1 BIOLOGY OF SMALL SMUTGRASS ( Sporobolus indicus var. indicus ) AND GIANT SMUTGRASS ( Sporobolus indicus var. pyramidalis ) AND L ONG TERM MANAGEMENT OF GIANT SMUTGRASS By NEHA RANA A DISSERTATION PRESENTED TO THE GRADUATE SCHOO L OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2012
2 2012 Neha Rana
3 To my husband Abhishant for his love, support, and encouragement
4 ACKNOWLEDGMENTS I w ould like to express my deepest gratitude to my major advisor, Dr. Brent Sellers, for accepting me as a graduate student, giving me invaluable advice, support and encouragement, and providing me a great working atmosphere throughout my graduate degree prog ram. Without his mentorship, I would not be the person I am today. I thank my committee members: Dr. Jason Ferrell, Dr. Greg MacDonald, Dr. Maria Silveira, and Dr. Joao Vend ramini for their suggestions and advice throughout my research program I would lik e to thank Dr Arnold Saxton and Dr. Salvador Gezan for their help in statistical analysis of my field experiments. In addition, I acknowledge the advice and suggestions of Dr. Hector Perez I extend my special thanks to Joseph Noel, Sushila Chaudhari Da niel Abe, Jose L uiz Cindy Holley Carlos Taniguchi and Adriana Guirado for their friendship and help in my research. I would also like to thank my friends at the weed shop, Anna Gr ies, Mike Durham, Sarah Berger, Courtney Stokes, and Sergio Morichetti for making me laugh and giving me company when I was alone. I am deeply thankful to my parents, Harish and Neelam Rana, for their blessing, love and support. Lastly, I would like to thank my husband, Abhishant, for his unconditional love, support and encoura gement, if it was not for him I would have never been able to finish my degree.
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 ABSTRACT ................................ ................................ ................................ ................... 10 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 12 2 LITERATURE REVIEW ................................ ................................ .......................... 17 Geographical Range ................................ ................................ ............................... 18 Smutgrass Problem Statement ................................ ................................ ............... 18 Taxonomy and Morphology ................................ ................................ .................... 19 Seed Biology ................................ ................................ ................................ ........... 20 Seed Dispersal ................................ ................................ ................................ ....... 21 Smutgrass in Forages ................................ ................................ ............................. 21 Management of Smutgrass ................................ ................................ ..................... 22 Cultural Control ................................ ................................ ................................ 23 Chemical Control ................................ ................................ .............................. 24 Hexazinone Usage and Mode of Action ................................ ................................ .. 25 Summary and Objectives ................................ ................................ ........................ 27 3 EFFECTS OF ENVIRONMENTAL FACTORS ON SEED GERMINATION AND EMERGENCE OF SMALL SMUTGRASS ( Sporobolus indicus var. indicus ) AND GIANT SMUTGRASS ( Sporobolus indicus var. pyramidalis ) ................................ .. 28 Materials and Methods ................................ ................................ ............................ 30 Results and Discussion ................................ ................................ ........................... 34 4 IMPAC T OF SOIL p H ON BAHIAGRASS COMPETITION WITH SMALL SMUTGRASS ( Sporobolus indicus var. indicus ) AND GIANT SMUTGRASS ( Sporobolus indicus var. pyramidalis ) ................................ ................................ ..... 46 Materials and Methods ................................ ................................ ............................ 48 Results and Discussion ................................ ................................ ........................... 51 Shoot biomass of Bahiagrass:Giant Smutgrass in the competitive mixtures. ... 51 Shoot biomass of Bahiagrass:Small Smutgrass in the competitive mixtures ... 53
6 5 INTEGRATED MANAGEMENT TECHNIQUES FOR LONG TERM CONTROL OF GIANT SMUTGRASS ( Sporobolus in dicus var. pyramidalis ) IN BAHIAGRASS PASTURES ................................ ................................ .................... 70 Materials and Methods ................................ ................................ ............................ 73 Experiment 1 ................................ ................................ ................................ .... 73 Experiment 2 ................................ ................................ ................................ .... 74 Experiment 3 ................................ ................................ ................................ .... 74 Results and Discussion ................................ ................................ ........................... 76 Experiment 1 ................................ ................................ ................................ .... 76 Experiment 2 ................................ ................................ ................................ .... 79 Experiment 3 ................................ ................................ ................................ .... 80 6 CONCLUSION ................................ ................................ ................................ ........ 86 LIST OF REFERENCES ................................ ................................ ............................... 89 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 97
