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1 H ERBAGE ACCUMULATION, NUTRITIVE VALUE AND TILLERING DYNAMICS OF BAHIAGRASS GENOTYPES UNDER GRAZING INTENSITIES By DANIEL REIS PEREIRA A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULF ILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 201 1
2 201 1 DANIEL REIS PEREIRA
3 To my parents Paulo and Rita
4 ACKNOWLEDGMENTS I gratefully acknowledge Dr. Lynn E. Sollenberger, chairman of the supervi sory committee, professor and true advisor. His influence in this degree program ranged from providing insight in the classroom guidance on experiment al procedures partic ipating in field activities, and being an example of professionalism and character. Thanks are extended to the members of the supervisory committee, Dr. Ann Blount, Dr. John Erickson, and Dr. Maria Sil veira, for r eviewing this document. Special thank is expressed to Dr. Mary C. Christman for agreeing on very short notice to participate o n the committee as the minor representative. I am especially thankful to Dr. Andr Brugnara Soares for his friendship and assistance with the research during the second year of the experiment. Our discussions on forage science certainly contributed to the m aturation of this work. Dwight Thomas and Richard Fethire assisted me during field and laboratory work, and for that I am thankful. I was very fortunate to share the day to day of graduate school with Miguel Castillo, Eduardo Alava, ChaeIn Na, Nick Krue ger, Kimberly Cline, Hermes Cuervo, Pedro Korndorfer, Andr Aguiar and Eduardo Gelcer, whose companionship and good nature have sustained me through the process. I would like to thank my girlfriend, Brbara, for her assistance and constant encouragement, motivating me through the toughest times and being my companion all along this journey. Lastly, I would very much like to acknowledge my parents, who have never once failed to offer their support.
5 TABLE OF CONTENTS page A CKNOWLEDGMENTS .................................................................................................. 4 LIST OF TABLES ............................................................................................................ 7 LIST OF FIGURES .......................................................................................................... 8 ABSTRACT ..................................................................................................................... 9 CHAPTER 1 INTRODUCTION .................................................................................................... 11 2 LITERATURE REVIEW .......................................................................................... 14 Origin and Characteristics of Bahiagrass ................................................................ 14 Taxonomy and Botanical Description ............................................................... 14 Center of Origin and Reproductive Behavior .................................................... 14 Uses of Bahiagrass in Florida ................................................................................. 15 Seasonal Yield Distribution ..................................................................................... 16 Bahiagrass Tille ring Characteristics ................................................................. 22 Size Density Compensation Theory ................................................................. 24 Summary and Project Objectives ............................................................................ 27 3 BAHIAGRASS GERMPLASM YIELD AND NUTRITIVE VALUE UNDE R TWO GRAZING INTENSITIES ........................................................................................ 29 Background ............................................................................................................. 29 Ma terial and Methods ............................................................................................. 30 Experimental Site ............................................................................................. 30 Experimental Design and Treatments .............................................................. 31 Environmental Conditions and Pasture Management ...................................... 32 Herbage Nutritive Value ................................................................................... 36 Statistical Analyses .......................................................................................... 36 Results and Discussion ........................................................................................... 37 Herbage Accumulation ..................................................................................... 37 Total season ..................................................................................................... 37 Seasonal .......................................................................................................... 39 Total season Herbage Harvested ..................................................................... 40 Conclusions ............................................................................................................ 44 4 TILLERING DYNAMICS OF BAHIAGRASS GERMPLASM UNDER GRAZING INTENSITIES .......................................................................................................... 47 Background ............................................................................................................. 47
6 Material and Methods ............................................................................................. 49 Tiller Population Density ................................................................................... 49 Tiller Mass ........................................................................................................ 49 Statistical Analysis ............................................................................................ 50 Results and Discussion ........................................................................................... 51 Tiller Density and Mass .................................................................................... 51 Across seasons .......................................................................................... 51 Seasonal patterns of tiller response ........................................................... 52 Leaf:stem Ratio and Leaf Characteristic s ......................................................... 58 Size density Compensation .............................................................................. 59 Conclusions ............................................................................................................ 60 SUMMARY AND CON CLUSIONS ................................................................................ 63 APPENDIX A DOUBLE SAMPLING EQUATIONS TABLE ........................................................... 67 B ENTRANCE HEIGHT TABL E ................................................................................. 68 C SOURCES OF VARIATION .................................................................................... 69 LIST OF REFERENCES ............................................................................................... 70 BIOGRAPHICAL SKETCH ............................................................................................ 77
7 LIS T OF TABLES Table page 3 1 Weekly average weekly maximum and minimum air temperatures and total precipitation ........................................................................................................ 33 3 2 Number of grazing events per treatment and year. ............................................ 34 3 3 Bahiagrass total season herbage accumulation as affected by year x height x genotype interaction ........................................................................................... 39 3 4 Bahiagrass total season herbage harvested as affected by year x height x genotype inter action ........................................................................................... 41 3 5 Bahiagrass weighted total season crude protein as affecte d by the year x genotype interaction ........................................................................................... 42 3 6 Bahiagrass weighted total season crude protein as affected by the stubble height x genotype interaction .............................................................................. 43 3 7 Bahiagrass total season weighted digestibility (IVDOM) as affected by year x height x genotype interaction .............................................................................. 44 4 1 Bahiagrass tiller density (tillers per 200 cm2) as affected by the stubble height X cultivar interaction ........................................................................................... 51 4 2 Bahiagrass tiller mass (size) as affected by cultivar ........................................... 52 4 3 Bahiagrass leaf stem ratio as affected by cultivar x st ubble height interaction. .. 58 A 1 Coefficients of the herbage mass double sampling equations ............................ 67 B 1 Entrance height applied by treatment and year .................................................. 68 C 1 Sources of variation for bahiagrass variables. .................................................... 69
8 LIST OF FIGURES Figure page 2 1 llustration of the multip hase mass density compe nsation in defoliated swards. ............................................................................................................... 26 3 1 Herbage accumulation at each grazing event for five bahiagrass genotypes during 2009 and 2010. ........................................................................................ 40 4 1 Bahiagrass population density as affected by year and genotype ...................... 55 4 2 Bahiagrass genotype tiller mass (g DM tiller1) a s affected by year and genotype ............................................................................................................. 57 4 3 Relationship between tiller mass (size) and tiller population density of bahiagrass cultivars ............................................................................................ 60
9 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science HERBAGE ACCUMULATION, NUTRITIVE VALUE AND TILLERING DYNAMICS OF BAHIAGRASS GENOTYPES UNDER GRAZING INTENSITIES By Daniel Reis Pereira August 2011 Chair: Lynn E. Sollenberger Major: Agronomy Grown on more than one million ha, bahiagrass ( Paspalum notatum Flgge) is the main pasture for horses ( Equus caballis ) and cattle ( Bos sp.) in Florida One of the limitations to its use is its str ong summer dominant herbage production pattern. B reeding efforts have now resulted in the development of less photoperiodsensitive bahiagrass cultivars that produce more forage for livestock during the cool season. Early evaluation of less photoperiodsen sitive (PCA) bahiagrass genotypes raised concerns regarding their persistence under grazing T he objectives of this study were: i) measure herbage accumulation and nutritive value responses of PCA genotypes to grazing management strategies; and ii) describe the relationships among tiller density, tiller mass, and ti ller size density compensation with bahiagrass persistence and herbage accumulation. Experiments were conducted in 2009 and 2010 in Gainesville, Florida. Two photoperiodsensitive diploid genoty pes (Pensacola and Tifton 9) and three PCA genotypes (UF Riata, Cycle 5, and Florida Hay) were evaluated at two grazing intensities (post graze stubble heights of 8 and 16 cm, with grazing initiated for all
10 treatments at a 30cm height). Herbage accumulati on in Year 1 was generally greater for the 16vs. the 8 cm treatment for PCA types Cycle 5 and UF Riata but in Year 2 there was no effect of stubble height for any genotype. The new bahiagrass cultivar, UF Riata had as great or greater herbage accumulation than any genotype at all stubble height x year combinations. L ack of stubble height effect in Year 2 was likely due to acclimation to the grazing treatments and clearly demonstrated no negative carryover effects from Year 1. UF Riata had crude protei n and in vitro digestible organic matter concentrations that were generally as great or greater than the other bahiagrasses. Tiller density varied relatively little among three diploid cultivars (Pensacola, Tifton 9, and UF Riata) in 2009, however, by the end of the 2010 grazing season, Pensacola had more tillers than the others for both stubble height treatments Tiller mass was greatest for Tifton 9 by October 2010 for both stubble height s. Greatest impact of close grazing on tiller mass occurred in Year 1 but by Year 2 the plants had adapted to the grazing treatments. The expected slope for the tiller size/density compensation (SDC) phenomenon was not observed for any of the cultivars. This may have been due to the relatively small range in grazing str ess associated with the treatments imposed. For these grazing treatments, PCA type UF Riata was as productive and often higher in nutritive value than existing cultivars. In addition, tiller responses provided no reason for major concerns for persistence under grazing of any cultivar evaluated. E stablishment of larger pastures and imposition of a broader range of grazing treatments is recommended so that the range of grazing management strategies that will be successful with PCA plants can be more clearly defined
11 CHAPTER 1 INTRODUCTION It is common in particular regions throughout the world that a single species contributes the bulk of forage production for livestock This holds true for the state of Florida, southern USA, where approximately one million beef cows ( Bos sp.), 120 000 dairy cows ( Bos taurus ), and 500 000 horses ( Equus caballus ) are fed, at least in part, from mor e than one million hectares of the rhizomatous w arm season perennial, bahiagrass ( Paspalum notatum Flgg) (Chambliss and Adjei, 2006) Bahiagrass is widely adapted throughout Florida. Because of its tolerance of heavy grazing and productivity in low fertility soils of varying drainage characteristics, it is the species of choice for most beef cattle producers. Grazing tolerance and a bility to withstand animal traffic also make it the most widely used grass for equine pastures. One of the limitations to the use of bahiagrass in Florida is its strong summer dominant herbage production pattern. Bahiagrass yield is concentrated between April and September and its growth during the cool season (October to April ) is minimal (Mislevy and Everett, 1981) Herbage accumulation rates of bahiagrass can be as low as 10 kg.ha1.d1 from October to April. Various factors contribute to the low herbage accumulation rate of bahiagrass in the cool season including low nighttime temperatures (Mislevy, 1985) freeze damage to tissue (Breman et al., 2008) and sensitivity to photoperiod. The latter response was documented by Sinclair et al. (2001, 2003) and led to initiation of breeding programs with the goal of developing less photoperiod sensitive bahiagrass cultivars (Blount et al., in review) Plants developed in this breeding program were selected visually for phenotype and inter mated afterward in
12 the g reenhouse, a breeding approach that is considered to be more than three times as efficient as traditional recurrent mass selection (Burton, 1992) Early evaluation of less photoperiodsensitive bahiagrass genotypes raised concerns regarding their long term persistence. Interrante et al. (2009a) imposed clipping treatments on experimental lines and released cultivars of bahiagrass and found that plant responses related to sward persistence (stem N content and stem base and rhizome mass) were lower for less p hotoperiodsensitive vs. standard entries. Such differences were most pronounced under close ( 4 cm stubble) or frequent (7 days rest period) defoliation. An important question is whether genotypes that are more productive later into autumn and earlier in spring i.e., the less photoperiodsensitive types, produce this cool season grow th at the expense of reserves that otherwise would be stored for winter survival and spring regrowth (Hirata et al., 2002; Interrante, 2008) If so, sward persistence or vigor of spring regrowth may be imperiled. Although carbohydrate reserve status plays an important role, bahiagrass persistence is also a consequence of high tiller, rhizome, and root densities (Hirata, 1993, 2001a) Thus, the detailed study of the dynamics of t iller characteristics should provide information on the mechanisms of sward persistence. One established theory that relates these variables is the sizedensity compensation (SDC) theory. The SDC theory, or self thinning rule, states that the mass (size) and abundance (density) of live entities in an environment are not related in a linear but in an exponential fashion (Kays and Harper, 1974) Numerous studies in the past 25 yr have shown that this phenomenon is also active in grazed swards (Sackville Ham ilton et al., 1995; Yu et al., 2008) Hirata and Pakiding ( 2002) for example, found minimal variation in tiller
13 population density throughout the growing season, but were still able to prove the rule to be true for grazed bahiagrass pastures. Because ther e are substantial data describing the tillering pattern of persistent cultivars of bahiagrass (Hirata, 2001) major deviations from the theoretical expectations may indicate genotypes prone to stand deterioration and lack of grazing tolerance. Based on the bahiagrass literature, and in particular the data indicating that less photoperiodsensitive bahiagrass types may be less persistent under defoliation than existing cultivars, there is need to evaluate the responses of these novel bahiagrass genotypes to grazing. Evaluation of tiller dynamics is also warranted because this information may provide a mechanistic understanding of the effect of defoliation on responses of novel bahiagrasses to grazing. Therefore, experiments were conducted with the objectives of: i) measuring herbage accumulation and nutritive value responses of new bahiagrass genotypes to grazing management strategies; and ii) describing the relationships among tiller density, tiller mass, and ti ller size density compensation with bahiagrass p ersistence and herbage accumulation. Research exploring herbage accumulation and nutritive value responses is described in Chapter 3 of the thesis, and characterization of tiller dynamics and relationships with persistence and production responses are desc ribed in Chapter 4.
