.10 Cow 0.20 0.30 0.51 0.10 Cow body condition score 4.7 5.0 0.47 0.24 Gain per hectare (kg ha 1 ) 168 203 0.18 23 Table 4 2: Month effects on herbage and animal responses of cow calf pairs grazing limpograss pastures and supplemented on creep feeding with 200 g d 1 of soybean meal (200) or control (no supplement) in 2011 (Experiment 1). Month Response v ariables June July August September P value SE Herbage r esponses Herbage mass (Mg ha 1 ) 6.2 a 5.0 b 4.3 c 2.8 d <.0001 0.8 Crude protein (g kg 1 ) 105 c 177 a 131 b 140 b < .0001 10 In vitro digestible organic matter (g kg 1 ) 582 b 652 a 606 b 660 a <.0001 15 Animal responses Herbage allowance (kg DM kg 1 LW) 3.2 a 2.5 b 2.1 c 1.4 d <.0001 0.3 Calf average daily gain (kg d 1 ) 1.1 a 0.7 b 0.3 c 0.2 d 0.0 2 0.1 Cow average daily gain (kg d 1 ) 1.0 a 0.2 c 0.1 c 0.3 b 0.34 0.1 Means followed by the same letter within rows are not different ( P > 0.10) P value for month effect
89 Table 4 3: Herbage and animal responses of cow calf pairs grazing limpograss pastures and supplemented by creep feeding with 400 g d 1 of soybean meal (400), 200 g d 1 of soybean meal (200), or control (no supplement) in 2012 (Experiment 2). Response v ariable Treatment Contrast Control 200 400 Linear Quadratic SE Herbage r esponses Herbage m ass (Mg ha 1 ) 6.4 7.1 6.7 0.28 0.95 0.7 Crude protein (g kg 1 ) 126 108 119 0.17 0.63 8 In vitro digestible organic matter (g kg 1 ) 544 521 497 0.31 0.99 35 Animal r esponses Herbage allowance (kg DM kg 1 LW) 3.3 3.6 3.7 0.43 0.59 0.4 Average daily gain (kg d 1 ) Calf 0.33 0.44 0.62 0.03 0.76 0.10 Cow 0.10 0.40 0.20 0.44 0.39 0.12 Gain per hectare (kg ha 1 ) 82 112 152 0.03 0.69 17
90 Conversely, there was greater HM throughout the experimental period in E xperiment 2, which likely resulted in greater proportion of stems and decreased nutritive value. It was observed by Inyang et al. (2010) that warm season grass pastures with lesser HM tended to have greater nutritive value. Newman et al. (2002) observed that limpograss canopies grazed at 60 c m stubble height had greater HM and decreased nutritive value compared canopies grazed at 40 cm. Animal R esponses Herbage allowance did not differ among treatments for E xperiments 1 and 2 (Table 4 2). Similar HA was expected because of similar stocking ra tes and HM among treatments. There was a decrease in HA from June to September (Table 4 2) i n E xperiment 1. In E xperiment 2, HA decreased from June to July and was similar in July and August (Table 4 4). The variation in HM was the main factor affecting HA considering that there was little variation in body weight and stocking rates among treatments throughout the experimental period. Fike et al. (2003) indicated that HA levels below 1.0 kg DM kg 1 LW may result in decreased forage intake and animal perfor mance. Inyang et al. (2010) observed that heifers grazing Mulato ( Brachiaria spp.) and bahiagrass had decreased ADG at HA below 1.4 kg DM kg 1 LW. The HA levels observed in this study were above 1.4 kg DM kg 1 LW, indicating that forage quantity likely did not limit animal performance. In E xperiment 1, there was no difference in ADG of calves receiving supplementation on creep feeding and control (Table 4 1). As a consequence, there was no difference in GHA between treatments. In E xperiment 2, there was a linear increase in ADG of calves from control to supplementation with 400 g soybean meal d 1
91 Table 4 4: Month effects on herbage and animal responses of cow calf pairs grazing limpograss pastures and supplemented by creep feeding with 400 g d 1 of soybean meal (400), 200 g d 1 of soybean meal (200) or control (no supplement) in 2012 (Experiment 2). Month Response variables June July August P value SE Herbage responses Herbage mass (Mg ha 1 ) 8.2 a 5.7 c 6.3 b <.0001 0.6 Crude protein (g kg 1 ) 133 a 117 a 96 b 0.003 6 In vitro digestible organic matter (g kg 1 ) 627 a 485 b 450 c <.0001 25 Animal responses Herbage allowance (kg DM kg 1 LW) 4.5 a 2.9 b 3.2 b <.0001 0.4 Cow average daily gain (kg d 1 ) 0.7 a 0.2 c 0.2 b 0.001 0.12 Means followed by the same letter within rows are not different ( P > 0.10) P value s for month effect The increased levels of soybean meal likely resulted in greater rumen degradable protein levels in the rumen and increased forage digestibility and intake (Vendramini et al., 2013). Lusby et al. (1985) reported an increase in ADG by 0.145 kg d 1 of calves grazing native pastures and 0.13 kg d 1 of calves grazing bermudagrass receiving on avera ge 0.31 kg d 1 of cotton seed meal over the non supplemented calves and no difference on cow ADG Calves supplemented with 0.45 kg d 1 of soybean meal grazing native pastures gained more weight than non supplemented calves during the winter when forage nu tritive value was low and when cows had limit ed milk production (Lusby and Wettemann, 1986). It is expected that calves receiving limited creep feed with a maximum of 0.6 kg d 1 of a 350 g kg 1 CP supplement decrease ruminal fiber and total tract NDF diges tibility (Cremin et al., 1990). However, positive results to limited creep fed CP supplement are not consistent ly achieved. Moriel and Arthington (2013a) reported two experiments testing creep feeding molasses plus urea supplements (0.18
92 kg d 1 ) to nursing calves and observed positive responses in only one experiment. Therefore, it is expected that the response to limit creep feeding CP supplements to nursing calves may be significantly affected by factors other than the supplement, such as forage quantity and quality, environmental factors, and cow milk production. There was a decline in ADG of calves from June to September (experiment 1) and June to August (experiment 2). The usual decrease in ADG in suckling calves generally occur s because of high rainfa ll and temperatures during the summer, which resulted in water standing on the pastures and likely depression in forage intake (Butris and Philips, 1987; Aiken et al., 1991). A linear increase in GHA was observed from control to 400 g soybean meal d 1 as a result of greater ADG There was no difference in ADG and body condition score of the cows from the creep feeding or control treatments in E xperiments 1 and no difference on ADG of cows in Experiment 2 (Table 4 1 and 4 3). Although there are few report s in the literature showing that creep feeding may increase performance of the cows (Sticker et al., 1979; Prichard et al., 1989), it is usually observed that suckling calves will not replace milk by concentrate feed, expecting the cow to have the same nut rient requirements in creep fed and control treatments. Tarr et al. (1994) found no difference in cow weight change between creep and no creep treatments (mean = 19.0 kg) when calves were fe d for 28, 56, or 84 d. Vendramini et al. (2012) observed that the re was no difference in ADG of cows between calves receiving 10 g kg 1 of concentrate supplementation in creep feeding and control. There was a month effect on ADG of the cows in E xperiment 1 and 2 (Table 4 2 and 4 4). The cows decreased ADG from June to July and subsequently increased from
93 July to August. The greater ADG in June may be result of filling effects caused by the transition of the cows from bahiagrass pastures to the experimental units with superior HM. The decrease in ADG in July may be a ne gative result of the greater rainfall and water standing on the pasture. Economics Analysis Calves supplemented with 200 and 400 g of soybean meal d 1 had an efficiency of added gain of 0.60 and 0.75, respectively (Table 4 5). Lower efficiency on calves c onsuming limited amount of protein supplemented feed have been reported i n the literature from 0.14 to 0.21 (Faulkner et al., 1994 and Tarr et al., 1994). Moriel and Arthington (2013a) reported an overall gain efficiency of 0. 3 8 for calves supplemented with molasses and urea. Greater values were reported by Lusby et al. (1985) when calves consumed limited amount of cottonseed meal grazing bahiagrass pastures 0.44 and native grasses, 0.36. The efficiency of added gain found in E xpe riment 2 is likely greater than the values found in the literature because the control calves in this study had decreased ADG, increasing the added gain and the efficiency of added gain. The cost of added gain was $ 0.80 and $ 0.64 kg LW 1 for the 200 and 4 00 treatment s, respectively (Table 4 5). There was a linear increase in income and return as supplement levels increased (Table 4 6). The linear increase in return was the result of the significant increase in added gain and limited feed amounts, which res ulted in decreased feed cost.
94 Table 4 5: Average daily gain, added gain, added BW, amount of feed, cost of feed, cost of added gain, and efficiency of added gain responses of suckling calves grazing limpograss pastures and supplemented on creep feeding 400 g d 1 of soybean meal, 200 g d 1 of soybean meal, or control in 2012 (Experiment 2). Treatment 1 Response v ariables 0 200 400 Average daily gain (kg d 1 ) 0.32 0.44 0.62 Added average daily gain (kg) 0.12 0.30 Added body weight (84 d) 10. 1 25.2 Amount of feed (kg /animal ) 16.8 33.6 Cost of feed ($, period) 2 8.1 16.1 Cost of added gain ($/kg) 3 0.80 0.64 Efficiency of added gain (kg SBM/kg BW) 4 0.60 0.75 1 0, 200, or 400 g/d of soybean meal, price ($/kg) = 0.48 2 A mount of feed*cost/kg 3 C ost of feed/added body weight 4 A mount of feed/added body weight Table 4 6: Economic analysis of cow calf pairs grazing limpograss pastures and supplemented on creep feeding with 400 g d 1 of soybean meal, 200 g d 1 of soybean mea l or control in 2012 (Experiment 2). Treatment 1 Response v ariables 0 200 400 Contrast P value SE Feed cost 1 0 24.2 48.4 Gross i ncome 2 278.0 418.0 522.0 L 0.03 43 Gross r eturn 3 278.0 393.8 473.6 L 0.06 55 1 0, 200, or 400 g/d of soybean meal, price ($/kg) = 0.48 2 Gain per area (kg ha 1 )*calf price ($/kg) = 3.3 3 Income feed cost Important Findings and Implications Limit creep feeding of 200 g soybean meal d 1 to cow calf pairs grazing limpograss pastures had no effect on ADG of the calves; however, increasing levels of limited creep feeding soybean meal from 0 to 400 g d 1 linearly increased the ADG of the calves in E xperiment 2. There was no effect of creep f eeding treatments on forage characteristics and performance of the cows i n both experiments. Forage quantity and nutritive value varied throughout the experiment periods, which may have affected the
95 performance of cows and calves. However, the excessive ra infall and water standing on the pastures during the summer in South Florida may be the main reason for the decrease in calf performance from June to August. It seems that greater levels of soybean meal supplementation may alleviate the decrease in ADG of the calves in the summer months, resulting in heavier calves at weaning. The economic analyses demonstrated that the 400 g soybean meal d 1 was the most efficient gain with the greatest economic return Due to the inconsistency in ADG of calves receiving c reep feeding, further studies on the duration and levels of creep feeding are necessary to verify the precision of creep feeding 400 g soybean meal d 1 as a minimum levels to efficiently increase weaning weights in cow calf operations in Florida.