14 CHAPTER 2 LITERATURE REVIEW Origin and C haracteristics of B ahiagrass Taxonomy and B otanical D escription Bahiagrass ( Paspalum notatum Flgg) is a perennial grass with shallow rhizomes [ also referred to as stolons (Sampaio et al., 1976; Beaty et al., 1977) ] glabrous leaves with blades varying from 3 to 30 cm in l ength and 3 to 12 mm in width and a characteristic inflorescence with two racemes. Paspalum is a large genus of the Poaceae family, comprising approximately 400 species. One of the Poaceae tribes, Paniceae, is characterized by grasses with single spikelet s and glumes resembling lemmas Within th is tribe is the genus Paspalum to which the closest gen us is Axonopus Plants in Axonopus differ subtly from Paspalum s in terms of sp ikelet shapes and position of the upper lemm a ( Zuloaga et al., 2004) There are two botanical varieties of P. notatum One is strong rooted, with larger leaves and slower spread, and is a tetraploid. C ommon bahiagrass is an example of this type, and it is classified as P. notatum var. notatum ( Zuloaga et al., 2004) The other form is a diploid and is taller, with longer and narrower leaves and faster rate of spread. These latter types are classified as P notatum var. saurae Parodi, often referred to as Pe nsacola types (Gates et al., 2004) because Pensacola is the cultivar name of the most widely used bahiagrass in the Southeast USA Plants used in the experiments described in this thesis belong to var. saurae. Center of O rigin and R eproductive B ehavior B ahiagrass is a morphologically diverse species indigenous to S outh America where it is found growing on light textured soils and is widely used in lawns, sports turf
15 and roadsides (Gates et al., 2004) Bahiagrass was first introduced into the USA by the Bureau of Plant Industry and grown by the Florida Agricultural Experiment Station in 1913 (Burton, 1967) The center of origin of Pensacola bahiagrass types is believed to be the region to the north of the Berduc Island, in the Paran River near Santa Fe, Argentina. It is believed that a seaborne cargo ship prior to 1926, brought bahiagrass to the region of Pensacola, Florida (Quarn, 1974; Gates et al., 2004) Most Paspalum species have a base chromosome number of x = 10, and known ploidy levels in the w ild range from diploid to pentaploid (Burton, 1955) The regular diploid races, including Pensacola types, are sexual and highly cross pollinated because most plants are self incompatible (Burton, 1948) The tetraploid races reproduce by obligate apomixi s (Gates et al., 2004) Uses of B ahiagrass in F lorida The uses of bahiagrass in Florida include turf, pasture, crop rotation, and hay. Pensacola types generally are not preferred for residential turf because they have rapid seedhead emergence following cutt ing which makes them less attractive. Bahiagrass is used as low maintenance turf because it is very persistent under close mowing and requires minimal nutrient and pesticide inputs. Highway rights of way have been planted with bahiagrass throughout the southeastern USA because the sod provides complete cover limits erosion and persists under low fertilizer inputs. Most of the agricultural land area planted to bahiagrass is used for pasture in extensive cow calf production systems (Gates et al., 2004) Ba hiagrass is particularly well suited to this use because of its persistence, even under low soil fertility, and tolerance of environmental stresses and severe grazing by livestock. Bahiagrass forage energy concentration decreases substantially as the season progresses, thus it is not
16 considered well suited to meet the nutritional requirements of young, growing animals or lactating cows (Gates et al., 2004) Bahiagrass can also be utilized as hay, but it tends to have low nutritive value when dry matter (DM) yield is sufficiently high to make a hay harvest practical (Kalmbacher 199 7) Recent research has documented the value of bahiagrass as a component of crop rotations. I nclusion of bahiagrass in a crop rotation increased earthworm population and subsequent cotton ( Gossypium hirsutum L.) crop yields due to improved rooting (Katsvairo et al. 2007) Including bahiagrass in the crop rotation reduced incidence of stem rot of peanut ( Arachis hypogaea L.) and increased peanut yield ( Franzluebbers, 2007) Season al Yield Distribution In the southeastern USA, bahiagrass is most productive from April to October (Mislevy and Dunavin 19 93 ; Ball et al. 2002 ) In Florida, 85 to 90% of bahiagrass forage was produced from April through September ( Mislevy and Everett, 1981, Kalmbacher, 1997) Stewart et al. ( 200 7 ) reported herbage accumulation rates of 30, 62, and 15 kg ha1 d1 in May, midJuly, and October, respectively, for continuously stocked Pensacola bahiagrass pastures receiving 120 kg N ha1 yr1. Similarly in su btropical Japan, aboveground bahiag rass pasture productivity decreased as daylength decreased in autumn (Hirata et al. 200 2 ) Low productivity of bahiagrass pastures during the cool season necessitates purchase of costly supplements and is a major obstacle to profitability of livestock enterprises. As a result, development of new genotypes that overcome the seasonal shortfall of forage is a primary objective of bahiagrass genetic improvement programs ( Blount et al., 2003)
17 Understanding the cause of thi s reduction in growth is important to facilitate the development of cultivars that are more product ive in cooler months. Pasture productivity in subtropical Japan decreased during short day months in part because plants allocated a greater proportion of nonstructural carbohydrates to storage organs ( Hirata et al., 2002) In the USA, decreased bahiagrass forage growth in autumn was observed even when soil moisture, soil fertility, and temperatures were adequate for much more rapid growth (Sinclair et al., 1997; Gates et al., 2004) implicating plant responses to daylength in the observed yield decrease. To address the question of the relative role of daylength in the reduction in yield, Sinclair et al. ( 2 001 ) artificially extended daylength during the winter months and measured the effect on forage yield of Pensacola bahiagrass, Tifton 85 bermudagrass ( Cynodon spp.), Florakirk bermudagrass [ Cynodon dactylon (L.) Pers.], and Florona stargrass ( Cynodon nlemfuensis Vanderyst) Yields of these grasses under extended daylength increased up to 6.2fold vs. that under natural daylength. In another study with these four grasses growth increased in an extended photoperiod treatment that was imposed during short daylength months (Sinclair et al., 2003) Pensacola bahiagrass exhibited the greatest increase, with forage yield for many harvests under the extendeddaylength treatment being more than double that of the natural daylength treatment ( Sinclair et al., 2003) Thus daylength sensitivity appears to be a criti cal component of the observed low productivity of bahiagrass during short daylength months (Blount et al., 2003) This observation led to efforts to select less photoperiod sensitive bahiagrass genotypes that extend growth into the cool season. Although t his approach has
18 potential to address a critical problem in the foragelivestock industry, o ne possible problem it may create is that by extending growth into cooler months, carbohydrate reserves may be reduced. This could result in reduced persistence of less photoperiod sensitive genotypes In a 2 yr study, Pensacola bahiagrass, Tifton 85 and Florakirk bermudagrasses, and Florona stargrass were heavily fertilized (67, 15, and 56 kg ha1 harvest1 of N, P, and K, respectively) and defoliated three times a year (Sinclair et al., 2003) They reported 1) no decrease in growth following the extended photoperiod treatment in either season, 2) no difference in below ground tissue mass throughout the season between extended and natural daylength treatments, and 3) no influence of extended daylength on total nonstructural carbohydrate (TNC) concentratio n of below ground tissue. Thus, in that study, extending daylength for daylengthsensitive types improved seasonal distribution of DM without negative effects on pers istence, at least in the short term. There is some research, however, that suggests that these improvements in forage yield may be detrimental to pasture persistence. For example, Tifton 9 bahiagrass is a taller, more erect, higher yielding bahiagrass cult ivar that produces forage earlier in spring and later in fall than the industry standard cultivars Pensacola and Argentine. Tifton 9 was selected from populations of Pensacola bahiagrass, a prostrate cultivar that partitions a large proportion of its dry weight to rhizomes (Beaty and Tan, 19 72) Pedreira and Brown (1996) reported that the selection process that led to Tifton 9 resulted in increased allocation of dry matter to harvestable foliage and possibly a greater production of nonroot biomass and resulted in taller plants with fewer rhizomes, a greater tendency for winter injury, and a greater susceptibility to
19 population shifts under close, frequent mowing. This research indicates that it may be difficult to maintain the same level of pasture p ersistence in bahiagrasses selected for improved cool season forage yield as in the cultivar Pensacola, and that testing of potential cultivars should include evaluation of persistence under grazing Bahiagrass Response to Defoliation Productivity Forage production of bahiagrass is generally thought to be lower than that of other regionally adapted grasses including bermudagrass and stargrass. Chambliss ( 200 3 ) reported total average annual yield of Pensacola, Argentine, and Tifton 9 bahiagrasses to be appr oximately 10 Mg DM ha1. M islevy ( 200 5 ) reported 3yr annual dry biomass yields ranging from 10.3 Mg ha1 for Pensacola to 12.1 Mg ha1 for Tifton 7 (a nonreleased breeders line) in c entral Florida when harvested monthly and fertilized with 56 kg N ha1 harvest1. For bahiagrass cultivars fertilized once annually with 56, 28, and 56 kg ha1 of N, P, and K, respectively, and cut to a 5cm stubble every 35 d in Sou thwest Florida, Muchovej and Mullahey ( 2000) reported annual DM yields ranging from 3 Mg ha1 for Paraguay to 4 Mg ha1 for Tift on 7. Hirata ( 199 3 ) reported greater yields of Pensacola with increasing stubble heights in the summer (8 Mg ha1 at 22cm stubble vs. 4.5 Mg ha1 at 2 cm), but greater yields with decreasing stubble heights in the fall (2 Mg ha1 at 2 cm stubble vs. 1 Mg ha1 at 22 cm) when harvested every 2 to 4 wk. Gates et al. ( 1999) reported bahiagrass DM yields were greater at a short (1.5 cm) than tall cutting height (10 cm) at 2or 4 wk regrowth intervals over 2 yr.
20 There has been rather limited research done to evaluate the effects of defoliation management on productivity of less photoperiodsensitive bahiagrass entries. Recent work has shown that a n upright growing, less photoperiodsensitive entry (Cycle 4 subsequently releas ed as UF Riata ) was generally no more productive during the May through October grazing season than other current cultivars of bahiagrass ( Interrante et al., 2009 a ) However, i t wa s considerably less productive when harvested frequently (7 d), suggesting t hat its upright growth habit makes it less tolerant of frequent defoliation. Data from other research has shown that yield of Cycle 4 in the coo l season was superior to that of Pensacola and Argentine (Blount et al., in review) Persistence Bahiagrass is valued for excellent persistence under severe grazing ( Gates et al., 2004) Pensacola bahiagrass forms a dense sward that contributes to its grazing tolerance (Hirata, 1993) It also maintains stable tiller density in terms of space and time, which contributes to its high persistence under grazing ( Hirata and Pakiding, 2004) Gates et al. ( 1999) reported that bahiagrass cover was not influenced by cutting interval (2, 4, and 8 wk) throughout 3 yr of the experiment and was not influenced by cutting height af ter the first year. In a 2yr grazing experiment, Pensacola maintained cover under continuous stocking of yearling heifers, while Tifton 9 cover was reduced after 2 yr. The importance of storage organs in defoliation tolerance response has been well docum ented in several C4 grass species ( Chaparro et al., 1996; Macoon et al., 2002) When subjected to continuous severe defoliation, bahiagrass tillers depend considerably on storage organs for energy for maintenance and growth of new leaves
21 ( Hirata and Pakidi ng, 2003) They reported that bahiagrass swards degraded when all regrowing laminae were removed every 1 to 4 d, resulting in a reduction of laminae production, mass of stubble, and rhizome mass. They concluded that rhizomes play a key role in bahiagrass defoliation tolerance. Gates et al. (1999) reported greater spring reserves (as estimated by total etiolated initial spring growth) for bahiagrass from plots cut every 8 wk the previous growing season than in those cut every 2 or 4 wk. After 3 yr of grazing every 2 wk, bahiagrass stem base TNC concentration during August to early September averaged less than 90 g kg1, which was 40 g kg1 lower than the overall average for 3, 5 and 7wk grazing frequencies Interrante et al. ( 2009a ) evaluated per sistence related responses of several bahiagrass entries to clipping defoliation every 7 or 21 d to a stubble height of 4 or 8 cm. Across defoliation treatments, stem base mass of Pensacola and Argentine averaged 60% greater than less photoperiod sensitive Cycle 4 while at 4and 8cm stubble height s, the numbers were 86 and 41% respectively Similarly, when defoliated every 7 d, average stem base mass of Pensacola and Argentine w as 125% greater than for Cycle 4 while the difference was only 23% for the 21d trea tment. Pensacola and Argentine also averaged 27% greater root + rhizome mass than Cycle 4 ; at 4 and 8cm stubble height s the values were 32 and 21%, respectively. When defoliated every 7 and 21 d, Pensacola and Argentine root + rhizome mass was 38 and 14% greater, respectively, than Cycle 4 These plant responses could result in the photoperiodinsensitive type being less persistent than Argentine and Pensacola under defoliation, particularly frequent or close defoliation. In the same experiment, harvesting every 21 d to an 8cm stubble height resulted in greatest cover for both Tifton 9 and Cycle 4 (83
22 and 77%, respectively), while cover for both entries was less than 40% if harves ted every 7 d to 4 cm. Interrante et al. (2009a ) reported that Cycle 4 often had less root + rhizome N and TNC content than Argentine and Pensacola. These differences also were most pronounced when the entries were harvested closely or frequently. They concluded that defoliation management of Cycle 4 will likely be more critical than for Pensacola and Argentine bahiagrass and that longer regrowth intervals (21 d or longer) and taller residual heights (8 cm or taller) may be required to ensure its persistence. Nutritive Value Another important plant response to defoliation is nutr itive value. Published differences in nutritive value among bahiagrass cultivars are generally small and inconsistent ( Kalmbacher, 1997; Mislevy and Dunavin, 1993; Chambliss and Adjei, 2006) Muchovej and Mullahey ( 2000) found no differences in in vitro di gestible organic matter ( IVDOM ) concentration among bahiagrass cultivars during summer while Mislevy et al. (2005) likewise reported few differences. Interrante et al. (2009b ) found that h erbage CP of Cycle 4 was comparable to that of other bahiagrasses a nd it was at least as digestible and sometimes more digestible than the existing bahiagrass cultivars. Differences in IVDOM among cultivars were relatively small (< 25 g kg1); however, and their impacts on cattle gains are not known. Tillering and the Siz e Density Compensation Theory Bahiagrass T iller ing Characteristics Research in Japan showed that density characteristics of Pensacola bahiagrass, such as tiller number and rhizome length, tended to increase when cutting hei ghts decreased from 22 to 2 cm (H irata, 1993) When Pensacola was defoliated daily to
23 remove all regrowing laminae from an initial 2cm cutting height, initial tiller density was maintained for 4 to 6 wk before declining (Pakiding and Hirata, 2002) In Japan, Pakiding and Hirata (2003) re ported that Pensacola responded to low N (50 kg ha1 yr1) and intense defoliation (2cm stubble at monthly harvest intervals) with increased tiller longevity, tiller appearance rate, and tiller density compared to high N (20 g m2 yr1) an d 12and 22 cm stubble heights Pensacola tiller formation was stimulated with high N (20 g m2 split applied annually) in May through June in a Japanese study on plots harvested monthly to a 3cm stubble when compared to plots receiving 5 g N m2 split applied annually (Islam and Hirata, 2005). Interrante et al. (2010) evaluated tiller responses of a several bahiagrass entries to clipping defoliation every 7 or 21 d to a stubble height of 4 or 8 cm. Tifton 9 and Cycle 4 decreased tiller number across treatments in the fi rst year of defoliation, likely indicating less rapid morphological adaptation to defoliation by these upright growing types than the more decumbent Pensacola. All cultivars showed a positive change in tiller number from the beginning of the first to the e nd of the second year and the authors concluded that over longer time frames phenotypic plasticity was possible even for the upright growing types. Less photoperiodsensitive Cycle 4 generally had the least or not different from the least tiller mass. In Florida, bahiagrass t iller appearance rate was greatest in spring and least in autumn, and t iller death rate was least in spring and greatest in midsummer through autumn and was not affected by entry (Interrante et al., 2010) Net tiller appearance rate w as great est and positive in the spring or early summer and tended to be close to zero or negative throughout the remainder of the growing season. The authors concluded
24 that the documented persistence of Pensacola bahiagrass under defoliation was explained in part by its ability to sustain a high number of tillers across a wide range of defoliation treatments. Tifton 9 and Cycle 4, which were shown to be less tolerant of frequent, close defoliation ( Interrante, 2009a) increase d tiller number under this type of management, but tiller mass was the least or not different from the least among the entries. S mall tillers may be indicative of a weakening stand and entries that are not as tolerant of frequent, close defoliation. This observation led the authors to t he conclusion that choice of defoliation management is more critical for Tifton 9 and Cycle 4 than for more decumbent Pensacola. S ize Density Compensation Theory Plant communities can optimize their LAI in response to defoliation by varying thei r morpholog y. This phenomenon, known as phenotypic plasticity ( West Eberhard, 1989; Schlichting, 2002) is reversible and occur s in pastures mainly through three sward characteristics: leaf size, number of leaves per tiller and tiller population density (Matthew et al., 2000) Of these three sward characteristics, tiller density is the most important, since leaf number per tiller is relatively constant for a given species ( Chapman, 1983) and leaf size is heavily dependent on leaf length which is controlled by defoliation intensity therefore dependent on grazing management. As a consequence, for short defoliation heights (or low leaf individual area), a higher population densit y of small tillers maximizes leaf area index and conversely, lower populations of larger t illers maximizes leaf area index when defoliation height is taller ( Matthew et al., 1995 ) This process of compensation between plant population densit y and plant size, termed SDC, was first
25 described by Japanese researchers for tree stands ( Yoda, 1963) a nd later recognized in grasslands (Kays and Harper, 1974) Over the past three decades, many have attempted to explain the SDC rule through geometric, allometric and dynamic growth arguments (Kikuzawa, 1999; Li et al., 2000) Among other things, the disc ussion revolved around the validity of self thinning as a universal ecological rule, and whether the slope of a size by density logarithmic plot is constant. Some authors have proposed a slope of 3/2 (Hernndez Garay et al., 1999; Bahmani et al., 2000; Ma tthew et al., 2000; Sbrissia, 2000, 2004) and others found 5/2 to more closely represent the observed response ( Davies, 1988) Matthew et al. ( 2000) proposed a dynamic solution for pastures unde r defoliation. According to those authors a four phase diagram of SDC is more appr opriate to explain the tiller dynami cs of swards under defoliation. The four phases (Figure 2 1) represent different states of a pasture maintained at constant herbage mass levels The first phase is where tiller appearance rate is i nsufficient for tiller population to reach the SDC boundary line. The second phase represents a condition where self thinning occurs, but leaf area is kept constant at a slope steeper than 1.5. The third phase is where the self thinning occurs at the 3/2 boundary line, and at the fourth, self thinning occurs at a slope of 1.