96 CHAPTER 5 EFFECT OF STOCKING R ATE ON HERBAGE RESPO NSES AND ANIMAL PERFORMANCE OF BEEF PASTURES O verview of the Research Problem Bermudagrass [ Cynodon dactylon (L.) Pers.] is an important warm season grass species for livestock production in the southeast USA (Hill et al., 2001) and can be used for grazing, hay, or silage (Taliaferro et al bermudagrass was the first hybrid bermudagrass released, and others followed (Taliaferro et al 2004). M ost of the bermudagrass cultivars released were suitable for livestock production, but the majority of the m are not adapted to poorly drained soils P roduction and persistence may be decreased under those conditions. Furness and Breen (1982) reported a de crease of 74% i n the area covered by bermudagrass after flooding periods of 161 d in South Africa. During flooding, plant respiration slows due to decreasing oxygen in the root zone followed by depletion in carbohydrates, and increase of toxic compounds, which can consequently affect growth and cause death of plants (Colmer and Voesenek, 2009). Jiggs is a bermudagrass that was distributed by a private company in Texas (Ocumpaugh and Stichler, 2000); however, there is no report of the release published in the literature. According to Vendramini (2008), Jiggs tolerates poorly drained soils and may be a productive bermudagrass cultivar for the poorly drained flatwoods soils in South Florida unlike other bermudagrass cultivars Vendramini et al. (2010) compare d four bermudagrass es Cynodon spp.), and Florakirk and reported that Jiggs had the greatest herbage accumulation among the bermudagrass cultivars during the summer in Florida. Mislevy et al. (2008) evaluated
97 herbage accumulation and nutritive value of Jiggs and Tifton 85 using the mob stocking technique and reported greater herbage accumulation for Jiggs than Tifton 85 (13.9 vs. 11.9 Mg ha 1 ). The same authors did not find difference s in crude protein (CP) among cultivars but Tifton 85 had greater in vitro digesti ble organic matter ( IVDOM ) conce ntrations than Jiggs (638 vs. 561 g kg 1 ). Despite the preliminary information presented by Mislevy et al. (2008), further information on the effects of grazing mana gement on Jiggs is not known. Stocking rate is one of the most important management factors in grazing systems and has a direct effect on herbage mass (HM) and animal performance (Burns et al., 1999). Hernndez Garay et al. (2004) evaluated the effects of stocking rates from 2.5 to 7.5 head ha 1 on stargrass ( Cynodon nlemfuensis Vanderyst) herbage mass (HM) nutritive value, and animal performance They observed that there was a linear decrease in HM and average daily gain ( ADG ) with increasing stocking ra tes. Conversely, there w ere linear increase s in CP and neutral detergent fiber (NDF) concentrations. Inyang et al. (2010) reported a linear decrease in HM and a quadratic increase in herbage accumulation rate (HAR) on bahiagrass ( Paspalum notatum Fl gge) a nd Mulato II ( Brachiaria spp.) pastures as stocking rate increased from 4 to 12 heifers ha 1 The authors also reported a linear decrease in ADG and a quadratic effect on animal liveweight gain per area as stocking rate increased. In a ddition to the effect s on herbage quantity, nutritive value, and animal performance, stocking rate and grazing intensity can affect persistence of warm season grass pastures. Inyang et al. (2010) observed that ground cover of Mulato II plots de clined from 87 to 74% as the harvest stubble height decreased from 12.5 to 2.5 cm.
98 Despite preliminary information presented by Mislevy et al. (2008), further information is needed on the effects of grazing on Jiggs bermudagrass. In addition, there are no known studies evaluating performance of beef cattle grazing Jiggs pastures. The objective of this study was to evaluate animal performance and forage characteristics of Jiggs bermudagrass pastures grazed at different stocking rates. Material and Methods The stud y was conducted at the UF/IFAS University of Florida Range Cattle Research and Education Center (RCREC), Ona, FL (27 o 26' N and 82 o 55' W) from May to August 2011 and 2012. The soil at the research site was a Pomona fine sand ( siliceous, hyperthermic, Ulti c Alaquod ) that is poorly drained with slow permeability. Prior to initiation of the grazing trial, mean soil pH (in water) was 6.4. Mehlich I (0.05 M HCl + 0.0125 M H2SO4) extractable P, K, Mg, and Ca concentrations in the Ap1 horizon (0 to 15 cm depth) were 48, 83, 361, and 2202 mg kg 1 Pastures were fertilized in March 2011 and 2012 with 40, 18, and 33 kg o f N, P, and K ha 1 respectively, followed by two applications of 40 kg N ha 1 in mid June and early August. The N fertilizer used was ammonium nitr ate. Pastures (0.25 ha experimental units) were established in August 2010 and grazing was initiated in May 2011 and 2012. The experimental period was from May to August of 2011 and 2012.Heifers were Angus sired (crossbred cows sired by Angus bulls) early weaned beef heifers ( Bos spp.) with initial body weight (BW) of 172 23 kg and 168 21 kg in 2011 and 2012 respectively. The final BW was 216 26 and 218 30 kg in 2011 and 2012, respectively. Calves were weaned at approximately 90 d of age and grazed annual ryegrass ( Lolium multif l orum Lam.) while receiving 10 g kg 1 BW
99 in concentrate supplement (140 g kg 1 CP and 780 g kg 1 TDN) supplement from January to May 2011 and 2012. Treatments were the factorial arrangement of three stocking rates [2 (low), 5 (medium), and 8 (high) heifers ha 1 ] in a randomized incomplete block design with three replicates for low and medium and two replicates for the high stocking rate treatment. The average stocking rates proposed in this study were the equivalent of 3.7, 8 .8, and 13.1 animal units (450 kg LW) ha 1 Pastures were grazed using a fixed and continuous stocking rate. Although the initial and final BW of the calves were similar among treatments and years, the final stocking rate for low, medium and high stocked p astures were 3.6, 8.9, and 14.2 AU ha 1 respectively. Herbage Measurements Pastures were sampled just prior to initiation of grazing and every 14 d during the grazing period. Herbage mass, herbage accumulation rate (HAR) herbage height, canopy light int erception, and herbage CP and IVDOM were measured. The double sampl ing technique was used to determine HM. The indirect measure was the settling height of a 0.25 m 2 aluminum disk, and the direct measure involved hand clipping all herbage to 2.5 cm above so il level using an electric clipper. Every 28 d, two or three double samples were taken from each of the eight experimental units for a total of 20 double samples per date Sites for double sampling were chose n to represent the range of herbage mass present on the pastures. At each site, the disk settling height was measure d and the forage under the disk was clip ped at ground level. Clipp ed forage was dr ied for 72 h and weighed. Indirect measures (disk heights) were taken every 14 d at 20 sites per pasture. Sites were selected by walking a fixed number of steps between each drop of the disk to ensure that all sections of the pasture were represented. The