26 Figure 21 llustration of the multiphase mass density compensation in defoliated swards. 1. Shoot sizedensity relations undefined. 2. Variable leaf area self thinning with log size: log density slope near 3. 3. Constant leaf area self thinning along the population boundary line at slope near 1.5. 4. Constant herbage mass self thinning at slope 1. d and s denote shoot population density and size (mass). Adapted from Matthew et al. (1995) and Scheneiter and Assuero (2010). Independent of the SDC slope value, there is a consensus that the mechanism operates in plant stands and that the main factor affecting SDC is light competition. Populations of few lar ge tillers appear as a consequence of the death of young tillers, which are usually located at the bottom of the canopy, not receiving enough radiation to thrive ( Sbrissia, 2004) Therefore, light may be the main determinant of the position of
27 the SDC line White ( 19 81) showed that reductions in light level resulted in lower values for the intercept of the SDC line, and proposed that nutrients do not alter its position directly but can influence it through their effect on photosynthesis and maintenance of foliar area. Within this resource competition context, plants with a greater proportion of tissue mass invested in leaves rather than stems are expected to be favored. Hence the use of the R ratio ( foliar area: tiller volume) helps to explain the sizedens ity dynamics in a plant population, giving an idea of the plant morpholog ical change s. Matthew et al. ( 1995 ) found higher R values for perennial ryegrass ( Lolium perenne L.) swards grazed to 160 mm when compared to 120 mm. This indicates a mechanism throug h which large shoots competitively exclude small shoots by changes in tiller geometry. Hernndez Garay et al. ( 1999) explained that the observed increase in R is a consequence of increased length of individual leaves, because at tall er cutting heights ther e are fewer live leaves per tiller and reduction in leaf number per tiller would tend to decrease R. Those authors also suggest ed that the 160 mm defoliation height le d to a situation where th e formation of new leaf tissue was accompanied by senescence of old leaves, with reduced tiller density. Hirata and Pakiding ( 2002) and Hirata (2002) when assessing the validity of the SDC rule in bahiagrass, did not report R ratio values, but their results suggest the existence of the self thinning rule in bahiagrass Summary and P roject O bjectives Based on this review of the literature, it was determined that information was needed on productivity, nutritive value, and tiller dynamics of less photoperiodsensitive bahiagrass types. The objectives of these studies wer e 1) to characterize herbage accumulation and nut ritive value of new and existing diploid bahiagrass genotypes
28 under grazing (Chapter 3), and 2) to describe tillering responses of these genotypes and evaluate the relationship between the mechanism of sizedensity compensation, persistence, and forage production in bahiagrass (Chapter 4)
29 CHAPTER 3 BAHIAGRASS GERMPLASM YIELD AND NUTRITIVE VALUE UNDE R TWO GRAZING INTENSITIES Background Bahiagrass ( Paspalum notatum Flgge) is used throughout Florida as forag e for cattle ( Bos sp.) and horses ( Equus caballus) b ecause of its tolerance of heavy grazing and productivity in low fertility soils of v arying drainage characteristics A limitation of the most widely used bahiagrass cultivar Pensacola is that annual he rbage accumulation occurs primarily from April to September (Mislevy, 1985) necessitating the feeding of costly supplements or hay to livestock during the cool season. Various factors contribute to the low herbage accumulation rate of bahiagrass in the c ool season including low nighttime temperatures (Mislevy and Everett 1985) freeze damage to tissue (Breman et al., 2008) and prioritization of C to storage instead of top growth under reduced daylength (Sinclair et al., 2001) The latter response was documented by Sinclair et al. ( 2001, 2003) and led to initiation of breeding programs with the goal of developing less photoperiodsensitive bahiagrass cultivars (Blount et al., in review). In order to develop bahiagrasses that are more cold tolerant and less photoperiod sensitive, recurrent selection was initiated with Pensacola ( Blount et al., 2003) Plants developed in this breeding program were selected visually for phenotype and inter mated afterward in the greenhouse a breeding approach that is considered to be at least three times as efficient as traditional recurrent mass selection ( Burton, 1992) Through this effort, less photoperiodsensitive and coldadapted (PCA) bahiagrass germplasm has been developed in order to extend the grazing season ( Blount et al., in review). The PCA types used in the current study are Cycle 4 and Cycle 5, and they
30 represent the fourth and fifth cycles of selection from Pensacola. Cycle 4 has been released as a cultivar with the name UF Riata, and it will be referred to by this name throughout the chapter. Early evaluation of less photoperiodsensitive bahiagrass genotypes raised concerns regarding plant persistence Interrante et al. ( 2009a) imposed clipping treatments on experimental lines and cultivars of bahiagrass They found that plant responses related to sward persistence (stem N content stem base and rhizome mass and cover ) were lower for less photoperiodsensitive vs. standard entries, particularly under close or frequent defoliation. An important question is whether genotypes that are more productive later into autumn and earlier in spring produce this cool season grow th at the expense of reserve storage. If so, sward persistence or at least vigor of spring regrowth and herbage accumulation the following year may be compromised Based on the bahiagrass literature, and in particular the data indicating that less photoperiodsensitive bahiagrass types may be less persistent than existing cultivars when def oliated there is need to evaluate these novel bahiagrass genotypes under grazing. As suggested by Bouton et al ( 1997) the use of grazing animals in cultivar selection and testing should happen as early as possible in the breeding program so pasture potential can be properly assessed. Therefore, an experiment was conducted with the objective of measuring herbage accumulation and nutritive value responses of new bahiagrass genotypes to grazing management strategies Material and Methods Experimental Site This study was conducted in Gainesville, FL in the U.S. Gulf Coast Region (2943 N lat; 8216 W long) The climate is humid subtropical ( Cfa under the Kppen c limate
31 classification), where the average maximum temperature during the summer is greater than 30 C, and multiple frosts and freezes are common during the winter. Yearly precipitation averages around 1300 mm, of which nearly half occurs in short duration rainfall events during June through Sept ember Soil at the location was an Adamsville fine sand (hyperthermic, uncoated Aquic Quartzipsamm ents) This soil is derived from sandy marine sediment and is described as having moderate drainage and rapid permeability. Average pH was 5.8, and MehlichI extractable soil P, K, Mg, and Ca concentrations w ere 67, 61, 89, and 685 mg kg1, respectively, in May of 2009, and they were 57, 43, 78, and 669 mg kg1, respectively, a year later. Bahiagrass entries used in the study were seeded into a prepared seedbed in September 2007. During the 2008 growing seaso n, prior to the initiation of this study, plots were fertilized and defoliated by clipping three times to encourage tillering and increase cover. Stands were fully established by the start of the experiment in spring of 2009. Experimental Design and Treatm ents Treatments were the factorial combinations of five bahiagrass genotypes and two post graze stubble heights of rotationally stocked pastures. Experimental units were 36m2 pastures (6 x 6 m) and treatments were arranged in a randomized complete block design with three replications. The f ive diploid bahiagrass genotypes evaluated were Pensacola, Tifton 9, Florida Hay, UF Riata, and Cycle 5. Post graze stubble heights were 8 and 16 cm, and grazing on all treatments was initiated throughout the grazing season when sward canopy height reached 30 cm.
32 The entrance height of 30 cm is justified on the basis that it corresponds to a leaf area index in the range of four to six, after which forage plant communities are not expected to show an increase in crop gr owth rate, and should be harvested for maximum efficiency (Bircham and Hodgson, 1984; Gardner et al., 1984) The stubble target of 8 cm was chosen because it corresponds to approximately 1000 kg ha1 of post graze herbage mass, a threshold after which catt le intake declines considerably in bahiagrass swards (Hirata et al., 2006) The five genotypes were selected to include existing cultivars, PCA types, and a range of plant growth habits. Entry UF Riata is a recently released cultivar (Blount et al., in rev iew ) that was selected for greater cool season production. Cycle 5 is one selection cycle beyond UF Riata from the same breeding program (Dr. A. Blount, personal communication, 2011). Both UF Riata and Cycle 5 have a more upright growth habit than Pensacol a. Pensacola and Tifton 9 are the industry standard diploid cultivars Together they occupy approximately 90% of the area planted to bahiagrass in Florida. Pensacola is characterized as decumbent in growth habit, while Tifton 9 has a more erect growth (Pedreira and Brown, 1996). Florida Hay is an experimental line specifically bred for use as a short rotation crop in integrated row croplivestock systems in Florida. It is characterized as having upright growth habit, rapid germination, i mproved resistance to dollar spot ( Sclerotinia homeocarpa) and ample seed production ( Dr. A. Blount, personal communication, 2011). Environmental C onditions and P asture Management Grazing was conducted during the growing seasons of 2009 and 2010, with pastures grazed from 2 June to 24 Oct. 2009 and 11 May to 9 Oct. 2010. Weather conditions i n both years of experiment ation are summarized i n Table 31. Yearling
3 3 Brahman by Angus crossbred heifers ( Bos sp.) served as grazing animals. At each grazing event, heifers were allocated to pastures for approximately 1 to 2 h in groups of two to six. Grazing was monitored and when average sward height reached the target stubble, cattle were removed. Table 3 1 Weekly average weekly maximum and minimum air temp eratures and total precipitation at the Beef Research Unit during the experimental period in 2009 and 2010. Week Minima Maxima Mean Rainfall 2009 2010 2009 2010 2009 2010 2009 2010 -------------------------------C -----------------------------------mm ----1 18.4 17.3 30.8 29.8 24.6 23.6 7 117 2 17.2 19.3 22.8 31.3 20.0 25.3 124 0.3 3 20.4 19.3 28.9 31.0 24.7 25.2 56 11 4 19.6 20.1 30.6 32.2 25.1 26.2 38 1 2 5 20.6 20.4 32.4 32.8 26.5 26.6 21 6 9 6 22.3 22.8 33.3 36.6 27.8 29.7 10 20 7 22 .2 22.0 34.8 33.4 28.5 27.7 2 5 1 8 23.1 22.9 31.8 31.9 27.4 27.4 35 58 9 22.3 21.1 30.0 31.8 26.2 26.5 80 65 10 22.4 22.7 31.3 32.3 26.9 27.5 84 20 11 21.8 23.1 31.7 33.6 26.7 28.3 6 98 12 21.7 23.8 31.7 34.8 26.7 29.3 33 7 13 21.8 24.2 32.4 34.8 27. 1 29.5 42 5 14 23.6 24.2 32.9 32.3 28.2 28.3 5 13 15 23.6 24.4 31.7 34.0 27.6 29.2 5 9 16 21.3 23.9 31.1 31.3 26.2 27.6 78 45 17 21.6 20.8 29.8 31.1 25.7 25.9 2 3 2 18 20.3 20.6 31.5 33.2 25.9 26.9 0. 3 5 19 21.9 19.4 31.6 33.3 26.7 26.4 96 2 20 22.0 17.2 31.9 32.8 26.9 25.0 1 0.8 21 14.9 19.4 29.7 29.4 22.3 24.4 3 0.3 22 22.4 10.1 31.3 27.0 26.9 18.6 32.5 21.3 23 20.8 13.9 30.8 30.7 25.8 22.3 1.0 0.0 24 10.3 8.9 26.3 29.7 18.3 19.3 9.7 0.0 Sum 779 661 Week 1 started on 10 May 2009 and 9 May 2010, and Week 24 represents the week beginning with 24 Oct. 2009 and 2010.