100 average disk height of the 20 indirect measures was entered into the regression equation developed from double sampling to predict HM. The average r 2 values for the equations were 0.70 and 0.88 for 2011 and 2012, respectively. Because these pastures were stocked continuously, a cage technique was used to measure H AR Three 1 m 2 cages were placed in the pasture at the initial sampling date. Placement sites were cho se n where the disk settling height was the same ( 1 cm) as the pasture average. Disk settling height was recorded at a specific site and the cage placed. After 28 d, the cage was removed and the new disk settling height recorded. Herbage accumulation rate was calculated as the change in HM during the 28 d that the cage was present. At the end of each 28 d period, cages were moved to new locations on the p asture with a current average disk settling height. Herbage allowance was calculated for each pasture as the average HM (mean across two sampling dates within each 28 d period) divided by the average total heifer live weight during that period (Sollenberge r et al., 2005). Herbage CP and IVDOM concentration were measured at the initiation of grazing and every 14 d thereafter for a composite of twenty h and plucked samples taken from each pasture. Samples were taken to the average stubble height of each pasture to attempted simulate what animals were consuming during each period of the collection. H and plucked samples were dried at 60C for 48 h in a forced air oven to constant weight and ground i n a Wiley mill (Model 4, Thomas Wiley Laboratory Mill, Thomas Scientific, Swedesboro, NJ) to pass a 1 mm stainless steel screen. Analyses were perform ed at the University of Florida Forage Evaluation Support Laboratory using the
101 micro Kjeldahl technique fo r N (Gallaher et al., 1975) and the two stage technique for IVDOM (Moore and Mott, 1974). Three forage samples per experimental unit were harvested from a 0.25 m 2 area at 2.5 cm stubble height and manually separated in leaf and stem every 14 d. The proportion of lea f stem, and senescent material in the canopy and CP and IVDOM of lea f and stem w as reported. Canopy l ight interception (LI) was measure d using AccuPAR LP 80 ceptometer (Decagon Devices, Pullman, WA). Eight readings were taken in each expe rimental unit at 1000 h every 14 d. The bea m fraction sensor was placed at the center of each half of the paddock and four readings were taken with a 90 distance from each othe r with the probe placed at ground level. The probe was placed in the same direc tion for all measurements. The measurements provided by the Accupar system were light transm itted, spread and incident, bea m fraction, zenith angle. Canopy LI was calculated by dividing transmitted by incident light times 100 and subtracted from 100. Mean undisturbed sward height was measured at eight sites per experimental unit every 14 d. Animal Measurements Body weight of the heifers was recorded at initiation of the experiment and every 28 d thereafter. Weights were taken at 0800 h following a 16 h shri nk period. Average daily gain was calculated each 28 d period through the entire grazing season. Gain per hectare (GHA) was calculated for each pasture over the entire grazing season. Animals received a concentrate supplement (140 g kg 1 CP and 780 g kg 1 TDN) at 10 g kg 1 body weight daily. Previous research showed that 10 g kg 1 of body weight of concentrate energy supplementation (146 g CP kg 1 and 780 g TDN kg 1 ) is necessary
102 for early weaned beef calves grazing warm season annual pastures to have satis factory performance (Vendramini et al., 2007). Statistical Analyses The response variables (ADG, GHA, HM, HAR, forage height, LI, HA, CP, and IVDOM ) were analyzed by fitting mixed effects models using the PROC MIXED procedure of SAS (SAS Institute Inc., 1996). Block, year, and its interactions were considered random effects. Months were analyzed as repeated measures. Treatments were considered differ ent when P < 0.10. Interactions not discussed were not significant ( P > 0.10). Single degree of freedom orthogonal contrasts were used to compare stocking rate effects. The means reported are least square s means and were separated st significant difference (LSD) at P < 0.10. Pearson correlation coefficients among LI, HA, and sward height were generated using PROC CORR of SAS (SAS Institute Inc., 2006). Results and Discussion Herbage Responses Herbage mass decreased linearly from 3.8 to 2.4 Mg ha 1 as stocking rate increased from low to high (Table 5 1). The effects of stocking rate on HM has been reported in the literature and a decrease in HM with greater stocking rates is a consequence of increased forage intake from a greater numb er of animals. Likewise Inyang et al. (2010) reported a decrease in HM from 5.9 to 3.2 Mg ha 1 as stocking rate increased from 4 to 12 heifers ha 1 There w as a decrease in HM from May to June and a subsequent increase in July. There was no difference in HM between July and August (Table 5 2 ). The greater HM in May was consequence of the HM accumulation from the spring to the time of the initiation of the study. Herbage mass decreased in June due to
103 grazing and subsequently increased in July likely because of the N fertilization in late June and favorable rainfall and temperature (Figure 5 1). Table 5 1: Effects of stocking rate on herbage responses of Jiggs bermudagrass pastures. Response v ariable Treatment Contrast Low Medium High Linear Quadratic SE Herbage m ass (Mg ha 1 ) 3.8 3.2 2.4 < 0.01 0.53 0.4 Light intercepted (%) 94 85 71 < 0.01 0.68 1 Height (cm) 17 12 9 < 0.01 0.62 1 Herbage accumulation rate (kg ha 1 d 1 ) 63 73 78 0.09 0.72 5 Crude protein (g kg 1 ) Hand plucked sample 158 158 158 0.97 0.98 6 Leaf 192 201 222 0.02 0.37 5 Stem 102 97 103 0.98 0.42 6 In vitro digestible organic matter (g kg 1 ) Hand plucked sample 486 482 516 0.23 0.35 18 Leaf 518 530 569 <0.01 0.20 18 Stem 520 507 510 0.97 0.21 12 Table 5 2: Month effects on forage responses of Jiggs bermudagrass pastures Data are means across three stocking rates. Month Response v ariable May June July August SE Herbage mass (Mg ha 1 ) 3.6 a 2.4 c 3.1 b 3.3 b 0.3 Herbage accumulation rate (kg ha 1 d 1 ) 30 c 43 c 98 b 114 a 9 Light interception (%) 93 a 82 b 80 b 78 b 2 Height (cm) 14 a 11 c 13 b 13 b 1 Crude protein ( g kg 1 ) 158 165 161 150 8 In vitro digestible organic matter ( g kg 1 ) 468 b 462 b 519 a 530 a 21 Within rows, means followed by the same lowercase letter are not different ( P > 0.10).