34 The three replicates of each genotype x stubble height treatment were grazed always on the same day during each grazing cycle of each year. Among treatments, number of gr azing events per year varied due to level of stubble height and to differences in growth habit and rate of regrowth among genotypes (Table 32). Table 3 2 Number of grazing events per treatm ent and year. Genotype Stubble height (cm) Grazing events 20 09 2010 C ycle 5 8 3 4 16 5 7 UF Riata 8 4 4 16 7 7 F lorida Hay 8 4 4 16 7 7 T ifton 9 8 3 4 16 5 7 P ensacola 8 3 3 16 3 6 Pastures were fertilized three times each year (5 May, 23 June and 9 Aug 2009, and 20 Apr 12 June, a nd 11 Aug 2010). At each fertilization 40 kg N ha1 and 30 kg K ha1 were applied using ammonium nitrate (NH4NO3) and muriate of potash (KCl). Herbage Accumulation and Harvested Herbage mass was measured before and af ter each grazing event using a double sampling technique, i.e., both a direct and an indirect meas ure. The indirect measure was a disk meter ( Santillan et al. 1979; Michell and Large, 1983) and the direct measure was hand clipping of herbage. Before animals entered the pasture, two 0.25m2 sites per pasture were selected that represented the range in herbage mass on that experimental unit. Disk settling height was determined at these two sites, and herbage
35 within the 0.25m2 quadrats was clipped to a 5cm SH bagged separately, dried at 60C in a forcedair oven to constant weight, and weighed. Before grazing was initiated, disk settling height was determined at 10 additional site s within each experiment al unit. Sites were selected systematically based on a predetermined number of steps between sites so that the entire experimental unit was represented. Average disk settling height was calculated. After grazing, the same procedure was repeated. Regression equations were developed that predict ed h erbage mass from disk settling using the double sampling data. Equat ions ( Table A 1) were generated by grouping the data by genotype since there was no added benefit in looking at preand post graze herbage mass data separately. Values for pre and post g raze herbage mass were obtained by entering the average settling height of the 10 disk height observations for a given preor post graze sampling event into the appropriate equation. The r squar e values ranged from 0.67 to 0.75 for the equations used. Her bage accumulation (HA) was calculated by subtracting post graze herbage mass at the end of a given grazing cycle from pre graze herbage mass of the following grazing cycle Herbage accumulation of Grazing Cycle 1 was considered to be pregraze herbage mass for this cycle Seasonal HA was calculated by summing HA across individual grazing cycles within each year Herbage harvested by grazing (H H ) was calculated by subtracting post graze herbage mass from pregraze herbage mass of the same graz ing cycle. Seasonal H H was calculated by summing H H across all grazing cycles within a year.
36 Herbage Nutritive Value Herbage crude protein (CP) and in vitro digestible organic matter ( IVDOM ) concentrations were measured to describe forage nutritive value. Handplucked herbage samples were collected from each experimental unit during pregraze sampling of each grazing cycle Herbage obtained in a grab sample was removed to the target stubble height at 1 0 locations per pasture to represent f orage grazed by the animals These samples were composited to a single sample per pasture per grazing cycle Samples were dried at 60C in a forceair drying oven to constant weight and ground in a Wiley mill (Model 4 Thomas Wiley Laboratory Mill, Thomas S cientific, Swedeboro, NJ) to pass a 1mm stainless steel screen. Nitrogen concentrations were measured using a modification of the aluminum block digestion procedure (Gallaher et al., 1975). Concentration of CP in herbage DM was calculated as N x 6.25. Her bage IVDOM was determined using a modification of the twostage technique (Moore and Mott, 1974). H erbage nutritive value for an entire grazing season was the weighted average across grazing cycles Statistical Analyses Statistical analys e s w ere perform ed using the MIXED procedure of SAS 9.2 ( SAS Institute, 2008) where means separation was based on Fishers LSD generated by the PDIFF option of the LSMEANS statement. A ssignment of letters for means separation was aided by the PDMIX800 macro (Saxton, 199 8) The framework suggested by Crawley ( 2007) was followed, where the significance level of 5% was adopted. Year, genotype, height, and their interactions were considered fixed effects, and block random. Year was considered fixed because treatments were im posed on the same
37 experimental unit in both years, and there was reason to expect carryover effects from Year 1 to Year 2. A summary of the analysis of variance is presented in Table C 1 Results and Discussion Herbage Accumulation Total s eason Total se ason HA was affected by genotype, year, and genotype X stubble height, genotype X year, and genotype X stubble height X year interactions, thus comparisons were made within year (Table 33 ). Focusing first on stubble height, t here was no effect of stubble on Year 2 HA but in Year 1 the response to stu bble height was significant for Cycle 5, UF Riata, and Pensacola (Table 33 ) There was no effect of stubble height for Florida Hay or Tifton 9 in either year The more upright growing Cycle 5 and UF Riata were favored by taller stubble height in Year 1, but the more decumbent growing Pensacola performed better when grazed to 8 vs. 16 cm. Th e first year difference due to stubble height suggests that Cycl e 5 and UF Riata were slow to adapt to close grazing but the lack of second year response indicates that over time they were able to do so. In the year prior to the start of the study, the plots were mowed approximately every 6 to 8 wk to a relatively lax 15cm stubble, so the negative effect of an 8 cm stubble in Year 1 likely occurred until plants adapted their growth habit (i.e., phenotypic plasticity) to more intensive defoliation. Evidence of greater adaptation to close grazing can be seen in the year to year changes in HA. For plots grazed to 8 cm, Cycle 5 HA increased 33% and UF Riata HA increased 34% from 2009 to 2010, while for plots grazed to 16 cm there was essentially no change in HA from year to year. In previous research with UF Riata, Tifton 9, and Pensacola, there was no effect of clipping stubble height on dry matter yield, but more upright growing
38 types like UF Riata and Tifton 9 perform ed better with longer regrowth intervals of 21 vs. 7 d (Interrante et al., 2009). In that study, the decu mbent Pensacola was not affected by interval between defoliation events In evaluating differences among genotypes, UF Riata had as great or greater HA than any other genotype for each stubble height by year combination (Table 33 ) Greatest differences were between UF Riata and Penscola (6390 vs. 3310 kg ha1) in pastures grazed to a 16 cm stubble height in 2009, but by 2010 the differences among genotypes were much less, appr oximately 1000 kg ha1. Most genotypes had greater HA in 2010 than in 2009 due to a somewhat longer growing season, and rainfall events that were better distributed throughout the season, which generally resulted in more grazing events per year (Table 31) in 2010 vs. 2009. The clipping study by Interrante et al. (2009) showed no differences in HA among diploid genotypes when defoliation occurred every 21 d, but Pensacola and Tifton 9 outyielded UF Riata when plots were defoliated every 7 d. Based on the ea rlier research of Interrante et al. (2009a ) showing less tolerance of frequent, close grazing i t was anticipated that HA of UF Riata would be reduced with the 8cm stubble height. However, it is well established that plant response to defoliation is affe cted by an interaction of defoliation intensity and frequency (Sollenberger and Newman, 2007). In the current study, although UF Riata was grazed more intensively with the 8cm stu bble height, this treatment was grazed less frequently than the 16 cm stubbl e treatment. This is reflected in the fact that there were four grazing events each year for the 8cm treatment but seven grazing events for the 16cm treatment. Thus, it is concluded that the better than expected performance of UF Riata
39 when grazed to an 8 cm stubble is due to the longer regrowth intervals between defoliation events which allowed more time for restoration of leaf area and carbohydrate reserves (Ortega et al., 1992). Table 3 3 Bahiagrass total season herbage accumulation as affected by year x height x genotype interaction ( P = 0.0015). Data are means of three replicates Genotype Evaluation year 2009 2010 Stubble height (cm) 8 16 P value 8 16 P value ------kg ha 1 yr 1 --------------------kg ha 1 yr 1 --------------C ycle 5 3990 a 5600 a b 0.0005 5310 c 5510 b 0.6456 UF Riata 4780 a 6390 a 0.0005 6420 a 6130 ab 0.4994 F lorida H ay 4590 a 5020 b 0.3236 5510 bc 5650 b 0.7571 T ifton 9 4630 a 4120 c 0.2381 6230 ab 6770 a 0.2116 P ensacola 4620 a 3310 c 0.0039 5710 abc 5 850 b 0.7375 SE 260 Means within a column followed by the same letter do not differ ( P > 0.05). P value for height comparisons within genotype and year. Standard error of the year x height x genotype interaction means. Seasonal Seasonal patterns of HA were generally similar among genotypes. Most notable exceptions were the extended fall growth period of UF Riata for the 8 cm stubble in 2009 and for UF Riata, Cycle 5, and Florida Hay for the 16cm stubble in both 2009 and 2010. Interrante et al. (2009) showed that diploid bahiagrass genotypes outyielded tetraploid types in spring but yielded less in summer, however, they found no differences among diploid types in seasonal patterns of HA Stewart et al. ( 2005, 2007) plotted seasonal pattern of HA for bahiagrass and showed greatest HA during early summer and lesser HA in spring and fall. Data collection was initiated somewhat later in
40 the current study and the lower HA in spring reported by Stewart et al. ( 2005, 2007) was not observed. Herbage accumulated k g h a 1 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 2009 Jun Jul Aug Sep Oct 2010 Jun Jul Aug Sep Oct 16-cm 8-cm Genotype Cycle-5 Florida-Hay Pensacola Tifton-9 UF-Riata Figure 31 Herbage accumulation at each grazing event for five bahiagrass genotypes during 2009 and 2010. Error bars denote standard errors of the mea n from three replicates. Total season H erbage Harvested The general pattern of response for total season H H (Table 34 ) was similar to that for HA (Tab le 3 3 ) A s observed for HA HH of UF Riata was as great or greater than that of all other genotypes for each year x stubble height combination. The main
41 difference between HH and HA responses occurred in 2010. During that year, stubble height affected or tended ( P < 0.10) to affect the HH for Cycle 5, UF Riata, and Pensacola, with all hav ing greater levels of HH at the shorter stubble. So even though HA did not differ between stubble heights in 2010, the 8 cm stubble allowed for greater harvest efficiency by grazing and caused the greater HH for three of five genotypes. Table 3 4 Bahiagrass total season herbage harvested as affected by year x height x genotype interaction ( P = 0.0008). Data are mea ns of three replicates Genotype Evaluation year 2009 2010 Stubble height (cm) 8 16 P value 8 16 P value ------kg ha 1 yr 1 --------------------kg ha 1 yr 1 --------------C ycle 5 3500 b 4760 ab 0.0036 4620 b 377 0 b 0.0430 UF Riata 4360 a 5550 a 0.0059 5740 a 5040 ab 0.0950 F lorida Hay 4020 ab 4170 b 0.7141 4740 b 4580 b 0.6912 T ifton 9 3870 ab 3200 c 0.1070 5650 a 5460 a 0.6290 P ensacola 3890 ab 2170 d 0.0001 5000 a b 4000 b 0.0177 SE 350 Means wi thin a column followed by the same letter do not differ ( P > 0.05). P value for height comparisons within genotype and year. Standard error of the year x height x genotype interaction means. Nutritive Value Crude protein There was year x genotype interaction for herbage CP. Interaction occurred because in 2009, Cycle 5 had greatest herbage CP but in 2010 Cycle 5 CP was not different than that of any other genotype (Table 35 ) The CP of UF Riata was less than only Cycle 5 in 2009 and was as great or greater than any other genotype in 2010. In research done at the same location as the current trial, C P varied little among three diploid and two tetraploid cultivars of bahiagrass that were harvested by clipping every 7
42 or 21 d, but UF Riata CP was a s great or greater than that of the other genotypes (Interrante et al., 2009). In that study the average CP ranged from 130 to 141 g kg1 for plants harvested weekly and from 112 to 117 g kg1 for plants harvested every 3 wk. These values are greater tha n those observed in the current study for two reasons. First, the defoliation interval in the earlier work was considerably shorter, and secondly the total season N rate was 160 kg ha1 in that study vs. 120 in the current research. Stewart et al. (2007) r eported herbage CP of Pensacola bahiagrass ranging from 85 to 150 g kg1 for continuously stocked bahiagrass pastures receiving from 40 to 360 kg N ha1 yr1. Table 3 5 Bahiagrass weighted total season crude protein as affected by the year x genotype i nteraction ( P < 0 .0001). Data are means across two stubble heights and three replicates (n = 6) Genotype Evaluation year P value 2009 2010 --------------g kg 1 ----------------------C ycle 5 103 a 86 a b < 0 .0001 UF Riata 94 b 91 a 0. 4617 F lorida H ay 90 bc 84 b 0.0743 T ifton 9 84 cd 90 a b 0.0979 P ensacola 77 d 86 a b 0.014 0 SE 2.6 Means within a year followed by the same letter do not differ ( P > 0.05). P value for year comparisons within genotype. Standard error of the year x genotype interaction means. There also was genotype x stubble height interaction for herbage CP ( P = 0.0084). Similar to the data already described, interaction occurred because Cycle 5 had the greatest CP, with the exception of UF Riata in 2009, but in 2010 it was greater than only Pensacola and similar to the other genotypes (Table 36 ) Herbage CP of UF Riata was as great or greater than all other genotypes in both years.
43 Based on data from Interrante et al. (2009b ), i t was anticipated that CP would be greater for the 16than the 8cm SH treatment in the current study because of shorter intervals between defoliation events for the 16 cm height However, there were no differences between stubble heights for any genotypes except Pensacola, where the 8cm treatment had greater CP than the 16cm height. Herbage CP occurred over a rather narrow range, such that significant differences in concentration were not detect ed. Table 3 6 Bahiagrass weighted total season crude protein as affected by the stubble height x genotype interaction ( P = 0.0084) Data are means across two stubble heights and three replicates (n = 6) Genotype Stubble height (cm) P value 8 16 --------------g kg 1 ----------------------C ycle 5 97 a 92 a bc 0.1376 UF Riata 90 a bc 94 a b 0.2934 F lorida H ay 86 c 87 b c 0.7881 T ifton 9 87 b c 87 c 0.9527 Pensacola 89 b c 75 d 0.0003 SE 0.26 Means within a year followed by the same letter do not differ ( P > 0.05). P value for heig ht comparisons within genotype. Standard error of the height x genotype interaction means. In vitro digestible organic matter Herbage IVDOM was affected by a year x height x genotype interaction ( P = 0.0008). As with herbage CP concen tration, there were few differences between stubble height treatments for IVDOM and no consistent effect of the shorter interval between defoliation events associated with the 16cm height treatment (Table 37 ) Herbage IVDOM of UF Riata was greater than any other genotype for both stubble height treatments in 2009. Cycle 5 was next high est in IVDOM for both height treatments and
44 was 5.2% less than UF Riata for the 8cm treatment and 4.5% less for the 16cm height. In 2010, UF Riata was greater in IVDOM than all but Tifton 9 and Pensacola for the 8cm height and all but Pensacola for the 16cm height. UF Riata IVDOM averaged 546 and 562 g kg1 in 2 yr of defoliation by clipping and was greater than Pensacola in both years and greater than Tifton 9 in 1 of 2 yr T hus t he IVD OM concentrations in the current study occur within the same range as those reported for UF Riata by Interrante et al. (2009b ). Table 3 7 Bahiagrass total season weighted digestibility (IV D OM ) as affected by year x height x genotype interaction ( P = 0 .0008). Data are means from three replicates Genotype Evaluation year 2009 2010 Stubble height (cm) 8 16 P value 8 16 P value ----------g kg 1 -------------------------g kg 1 ---------------C ycle 5 508 b 515 b 0.5085 519 b 527 b 0.4754 UF Riata 536 a 539 a 0.8106 564 a 571 a 0.485 0 F lorida H ay 506 b 512 b 0.5718 540 b 540 b 0.9504 T ifton 9 508 b 513 b 0.6454 577 a 504 c < 0 .0001 P ensacola 471 c 494 b 0.0396 569 a 568 a 0.933 0 SE 7.56 Means wit hin a column followed by the same letter do not differ ( p > 0.05). P value for height comparisons within genotype. Standard error of the year height genotype interaction means. Con c l usions Five diploid genotypes of bahiagrass were compared under grazi ng to evaluate their herbage accumulation and nutritive value. Of particular interest was the response of the less photoperiod sensitive, cold tolerant genotypes that have been referred to as PCA types. T wo defoliation treatments were tested. Grazing was initiated when canopy height reached 30 cm and ended when stubble height was either 8 or 16 cm.
45 Most productivity and nutritive value responses were affected by twoor three way interactions. Herbage accumulation in Year 1 was generally greater for the 16vs. the 8 cm treatment for PCA types Cycle 5 and UF Riata This response was anticipated based on previous report s of defoliation sensitivity of the PCA bahiagrass es. However, in Year 2 there was no effect of stubble height for any of the genotypes, as P CA and nonPCA types did equally well with short as with taller stubble. UF Riata had as great or greater herbage accumulation than any genotype at all stubble height x year combinations. It seems likely that the lack of stubble height effect in Year 2 wa s due to acclimation to the grazing treatments and also to the length of the regrowth interval associated with each height treatment. Grass grazed to 8 cm required considerably longer to regrow to 30 cm (an average of 37 d between defoliation events) compared to those plants grazed to 16 cm (average of 25 d between defoliation events) Thus, plants grazed to shorter stubble had longer recovery periods than those grazed to tall stubble, and it is likely that neither treatment imposed high levels of stress on the plants. As a result it is difficult to make conclusive statements about the grazing tolerance of the PCA genotypes however, based on the work done by Interrante et al. (2009a ) it seems likely that PCA types can tolerate moderate levels of defoliation stress, by grazing or clipping, but because of their more upright growth habit they are less likely to perform well under conditions of frequent, close defoliation. The PCA types had crude protein and in vitro digestible organic matter concentrations that were as great or greater than other diploid bahiagrasses. Thus, it appears that along with superior cool season production (Blount et al., in review), they
46 may increase nutritive value, especially digestibility, over current cultivars. This conclusion is also supported by Interrante et al. (2009b ). With availability of seed of PCA types no longer limiting, it appears that establishment of larger pastures and imposition of a broader range of grazing treatments is warranted to more clearly define the limits of grazing management that will be successful with these plants.
47 C HAPTER 4 TILLERING DYNAMICS O F BAHIAGRASS GERMPLASM UNDER GRAZING INTENSITIES Background Bahiagrass ( Paspalum notatum Flgg ) is the predominant pasture grass utilized by the beef cattle industry in southern Alabama, southern Georgia, and throughout Florida ( Blount, 2003 ). Its tolerance of heavy grazing and productivity in low fertility soils of va rying drainage characteristics makes it the choice for most beef cattle producers Sinclair et al. (2001, 2003) showed that bahiagrass responds to photoperiod, resulting in minimal growth during the cool season. As a consequence, breeding programs were initiated with the goal of developing less photoperiod sensitive bahiagrass cultivars (Blount et al., 200 3 ). Plants developed in this breeding program were selected visually for phenotype and inter mated afterward in the greenhouse, a breeding approach that is considered to be more than three times as efficient as traditional recurrent mass selectio n (Burton, 1992). Because the product of this breeding effort is less photoperiod sensitive, and yields better in the cool season, they are referred as PCA (pho toperiod insensitive, cold adapted) types Early evaluation of PCA genotypes was made by Interr ante (2009 a ), who imposed clipping treatments on experimental lines and released cultivars of bahiagrass and found that plant responses related to sward persistence (stem N content and stem base and rhizome mass) were lower for less photoperiodsensitive v s. standard entries. Such differences were most pronounced under close or frequent defoliation. An important question is whether genotypes that are more productive later into autumn and earlier in spring produce this cool season grow th at the expense of re serves that are stored for
48 winter survival and spring regrowth (Hirata et al., 2002; Interrante, 2008) If so, sward persistence or vigor of spring regrowth may be imperiled. Although reserve status may play an important role, bahiagrass persistence is al so a consequence of high tiller densities (Hirata, 1993, 2001; Hirata et al., 2002) Thus, the detailed study of the dynamics of these tiller characteristics should provide information on the mechanisms of sward persistence. One established theory that rel ates these variables is the sizedensity compensation (SDC) theory. The SDC theory, or self thinning rule, states that the mass (size) and abundance (density) of live entities in an environment are not related in a linear but in an exponential fashion (K ays and Harper, 1974) Numerous studies in the past 25 yr have shown that this phenomenon is also present in grazed swards (Sackville Hamilton et al., 1995; Yu et al., 2008) Hirata and Pakiding ( 2002) for example, found minimal variation in tiller popula tion density throughout the growing season, but were still able to prove the rule to be true for grazed bahiagrass pastures. Because there are substantial data describing the tillering pattern of persistent cultivars of this species (Hirata, 2001) major deviations from the theoretical expectations may indicate genotypes prone to stand deterioration and lack of grazing tolerance. Tiller dynamics of a PCA type, UF Riata, was studied by Interrante et al. ( 2010) but there are no studies evaluating tiller responses of less photoperiod sensitive, coldadapted bahiagrass under grazing, the most common use of bahiagrass Therefore, the objectives of this study were i) to determine the e ffects of grazing intensity (defined in this study by stubble height) on tiller population density, tiller mass, leaf length, and leaf -
49 stem ratio of three bahiagrass cultivars, and (b) to relate such results with sward persistence of these pastures. Material and Methods This experiment was conducted on the same experimental units described for Chapter 3. The exception was that only three bahiagrass genotypes were evaluated for tiller responses. All three are released cultivars and will be referred to as such in this chapter. The three chosen were industry standards Pensacola and Tift on 9 and the PCA type UF Riata. Tiller P opulation Density Tiller population density was determined three times (June, August, October) by counting tillers inside 10by 20cm quadrat s. Th ese times were chosen to show the pattern of tiller density in the early, mid and late growing seas on. More frequent measures are often not useful because no differences in tiller population are expected to be observed in periods shorter than 60 to 90 d (Hirata, 2001a). Ten randomly chosen quadrats (sampling units) were counted i n each experimental unit at each date. Because grazing intervals were different for each treatment and tiller populations were assessed at the same date for all treatments, simple interpolation was used to determine tiller populations at a given date. Tiller M ass Tiller mass was determined before each grazing event. Ten tillers per e xperimental unit were randomly chosen and cut so that rhizomes and white portions of the sheath were not considered part of the tiller entity. Tillers were placed in p lastic bags containing ice, transported to the laboratory, and separated in to leaf and stem fractions by cutting at the ligule. Components were dried separately at 60C until
50 constant weight. To facilitate presentation of the data and relating of tiller mass and density, some sampling dates were combined for treatments that were grazed more than three times per year This allowed presentation of the tiller mass and density data for the same three seasons. Leaf N umber and L ength Leaf number was determined by counting attached leaves, grazed or not, comprised of more than 50% live tissue o n each tiller. Leaf length was measured from the ligule to the tip of leaves with a millimeter scale and dimensions were recorded in centimeters with one decimal place. Both measurements were made before tissues were dried. Statistical A nalysis Statistical analys e s w ere performed using the MIXED procedure of SAS (Saxton, 1998) where means separation was based on Fishers LSD, generated by the PDIFF option of the LSMEANS statement. Assignment of letters for means separation was aided by the PDMIX800 macro (Saxton, 1998) The framework suggested by Crawley ( 2007) was followed, where the significance level of 5% was adopted. Year, cultivar height, and their interactions were considered fixed effects, and block random. Year was considered fixed because treatments were imposed on the same experimental unit in both years, and there was potential for carryover effects from Year 1 to Year 2. A summary of the analysis of variance is presented in Table C 1
51 Results and Discussion Tiller Density and Mass Across seasons T iller density and mass of three bahiagrass cultivars were measured dur ing three seasons per year. I nitially data will be presented averaged across seasons to descri be overall responses to genotype, stubble height, and year. A cross seasons, there was cultivar x stubble height interaction. T iller density was not affected by cultivar when post graze stubble height was 8 cm (Table 41), but for the 16cm stubble height P ensacola had 14% more tillers than UF Riata and 23% more than Tifton 9. More upright growing cultivars had more tillers (Tifton 9; P = 0. 0 476) or tended to have more tillers (UF Riata; P = 0.1034) when grazed to an 8than 16cm stubble, while more decumbent Pensacola tended to have more tillers when grazed to 16 than 8 cm ( P = 0.0979). Table 41. Bahiagrass tiller density (tillers per 200 cm2) as affected by the stubble height X cultivar interaction ( P = 0.0094) Data are means from three replicates and tw o years. Cultivar Stubble height (cm) P value 8 16 ------tiller s quadrat 1 --------------UF Riata 17.7 a 16.3 b 0.1034 Tifton 9 16.3 a 14.5 c 0.0476 Pensacola 17.2 a 18.9 a 0.0979 SE 1.0 Means within a colum n followed by the same letter do not differ ( P > 0.05). P value for height comparisons within cultivar Standard error of the year height cultivar interaction means. Bahiagrass tiller mass across seasons was affected only by cultivar with the upright Tifton 9 having greater tiller mass than Pensacola and UF Riata tiller mass
52 being intermediate (Table 42). Tiller mass was greater in 2010 than in 2009 (0.458 vs. 0.408 g tiller1) across cultivar s and stubble heights. Table 42. Bahiagrass tiller mass (size) as affected by cultivar Data are means from three replicates two stubble heights and two years Cultivar ----g tiller 1 ----UF Riata 0.42 5 ab Tifton 9 0.459 a Pensacola 0.41 6 b SE 0.025 Means w ithin a column followed by the same letter do not differ ( P > 0.05). Standard error of the means. Seasonal patterns of tiller response T iller density generally decreased as the grazing season progressed in both 2009 and 2010, with the exception of UF Riata pastures grazed to 8 cm in 2009 (Fig. 4 1). Based on previous research with bahiagrass this pattern of response is not considered to be indicative of poor persistence. When several bahiagrass genotypes were defoliated by clipping, net tiller appearanc e rate decreased throughout the growing season each year, with rates as low as 7.4 tillers m2 d1 in August 2005 and 2.1 tillers m2 d1 in October 2006 (Interrante et al., 2010). Hirata and Pakiding (2001) and Pakiding and Hirata (2003) reported that net tiller appearance rate in Pensacola bahiagrass was generally positive in May or June, and was usually close to zero or negative in other seasons. These seasonal patterns in net tiller appearance likely explain the general overall decrease in tiller dens ity as the season progressed in the current study.