104 Figure 5 1: Average monthly precipitation from 1942 to 2012 and during the experimental period 2011 to 2012 at the Range Cattle Research and Education Center, Ona, F L There was a linear increase in HAR as stocking rate increased (Table 5 1). Pastures grazed at low stocking rate had decreased HAR because the excess HM, which resulted in self shading accumulation of non photosynthetic residue especially on the young basal tillers (Adjei et al., 1980), and reduced photosynthesis (Parsons et al., 1988; Hernandez Garay et al., 200 0 (1987), which demonstrated that there was an increase in HAR from 51.3 to 68.2 kg ha 1 d 1 as stocking rate of dry jersey cows grazing perennial ryegrass ( Lolium perenne L.) and white clover ( Trifolium repens L.) pastures increased from 2.77 to 4.28 cows ha 1 during the summer. Additionally there was a month effect (Table 5 2 ) in HAR. The HAR was similar in May and June and increased in July and August (Table 5 2 ). The 0 50 100 150 200 250 May June July August Precipitation (mm) Month Average Preciptation 1942 2012 Average preciptation 2011 2012
105 increase in HAR in July and August occurred because of the N fertilization and greater rainfall in those months (Figure 5 1). There was no difference among treatments in proportion of leaf, stem and senescent material, with averages of 14, 45, and 41 % respectively; however there was monthly variation in plant morphology The leaf:stem ratio increased from May to June before decreasing in July and August ( Table 5 3 ). The reason for increasing proportion of stems is likely due to greater HAR resulted from greater temperature and rainfall. According to Mislevy et al. (2001) and Ezenwa et al. (2006), higher temperatures may result in rapid growth and greater contribution of lignified tissues with reduced digestibility. In addition, animals likely selected leaves with greater nutritive value (Table 5 2 ) and decreased the proportion of leaves. Table 5 3 : Month effect on leaf, stem and dead material prop ortion o f Jiggs bermudagrass pastures Data are moans across three stocking rates Month Response v ariable May June July August SE -------------% -----------Leaf 8 c 16a 16 a 13 b 2 Stem 41 b 35 c 53 a 51 a 2 Dead 51 a 48 a 31 c 36 b 2 Within rows, means followed by the same lowercase letter are not different ( P > 0.10). There was a linear decrease in canopy LI and forage height as stocking rate increased from low to high (Table 5 1). Light interception decreased from May to June and remained constant until August (Table 5 2 ). The low stocking rate treatment had 93% LI, which is similar to the 95% LI i ndicated by da Silva and Nascimento Junior (2007) as optimum levels for herbage ac cumulation and nutritive value of warm season
106 grasses. The medium and high stocking rates had LI below 95% but with greater HAR than for low stocking rate. This may indicate that 95% LI does not always maximize HAR across a range of species Forage height decreased from May to June, increased in July, and remained constant in August, corroborating the HM and HAR data (Tab le 5 2 ). Forage height was greater in May likely because of the extended growing period before the initiation of the experiment and decr eased in June as result of grazing and decreased HAR. Correlations between LI and forage height were significant (Table 5 4) however, only 31 % of the relationship between the variables was explained by the model (r = 0. 31 ). da Silva and Nascimento Junior (2007) stated that forage height and light interception are well correlated, therefore it is practical to have a constant grazing height to manage warm season grasses for optimum herbage accumulation and nutritive value. The low correlation coefficient bet ween height and LI in this experiment calls into question this conclusion Management practices and climatic conditions may affect plant structure, especially density and leaf angle (Fagundes et al., 1999) and may be responsible for variation in the relati onship between LI and forage height. Table 5 4 : Correlations among herbage mass, light interception, and forage height of Jiggs bermudagrass pastures grazed at different stocking rates Response Variable Herbage mass Light interception Forage height Herbage mass r = 0.11 r = 0. 71 P = 0.37 P < 0.01 Light interception r = 0.11 r = 0. 56 P = 0.37 P < 0.01 Forage height r = 0. 71 r = 0. 56 P < 0.01 P < 0.01
107 Nutritive Value and B otanical C omposition There was no difference in hand plucked herbage CP and IVDOM concentrations among treatments (Table 5 2) It was observed in previous studies with warm season grasses that increasing stocking rate increase d forage nutritive value (Inyang et al., 2010; Hernandez Gray et al., 2004) ma inly due to the more frequent appearance of new tissue with greater nutritive value. Crude protein concentrations were also similar across months; however, there was an increase in IVDOM from May to August The l ong regrowth period from the start of the gr owing season to the start of the experimental period (May) decreased IVDOM Vendramini et al. (2007) reported an increase in IVDOM from May to June (from 573 to 681 g kg 1 ) but a decrease in July and August, 603 and 640 g kg 1 respectively on Tifton 85 pa stures; however, there was no month effect on CP concentration. There was a linear decrease in Jiggs ground cover with increasing stocking rate after the 2 yr experiment. Conversely, there was a linear increase in common bermudagrass and a quadratic increase i n broadleaf weeds (Table 5 5 ). Greater stocking rate treatments result ed in shorter forage height and lesser canopy LI, which likely resulted in insufficient leaf area to optimize photosynthesis and restore plant reserves Common bermudagrass to lerates shorter stubble heights and likely occupied the open spaces in the pastures. Mislevy et al. (1998) reported an increase in common C ynodon nlemfuensis Vanderyst var. nlemfuensis) was grazed at a 2 wk interval for 3 yr. Interrante et al. (2009) found that cover to < 40%, indicating that even more persistent warm season grass species may decrease ground cover under se vere defoliation.
108 Table 5 5: Botanical composition of Jiggs bermudagrass pastures Treatment Polynomial Contrast Response Variable Low Medium High Linear Quadratic SE ---------% ---------Jiggs 95 78 39 <0.01 0.16 8 Common Bermuda 4 17 36 0.02 0.67 9 Weed 2 5 25 <0.01 0.01 2 Animal Responses There was a linear decrease from 2.3 to 0.4 kg DM kg 1 LW in HA from low to high stocking rates (Table 5 6 ). The decrease in HM and greater stocking rates were the main factors influencing the decrease in HA. Inyang et al. (2010) reported a decrease in HA from 2.8 to 0.6 kg DM kg 1 LW as stocking rates increased from 4 to 12 heifers ha 1 grazing bahiagrass and Mulato pastures. There was a month effect on HA, which was similar to the variation i n HM. The greatest HA was observed in May with subsequent decline in June and similar HA from June to August (Table 5 7 ). Table 5 6: Animal responses of heifers grazing Jiggs bermudagrass pastures Response v ariable Treatment Polynomial Contrast Low Medium High Linear Quadratic SE Herbage allowance (kg DM kg 1 LW) 2.3 0.8 0.4 <0.01 0.04 0.1 Average daily gain (kg d 1 ) 0.7 0.4 0.3 <0.01 0.13 0.04 Gain per hectare (kg ha 1 ) 692 975 1064 0.01 0.20 72 Average daily gain decreased linearly from 0.7 to 0.3 kg d 1 as stocking rate increased from low to high (Table 5 6 ). The decrease in ADG was likely caused by the decrease in HA with increasing stocking rates. Inyang et al. (2010) observed that HA of less than 1.4 kg DM kg 1 LW decreased ADG of he ifers grazing bahiagrass and Mulato pastures. In this study, calves were supplemented with 10 g kg 1 of body weight which
109 likely decreased forage intake and the required levels of HA. Vendramini et al. (2013) observed that 10 g kg 1 of body weight of conc entrate supplementation is approximately 33% of the total dry matter intake of early weaned calves receiving stargrass. However, Vendramini and Arthington (2008) reported that there was a decrease in ADG of early weaned calves grazing stargrass pastures wh en HA decreased from 1.0 to 0.7 kg DM kg 1 LW. In addition, the decrease in HA with greater stocking rates likely decreased the opportunity for calves to select plant par ts with greater nutritive value. This resulted in intake of forage with lesser nutritive value and resulted in reduced ADG There was a month effect on ADG (Table 5 7) Animal performance declined from May to June, increasing again in July and declining in August. The greater ADG in May occurred due to greater gut fill result ing from the transition of the calves from annual ryegrass pastures to bermudagrass in May, which was also observed in previous studies (Vendramini and Arthington, 2008). The decline in ADG in June and subsequent incr ease in July may be related to the variatio n in HA in those months (Table 5 7 ). In August, greater temperature and rainfall resulted in water standing on the pastures and decreased performance of the calves. Such conditions decrease grazing time, forage in take and animal performance (Butris and Philips, 1987; Aiken et al., 1991). Table 5 7: Month effects on animal responses of Jiggs bermudagrass pastures Data are means across three stocking rates. Month Response v ariable May June July August SE Herbage allowance (kg DM kg LW 1 ) 1.6 a 0.9 c 1.1 b 1.1 b 0.2 Average daily gain (kg d 1 ) 0.9 a 0.4 c 0.6 b 0.0 d 0.1 Within rows, means followed by the same lowercase letter are not different ( P >0.10).
110 There was a linear increase from 692 to 1064 kg ha 1 in GHA as stocking rate increased from low to high (Table 5 6 ). Despite greater ADG with low stocking rates, the increased number of animals in the high stocking rate resulted in greater GHA. Derner et al. (2008), working with mixed grass prairie and yearlings steers (247 24 kg) reported a linear increase in GHA from ~10 to 60 kg ha 1 as stocking rate increased from 0.20 to 0.44 steers ha 1 According to Mott and Moore (1985 ) there should be a linear decrease in ADG and quadratic relationship of G HA with increasing stocking rates. In this study, the inclusion of concentrate in the diet of the calves may have influenced the shape of the response of GHA to increasing stocking rates. Important Findings and Implications S tocking rate had significant effects on forage and animal responses on Jiggs bermudagrass pastures. Increasing stocking rates decreased HM, forage height, canopy LI, and HA, which decreased ADG of calves. However, GHA increased linearly with increasing stocking rate. The greate st detr imental effect of increasing stocking rates was the reduction in forage height and LI, which resulted in significant reduction in Jiggs ground cover after 2 yr of grazing Considering that persistence is one of the most important attributes of warm season perennial grasses, Jiggs pastures must be managed as the low SR treatment in this experiment and should not be grazed below 17 cm stubble to maintain the ground cover of the desirable forage species.
111 CHAPTER 6 SUMMARY AND CONCLUSI ONS Summary Warm season grasses are the main source of forage for cow calf operations in Florida. Although warm season grasses have rapid forage accumulation during the growing season (spring, summer, and early autumn) in tropical and subtropical regions, herbage accumulation and nutritive value are reduced during the winter. Therefore, supplementation may be necessary to maintain the productivity of the cattle herd during the months with shortage of forage quantity and limited quality. Stockpil ing forage is a strategy that allow s forage to accumulate during the growing season to be grazed at a later date usually during the winter when growth ceases of most warm season grasses. Limpograss [ Hemarthria altissima ( Poir. ) Stapf & C.E. Hubb. ] is commonly used for stockpiled forage mainly because it has greater herbage accumulation and digestibility than other warm season grasses in the winter; however, crude protein (CP) concentration is usually limiting Therefore, a protein supplementation pro gram is necessary to overcome the reduced CP concentrations of stockpiled limpograss. The most common protein supplements are soybean [ Glycine max (L.) Merr.] meal, cottonseed ( Gossypium spp.) meal, and urea mixed with molasses based supplement. Studies h ave been conducted to evaluate different sources of protein for animals grazing warm season grasses; however there are few studies that have test ed the effects of different sources of rumen degradable protein (RDP) on performance of cow calf pairs grazing stockpiled limpograss.