53 T iller density varied relatively little among cultivars in 2009 (Fig. 4 1) however, i n 2010 cultivar effects were much more pronounced. In 2010, Pensacola generally had a greater tiller density than the other cultivar s (Fig. 4 1) This result is similar to that reported by Interrante et al. (2010) who found that upright growing bahiagrass genotypes generally had the fewest tillers for four defoliation treatments, while the more decumbent genotypes like Pensacola had the most tillers for each defoliation treatment. In addition, they observed that Pensacola tiller density did not respond to clipping frequency or intensity. In contrast, tiller number of both Tifton 9 and UF Riata varied widely across defoliati on treatments and was greatest when those cultivars were defoliated frequently and closely (Interrante et al., 2010) In 2010 of the current study, Tifton 9 and UF Riata generally had fewer tillers than Pensacola, and the effect of stubble height on t iller density was more pronounced for Tifton 9 and UF Riata than Pensacola. The pattern of response to stubble height described earlier in the across seasons comparisons (i.e., greater tiller density at 8cm stubble for UF Riata and Tifton 9 and greater density at 16cm stubble for Pensacola) started to emerge in 2009 By 2010, Pensacola had more tillers than the other cultivars by seasons end for the 8cm stubble height and at all dates for the 16cm stubble. Tiller density was similar at all 2010 dates for UF Riata and Tifton 9 when grazed to an 8 cm stubble, but when grazed to a 16cm stubble it was greater for Tifton 9 than UF Riata in June and August The advantage in tiller density of the 16vs. 8 cm stubble height was evident for Pensacola throughout 201 0, while the advantage of 8vs. 16 cm stubble was pronounced only in June and August for Tifton 9 and UF Riata.
54 Previous r esearch showed that Pensacola bahiagrass tiller number increase d as cutting height decreased from 22 to 2 cm (Pakiding and Hirata, 2 002) They also found that Pensacola responded to intense defoliation with increased tiller longevity, tiller appearance rate, and tiller density compared to 12and 22 cm stubble heights (Hirata and Pakiding, 2004; Hirata, 2004 ) In another study, t iller number of highly persistent Pensacola was unaffected by four defoliation treatment s and g enerally greatest among genotypes tested (Interrante et al., 2010) In the current study, Pensacola tiller density increase d with taller stubble instead of with short er stubble height. This response was unexpected, but it can be explained based on conditions of the experiment. This may be due in part to intervals between grazing events not being consistent across stubble height treatments i.e., pastures grazed to shor t stubble (8 cm) required a longer regrowth period to reach a 30cm height than tall stubble (16 cm) pastures. Thus, short stubble treatments had greater recovery time, and the anticipated greater level of defoliation stress that would lead to more and sm aller tillers for the 8 cm stubble treatment did not materialize. Additionally, the longer regrowth periods increased the time period when the base of the dense Pensacola canopy was shaded, and tiller density is reduced in low light environments (White 1 9 81) Tifton 9 and UF Riata likely responded differently because their growth habit is more bunchlike, and the degree of shading lower in the canopy is likely much less relative to Pensacola.
55 Figure 41 Bahiagrass populat ion density as affected by year and genotype. Error bars denote standard errors. T iller mass decreased for the 8 cm stubble height throughout the entire 2009 grazing season (Fig. 4 2) For the 16 cm height tiller mass increased (UF Riata and Tifton 9) or remained relatively constant (Pensacola) from August to October after decreasing from June to August. Tiller mass was not different between stubble heights during June and August 2009 (Fig. 42), but for all genotypes there was greater tiller mass for the 16 than the 8cm stubble treatment in October. In 2010, there was less seasonal change in tiller mass and patterns of response were generally similar for the two stubble heights. For example, UF Riata tiller mass decreased from 0.52 to 0.19 g
56 from June to October 2009, but in 2010 the decrease was from 0.49 to 0.28 g. For Pensacola, the changes were 0.52 to 0.30 g in 2009 vs. 0.43 to 0.31 in 2010. These data suggest that greatest impact of close grazing on tiller mass occurred in Year 1 and that by Year 2 the plants had adapted to this grazing treatment. Interestingly, although tiller mass for the 8cm stubble treatments did not decrease as much as the season progressed in Year 2 as in Year 1, the changes in tiller density were more pronounced for UF Riata and Tifton 9 in 2010 than 2009 (Fig. 41), suggesting some mass/density compensation. For the 16cm stubble in 2010, neither Pensacola nor UF Riata tiller mass varied widely throughout the year, but mass of Tifton 9 tillers increased from June to October. Tiller mass was not different among cultivar s in June and August 2009, but by October Pensacola had the smallest tillers for the 16cm stubble. In 2010, June tiller mass for all cultivar s approximated that observed in 2009, and again there were no cultivar differences in June and August (Fig. 4 2). At the end of the grazing season each year for each stubble height, Pensacola tillers had the lowest or equal to the lowest mass and Tifton 9 tillers were as heavy or heavier than the other genotypes. Mass of U F Riata tillers in October was generally not different than Pensacola, with the exception of the 16cm stubble height in 2009 (Fig. 4 2). Interrante et al. ( 2010) evaluated tiller responses of several bahiagrass entries to clipping defoliation every 7 or 2 1 d to a stubble height of 4 or 8 cm. They found that Tifton 9 and UF Riata decreased in tiller number across treatments in the first year of defoliation, likely indicating less rapid morphological adaptation to defoliation by these upright growing types t han the
57 more decumbent Pensacola. Less photoperiodsensitive UF Riata generally had the least or not different from the least tiller mass as well in that study Figure 42 Bahiagr ass genotype tiller mass (g DM tiller1) as affec ted by year and genotype. Error bars denote standard errors
58 Leaf:stem Ratio and Leaf Characteristics There was cultivar x stubble height interaction for tiller leaf:stem ratio (Table 43). Leaf:stem r atio was not affected by cultivar for the 8cm stubble, but when grazed to a 16cm stubble it was 25% gr eater for Pensacola than Tifton 9 and 15% greater for Pensacola than UF Riata (Table 43). Leaf:stem ratio was greater for the 8than 16 cm stubble for Tifton 9 and tended to be greater for UF Riata, but the reverse was true for the more decumbent Pensacola. Table 43. Bahiagrass leaf stem ratio as affected by cultivar x stubble height interaction. Data are means across three replicates. Cultivar Grazing height (cm) 8 16 P value ------------------g g 1 --------------------------UF Riata 1.84 a 1.46 b 0.0184 Tifton 9 1.78 a 1.07 c <.0001 Pensacola 1.99 a 1.82 a 0.404 SE 0.12 Means within a column followed by the same l etter do not differ ( P > 0.05). P value for height comparisons within cultivar Standard error of the year height cultivar interaction means. Leaf characteristics were generally unaffected by treatments with the exception of leaf width (P = 0.0061). Le af width was marginally greater for Tifton 9 (0.36 cm) than for UF Riata (0.34 cm) and Pensacola (0.33 cm). There was no effect of stubble height ( P = 0.5526) or cultivar ( P = 0.3353) on the number of leaves per tiller. Number of leaves ranged from 3.0 to 6.2 and averaged 4.8. There also was no effect of stubble height ( P = 0.8162) or cultivar ( P = 0.6390) on leaf length. Leaf length ranged from 18 to 44 cm and averaged 32 cm Neither was t here an effect of stubble height ( P = 0.4466) or cultivar ( P = 0.344 4) on leaf area per tiller Overall values of leaf area per tiller (cm2 tiller1) wer e in the range 23 to 97 and averaged 53.