112 Creep feeding is another supplementation strategy that can be used to provide supplement to calves on pasture Creep feeding is a management practice used to provide extra nutrients to suckling calves in a sectioned off part of the pasture, which prevents the mother from gaining access to the feed. Creep feeding has been used extensively in cow calf production systems; however, low feed efficiency generally results in reduced economic feasibility and limited interest by producers. C reep feeding programs that target nutrients in limited supply for calves ha s been somewhat effective in improv ing performance when forages are of poor nutritive value. S upplementing limited sources of RDP to calves in creep feeding may be an effective mana gement practice to increase productivity of cow calf systems. The effects of creep feeding limited amounts of RDP to cow calf pairs grazing limpograss pastures during the summer in Florida are not known. Bermudagrasses [ Cynodon dactylon (L.) Pers.] are the most planted warm season grass in the southeastern USA. They are characterized by high yields, good nutritive value, and persistence under grazing. The most planted bermudagrass es in the ( Cynodon spp.); howev er, they are not persistent and productive on the poorly drained soils commonly f ound in South Florida. Jiggs bermudagrass has generated interest from producers in South Florida because it tolerates poorly drained soils. Clipping studies have been conducte d in South Florida and show ed superior herbage accumulation and comparable nutritive value of Jiggs when compared with other warm season grasses. However, there is limited information on grazing management of Jiggs bermudagrass pastures.
113 In order to gen erate information about supplementation strategies on limpograss pastures and grazing management of Jiggs pastures, three studies were conducted. The first study (Chapter 3) evaluated the effect of feeding different sources of RDP supplement cottonseed me al vs. urea, on performance of cow calf pairs grazing stockpiled limpograss pastures during the winter. The second study (Chapter 4) evaluated the effects of creep feeding limited protein supplement on performance of cow calf pairs grazing limpograss pastu res during summer. The third study (Chapter 5) assessed the effect of stocking rate on herbage responses and performance of beef heifers grazing Jiggs pastures during summer. The studies were conducted at the UF/IFAS Range Cattle and Education Center, Ona, FL, in 2011 and 2012. The general objective of these studies was to improve the efficiency of forage utilization and performance of beef cattle production in South Florida. Stockpiled Limpograss Stud ies Grazing study This experiment was conducted fro m January to March 2011 and 2012. Treatments were two sources of RDP supplement, urea or cottonseed meal, in addition to a molasses based supplement. Treatments were isonitrogenous (750 g CP d 1 ) with the same amount of RDP (480 g d 1 ) and rumen undegrada ble protein ( RUP; 270 g d 1 ), and isocaloric (2.57 kg total digestible nutrients d 1 ). Each treatment was replicated four times in a completely randomized design. The supplement was offered three times a week, Monday, Wednesday and Friday. Pastures (experimental units) were stocked continuously using a fixed stocking rate with three cow calf pairs per pasture. Cows averaged 418 59 and 413 46 kg of body weight (BW) and calves 100 19 in 2011 and 78 12 kg of BW in 2011 and 2012, respectively. Pasture s (1 ha experimental
114 unit s ) were clipped at 10 cm stubble height in October of 2010 and 2011, fertilized with 90 kg N ha 1 and stockpiled for ~ 90 d. Herbage mass (HM), allowance (HA), and nutritive value were similar on stockpi led limpograss pastures grazed by cow calf pairs supplemented with two sources of RDP There was a year and month effect on all herbage responses. Different rainfall patterns and number and timing of freezing events during the stockpiling and grazing perio d resulted in greater HM in 2012 compared to 2011. T here were no differences between treatments in cow calf performance h owever there were year and month effects on cow and calf average daily gain (ADG). Animal p erformance decreased over time and followed the same pattern of decrease as HM and HA. Blood urea nitrogen (BUN) increased over time as consequence of the protein supplementation. Drylot s tudy A drylot study was conducted to evaluate the dry matter intake (DMI) of cows receiving the same treatment s described in the grazing phase. Cow calf pairs (two pairs per pen, four replicates per treatment) received ground limpograss hay (63 g kg 1 CP and 520 g kg 1 TDN) with 10% refusals. Only the cows had access to the supplement and hay, and calves were fed separately. There were 10 d of adaptation period and 7 d of DMI collection in 2011 and 2012. Dry matter intake was determined daily. There were no differences i n forage and total DMI between treatments s upporting the data from the grazing study where no d ifferences were found on cow and calf performance. Metabolic s tudy Two rumen fistulated steers were allocated in one of two metabolic cages and received the same treatments described in the grazing study in a 2 x 2 Latin square design in 2011 and 2012. The re were 10 d of adaptation, followed by 2 d of blood and
115 rumen fluid collection in a 2 h interval for the first 24 h and every 4 h for the next 24 h after the supplement was offered. There were no differences in BUN, ruminal pH and ammonia total volatile fatty acids concentration, propionic, acetic, and butyric acid concentrations, and branched chain fatty acids between treatments. There was a time effect on ruminal ammonia and pH, and BUN. Blood urea nitrogen and ruminal ammonia increased after supplemen tation and decrease d thereafter, while ruminal pH decreased after supplement was offered and increased subsequently. Urea was as effective as cottonseed meal as a source of RDP for cows grazing stockpiled limpograss when fed with molasses supplements. Creep feeding Study Two experiments were conducted to test the effect of limit creep feeding protein supplements to calves grazing limpograss pastures during summer 2011 (June to September, E xperiment 1) and 2012 (June to August, E xperiment 2). In E xperime nt 1, treatments were calves receiving 0 or 200 g d 1 of SBM (480 g CP kg 1 ) in a randomized complete block design with four replicates. In E xperiment 2, the treatments were 0, 200 and 400 g d 1 of SBM in a randomized incomplete block design with three re plicates for Control and 200 treatments, and two replicates for the 400 treatment. There were eight limpograss pastures (1 ha experimental units) with three cow calf pairs per pasture Calves were approximately 6 mo of age at the initiation of the study. P astures were fertilized with 90 kg N ha 1 in April 2011 and 2012. No treatment differences were found for HM, HA and nutritive value i n either experiment. There was no difference i n cow calf performance in E xperiment 1 but in E xperiment 2, there was a li near increase i n calf ADG and G HA as supplement
116 increased from 0 to 400 g d 1 There were no differences in cow performance. During both experiments, cow calf performance decreased over time, as a consequence of the decline in HM and HA, a s well as advers e environmental condition s including water standing on the pastures and high temperature and humidity. I n E xperiment 2 there was a linear increase in income and return as supplementation increased from 0 to 400 g of SBM d 1 Jiggs B ermudagrass Grazing Study The study was conducted during the summer 2011 and 2012. Treatments were a factorial arrangement of three stocking rates (SR) [2 (low), 5 (medium), and 8 (high) heifers ha 1 ) in a randomized incomplete block design with three replicates for low and m edium and two replicates for the high stocking rate treatment. The equivalent stocking rates of 3.7, 8.8, and 13.1 animal units (450 kg LW) ha 1 were targeted in this study. Eight 0.25 ha Jiggs pastures were used as experimental units. Fertilization manage ment was 40, 18, and 33 kg o f N, P, and K ha 1 respectively, applied in March 2011 and 2012 followed by two applications of 40 kg N ha 1 in the middle of June and early August each year Animals were Angus sired heifers (crossbred cows sired by Angus bul ls) with initial BW of 172 23 kg and 168 21 kg in 2011 and 2012 respectively and final BW of 216 26 and 218 30 kg in 2011 and 2012, respectively. Pastures were grazed using a continuous and fixed stocking rate during the experimental period. Heife rs received 10 g kg 1 BW in concentrate supplement (140 g kg 1 CP and 780 g kg 1 TDN) daily. There was a linear decrease in HM, canopy light interception, and forage height as stocking rate increased from low to high. Herbage mass and forage height decreas ed i n the first month and increased thereafter, while light interception decreased
117 i n the first month and was constant until the end of the trial. On the other hand, herbage accumulation rate (HAR) increased with increasing stocking rate There was a month effect on HAR, where it was similar i n May and June and increased in July and August, likely due to N fertilization and favorable rainfall. No differences were found i n forage nutritive value and leaf, stem and senescent material proportion in the sward. Jiggs ground cover decreased over the 2 yr of grazing as stocking rate increased. Herbage allowance also decreased as stocking rate increased, both because of the decrease in HM and an increase in animal liv e weight per area. Average daily gain decreased as stocking rate increased Herbage allowance and climatic conditions were likely the main factors affecting ADG throughout the experimental period. Additionally, there was a linear increase i n G HA as stockin g rate increased. Heifers i n the high stocking rate treatment had decreased ADG; however, a greater number of animals resulted in a greater gain per area. Conclusions Urea was as affective as cottonseed meal as the main source of RDP when supplemented wi th molasses to cows calf pairs grazing stockpiled limpograss pastures. Urea has greater solubility in the rumen than cottonseed meal, which may lead to inefficient use of N in the rumen. However, the slower intake of urea fed with molasses likely decreased levels of urea be ing consumed over time, promot ing greater synchrony of N and energy in the rumen It also likely decreased the amount of N be ing absorbed by the rumen epithelium, and increased microbial protein formation. It is important to state that th e supplements were adjusted for similar RUP and energy concentrations with additional sources of feed (feather and corn meal), and this practice may be difficult to implement because most producers do not have the capability of storing and mixing
118 commoditi es in addition to molasses on the farm. Therefore, this study provides important information for future studies to investigate including whether the additional levels of energy and RDP provided by the cottonseed would impact the performance of cow calf pai rs grazing stockpiled limpograss pastures at the levels of supplement used in this study. Creep feeding 400 g kg 1 of soybean meal to suckling calves grazing limpograss pastures was effective in increas ing performance of the calves with superior gain:feed ratio. This implies that this management practice could be readily adopted by producers with potential positive impacts in cow calf production. The efficient extra gain provided by creep feeding would be beneficial to increase weaning weights, which would increase the market value of the calf. In addition, the greater weaning weight may also decrease age of puberty and reduce the time to first conception. Although raising and breeding heifers at 14 15 mo may be costly, the current market price for a bred heifer may be twice the price offered for a weaned heifer. This study also generates us eful information for further research to test the effect of creep feeding other RDP sources to calves grazing limpogr ass pastures. Finally Jiggs bermudagrass show ed similar production and nutritive value characteristics to other bermudagrasses; however, grazing Jiggs to stubble heights shorter than 15 to 20 cm is detrimental to the stand favoring the appearance of undes irable plant species in the pasture. Jiggs should be used in rotational stocking, primarily because of the option to move animals to a different pasture and the opportunity to exercise greater control over stubble heights. On the other hand, producers with flexibility to adjust stocking rate using continuous stocking may also be
119 able to maintain a desirable stubble height and promote production and persistence of Jiggs. The levels of decrease in stand presented at the high stocking rate treatment indicated that Jiggs would need to be re established after a few years of grazing. Considering that establishment is one of the most costly factors in warm season perennial grass grazing systems, the utilization of Jiggs should be avoided in extensive grazing system s with limited inputs and management practices.
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138 BIOGRAPHICAL SKETCH Andr De Stefani Aguiar was born in So Pa ulo SP, Brazil He is the oldest son of Heliodoro Aguiar, and Renata Aguiar; and brother of Priscila Aguiar He graduated with in Brazil in 2007. After his undergrad uate st udies he moved to Texas in the Spring of 2007 to attend an internship program and in the Fall of 2007 he started his M.S. program in Animal Science at Texas A&M University with emphasis in Ruminant Nutrition He received his M.S. degree in May of 2010. In the Spring of 2010 he moved to Florida to work on his Ph.D. under the guidance of Dr. Vendramini in the area of forage management and supplementation strategies with beef cattle. After graduation Andr hopes to pursue a career in the beef cattle industr y. He received his Ph.D. from University of Florida in the fall of 2013.