59 Size density C ompensation Figure 43 shows the relationship between tiller mass (size) and tiller population density of the culti vars studied. Each data point in the plot corresponds to one observation from an experimental unit, where the value of tiller mass was determined immediately before a grazing event For these cultivars grazed in the manner of this experiment, the relations hip does not correspond to theoretical expectations of SDC. This is in contrast to results of Hirata and Pakiding (2002) who found the relationship to be present in bahiagrass swards stocked continuously throughout the year. Neither do t hese results suppor t the theory suggested by Sackville Hamilton et al. (1995) since none of the slopes is different from zero. Separating the data by year, or seasons (Sbrissia, 2004) does not change the interpretation of these data. The se results may not provide conclusive evidence of whether SDC occurs for the bahiagrass genotypes evaluated. The SDC is best observed across a wide range of grazing management treatments, and in this experiment the range of defoliation stress imposed was relatively narrow due to the structure of the treatments. As indicated in Chapter 3, g rass grazed to 8 cm required an average of 37 d between defoliation events while that grazed to 16 cm required an average of 25 d between defoliation events Longer recovery time ameliorates the effects of s hort stubble heights, t hus, neither treatment imposed high levels of stress on the plants Levels of stress imposed were likely quite similar, as reflected in a lack of differences in 2010 herbage accumulation between stubble heights within a cultivar
60 Figure 43 R elationship between tiller mass (size) and tiller population density of b ahiagrass cultivar s. The s haded area comprises standard error s of predictions. The dashed line (slope = 1.5; intercept = 2 ), was plotted to rep resent what a curve with the expected inclination would be in the neighborhood of the collected data Each data point corresponds to one observation from one experimental unit. Conclusions Tiller density, mass, and leaf characteristics of three diploid cultivars (Pensacola, Tifton 9, and UF Riata) of bahiagrass were compared to make inference regarding plant persistence under grazing. T wo defoliation treatments were imposed during 2009 and
61 2010, with g razing events initiated when canopy height reached 30 cm and terminated when stubble height was either 8 or 16 cm. T iller density generally decreased as the grazing season progressed in both 2009 and 2010, but this response was primarily due to normal seasonal patterns of bahiagrass growth and less a respons e to treatments Tiller density varied relatively li ttle among cultivar s in 2009, however, i n 2010 cultivar effects were much more pronounced. By the end of the 2010 grazing season, Pensacola had more tillers than the other cultivars for both stubble heigh t treatments Tiller density was similar at all 2010 dates for UF Riata and Tifton 9 when grazed to an 8 cm stubble, but when grazed to a 16cm stubble it was greater for Tifton 9 than UF Riata in June and August T iller density was greater for the 16 vs. 8 cm stubble height for Pensacola throughout 2010, while for Tifton 9 and UF Riata there was an advantage of the 8 vs. 16 cm stubble at two of three dates Tiller mass was not different between stubble heights during June and August of the 2 yr but Tif ton 9 had the heaviest tillers in October 2010 for both stubble height treatments In 2010, there was less seasonal change in tiller mass and patterns of response were generally similar for the two stubble heights. G reatest impact of close grazing on tille r mass occurred in Year 1, and it appears that by Year 2 the plants had ada pted to the grazing treatment s. The tiller SDC was not observed for any of the bahiagrass cultivars in the current study. The relatively small range in grazing stress imposed by the treatments in the study may have been responsible for the lack of measureable SDC.
62 In conclusion, changes in tiller density and mass generally were more pronounced in the first than the second year of grazing, suggesting that all three bahiagrass cultivar s have the ability to adapt to new defoliation regimes, at least within the range of stress imposed by these specific grazing treatments. For these low to moderatestress grazing treatments, patterns in tiller density and mass varied among cultivars but d id not suggest major concerns for persistence of any of the cultivars evaluated. This is also supported by the absence of grazing treatment effects on herbage accumulation in the second year of the study (2010; Chapter 3). With seed supply of UF Riata no l onger limiting, e valuation of this new cultivar under grazing should continue, and a much wider range of grazing intensities should be imposed to better assess its persistence under defoliation by livestock
63 CHAPTER 5 SUMMARY AND CONCLUSI ONS Grown on more than one million ha, b ahiagrass ( Paspalum notatum Flgge) is the main pasture for horses ( Equus caballis ) and cattle ( Bos sp.) in Florida ( Chambliss and Adjei, 2006). One of the limitations to its use is its strong summer dominant herbage production pattern. Bahiagrass yield is concentrated between April and September and its growth during the cool season is minimal (Mislevy and Everett, 1981) The reasons for minimal herbage accumulation during the cool season were established by Sinclair et al. (199 7, 2001, 2003), and include the negative response of bahiagrass aboveground herbage productivity to shorter daylength. Such findings led to initiation of breeding programs with the goal of developing less photoperiod sensitive bahiagrass cultivars that wo uld produce more forage for livestock during the cool season (Blount et al., in review) Early evaluation of less photoperiodsensitive bahiagrass genotypes raised concerns regarding their long term persistence, since Interrante et al. (2009a) found that plant responses related to sward persistence (stem N content and stem base and rhizome mass) were lower for less photoperiodsensitive vs. standard entries Given that Interrantes study did not include animals and the response of new entries under grazing is of great interest (Bouton, 1997) the objectives of this study were: i) measur e herbage accumulation and nutritive value responses of new bahiagrass genotypes to grazing management strategies; and ii) describe the relationships among tiller density, ti ller mass, and ti ller size density compensation with bahiagrass persistence and herbage accumulation.
64 Experiments were conducted in 2009 and 2010 in Gainesville, Florida. Two photoperiodsensitive diploid genotypes (Pensacola and Tifton 9) and three coldadapted, less photoperiod sensitiv e genotypes (UF Riata, Cycle 5, and Flo rida Hay) were evaluated under mobgrazing at two grazing intensities (post graze stubble heights of 8 and 16 cm, with grazing initiated for all treatments at a 30cm height) Measure ments of interest were herbage accumulation, forage nutritive value, and tiller dynamics. Herbage accumulation in Year 1 was generally greater for the 16vs. the 8 cm treatment for PCA types Cycle 5 and UF Riata However, in Year 2 there was no effect of stubble height for any of the genotypes The new bahiagrass cultivar, UF Riata had as great or greater herbage accumulation than any genotype at all stubble height x year combinations L ack of stubble height effect in Year 2 was likely due to acclimation to the grazing treatments and clearly demonstrated no negative carryover effects from Year 1. In addition, neither grazing treatment imposed high levels of defoliation stress because plants grazed to the 8cm stubble had average regrowth periods of 37 d t o reach a 30cm height, and plants grazed to 16 cm had average regrowth periods of 25 d. As a result it is difficult to make conclusive statements about the grazing tolerance of the PCA genotypes. H owever, based on the work done by Interrante et al. (2009 a ) it seems likely that PCA types can tolerate moderate levels of defoliation stress, by grazing or clipping, but because of their more upright growth habit they are less likely than existing cultivars like Pensacola and Argentine to perform well under con ditions of frequent, close defoliation.
65 UF Riata had crude protein and in vitro digestible organic matter concentrations that were generally as great or greater than other diploid bahiagrasses. Thus, it appears that along with superior cool season product ion (Blount et al., in review), it may increase nutritive value, especially digestibility, over current cultivars. This conclusion is also supported by Interrante et al. (2009b ). Tiller density varied relatively little among three diploid cultivars (Pensac ola, Tifton 9, and UF Riata) in 2009, however, by the end of the 2010 grazing season, Pensacola had more tillers than the other genotypes for both stubble height treatments Tiller mass was not different between stubble heights during June and August of th e 2 yr but Tifton 9 had the heaviest tillers by October 2010 for both stubble height treatments. G reatest impact of close grazing on tiller mass occurred in Year 1 and it appears that by Year 2 the plants had adapted to the grazing treatments. The tiller size/density compensation phenomenon was not observed for any of the bahiagrass cultivars in the current study. The relatively small range in grazing stress imposed by the treatments in the study may have been responsible for the lack of measureable SDC. Overall, changes in tiller density and mass generally were more pronounced in the first than the second year of grazing, suggesting that all three bahiagrass cultivars have the ability to adapt to new defoliation regimes, at least within the range of stres s imposed by these specific grazing treatments. For these grazing treatments, tiller responses did not give reason for major concerns for persistence of any cultivar evaluated. At the time of establishment of this study, limited seed availability of photoperiodinsensitive, coldadapted (PCA) types reduced plot area and number of treatments that
66 could be imposed. With the subsequent release of UF Riata as a cultivar, availability of seed of this PCA type is no longer limiting, and establishment of larger pastures and imposition of a broader range of grazing treatments is now possible and is warranted to more clearly define the range of grazing management strategies that will be successful with PCA plants
67 APPENDIX A DOUBLE SAMPLING EQUATIONS TABLE Table A 1 Coefficients of the herbage mass double sampling equations with their respective coefficient of determination and confidence intervals for the slope Genotype 0 1 r 2 PEN 8.1 4.4 0.3 *** 0.73 (3. 8 ; 4 .9) T9 7.6 4.0 0.2 *** 0.72 (3.6; 4.5) FH 7.3 3.7 0.2 *** 0.67 (3.3; 4.1) C 4 6.9 3.9 0.2 *** 0.72 (3.5; 4.3) C5 6.7 3.5 0.2 *** 0.75 (3. 2; 3 9 ) Confidence intervals for the sl opes are based on a 5% significance level Signif. codes: 0 *** 0.001 ** 0.01 * 0.05 . 0.1 1
68 APPENDIX B ENTRANCE HEIGHT TABL E Table B 1 Entrance height applied by treatment and year Genotype Stubble height (cm) Entrance height (cm) 2009 2010 C5 8 33.2 (3. 8 ) 31.4 (2.3 ) 16 32.5 (5.1) 31.4 (2.5 ) C4 8 28.3 (1.7 ) 31.2 (1.8 ) 16 30.4 (2.5 ) 32.2 (2.5 ) FH 8 34.7 (4.8 ) 30.9 (1.8 ) 16 31.8 (6.4 ) 31.8 (2.4 ) T9 8 31.2 (1.6 ) 31.6 (1.0 ) 16 30.0 (1.8 ) 31.1 (2.8 ) PEN 8 31.4 (1.2 ) 29.7 (1.9 ) 16 30.6 (1.9) 30.7 (2.4) Numbers in parenthesis denote the standard deviation of the mean entrance height
69 APPENDIX C SOURCES OF VARIATION Table C 1 Sources of variation for bahiagrass variables. Source of variation Response variable Entry SSH Year Entry X SSH Entry X Year SSH X Year Entry X SSH X Year HA P = 0.0002 P = 0.0661 P < 0.0001 P = 0.0123 P = 0.0013 P = 0.0201 P = 0.0015 HD P < 0.0001 P = 0.0416 P < 0.0001 P = 0.0201 P < 0.0001 --P = 0.0019 CP P = 0.0001 P = 0.0669 --P = 0.0084 P < 0.0001 ----IVOMD P < 0.0001 P < 0.0001 P < 0.0001 P = 0.0007 --P = 0.0045 P = 0.0020 Tiller density P = 0.0002 P = 0.3472 --P = 0.0094 ------Tiller mass P = 0.0911 --P = 0.0033 -------Leaf:stem ratio P = 0.0016 P =0.1417 P < 0.0001 P = 0.0894 ------Leaf length P = 0.0061 P = 0.3664 --P = 0.2659 ------Leaves per tiller P = 0.3353 P = 0.5526 --P = 0.5813 ------Leaf area per tiller P = 0.3444 P = 0.4466 --P = 0. 0497 ------
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77 BIOGRAPHICAL SKETCH Daniel Reis Pereira was born on 29 May 1985 in Braslia, Distrito Federal, Brazil. He moved several times throughout the country, but most of h is summer v acations were spent on his family dairy, in the Paraba valley region, where he developed his interest for agriculture. In 2008 he graduated from the University of So Paulo (Brazil) with a B.S. in A gronomy and in the summer of 2011 he completed his M.S. in Agronomy at the University of Florida.