7 LIST OF TABLES Table page 4 1 Effect of pH on bahiagrass and giant smutgrass ( Sporobolus indicus var. pyramidalis ) shoot biomass when grown in monoculture at low (4 plants pot 1 ) and high (8 plants pot 1 ) densi ty levels. ................................ .......................... 57 4 2 Effect of pH on aggressivity index values as determined from bahiagrass and giant smutgrass ( Sporobolus indicus var. pyramidalis ) relative shoot weight data at low (4 plants pot 1 ) and high (8 plants pot 1 ) density levels ................... 58 4 3 Effect of pH on bahiagrass and small smutgrass ( Sporobolus indicus var. indicus ) shoot biomass when grown in monoculture at low (4 plant s pot 1 ) and high (8 plants pot 1 ) density levels. ................................ .............................. 59 4 4 Effect of pH on aggressivity index values as determined from bahiagrass and small smutgrass ( Sporobolus indicus var. indicus ) relative shoot weight data at low (4 plants pot 1 ) and high (8 plants pot 1 ) density levels of mixtures. ........ 60 5 1 Effect of hexazinone herbicide, renovation, and fall roller chopping on smutgrass densit y from 2009 to 2011. ................................ ................................ 83 5 2 Summary of rainfall by months at Ona (south Florida) during the duration of all experiments. Rainfall data was collected by the Florida Automated Weather Network. ................................ ................................ ............................... 84 5 3 Effect of hexazinone and nitrogen application on smutgrass density from 2009 to 2011. ................................ ................................ ................................ ...... 84 5 4 Effect of sequential applicat ion of hexazinone on smutgrass density 24 MAT.. 85
8 LIST OF FIGURES Figure page 3 1 Effect of light on germination of small smutgrass ( Sporobolus in dicus var. indicus ) and giant smutgrass ( Sporobolus indicus var. pyramidalis ) incubated at 30/20 C in continuous darkness or a 16h light/8h dark period.. ..................... 42 3 2 Effect of constant temperature on small smutgrass ( Sporobolus indicus var. indicus ) and giant smutgrass ( Sporobolus indicus var. pyramidalis ) seed germination. ................................ ................................ ................................ ........ 43 3 3 Effect of diurnal temperature flux ( 27/15, 33/24, 29/19, and 22/11 C) on germination of small smutgrass ( Sporobolus indicus var. indicus ) and giant smutgrass ( Sporobolus indicus var. pyramidalis ) in a 12 h photoperiod.. ........... 44 3 4 Seed germination of s mutgrass varieties at different levels of pH incubated at 30/20 C alternating day/night temperature in 16h light/8 h dark.. ....................... 45 4 1 Influence of hydrated lime on the pH of a Smyrna Sand.. ................................ .. 61 4 2 Relative shoot weight of bahiagrass and giant smutgrass ( Sporobolus indicus var. pyramidalis ) at pH 4.5 in competitive mixtures for (A) the low planting density (4 plants pot 1 ), and (B) the high planting density (8 plants pot 1 ). ......... 62 4 3 Relative shoot weight of bahiagrass and giant smutgrass ( Sporobolus indicus var. pyramidalis ) at pH 5.5 in competitive mixtures for (A) the low planting density (4 plants pot 1 ), and (B) the high planting density (8 plants pot 1 ). ......... 63 4 4 Relative shoot weight of bahiagrass and giant smutgrass ( Sporobolus indicus var. pyramidalis ) at p H 6.5 in competitive mixtures for (A) the low planting density (4 plants pot 1 ), and (B) the high planting density (8 plants pot 1 ). ......... 64 4 5 Mean shoot weight of bahiagrass and giant smutgrass ( Sporobolus indicus var. pyramidalis ) at three pH levels in competitive mixtures at the 50:50 proportion. ................................ ................................ ................................ .......... 65 4 6 Relative shoot weight of bahiagrass and small smutgrass ( Sporobolus indicus var indicus ) at pH 4.5 in competitive mixtures for (A) the low planting density (4 plants pot 1 ), and (B) the high planting density (8 plants pot 1 ).. ........ 66 4 7 Relative shoot weight of bahiagrass and small smutgrass ( Sporobolus indicus var. indicus ) at pH 5.5 in competitive mixtures for (A) the low planting density (4 plants pot 1 ), and (B) the high planting density (8 plants pot 1 ).. ........ 67 4 8 Relative shoot weight of bahiagrass and small smutgrass ( Sporobolus indicus var. indicus ) at pH 6.5 in competitive mixtures for (A) the low planting density (4 plants pot 1 ), and (B) the high planting density (8 plants pot 1 ).. ........ 68
9 4 9 Mean shoot weight of bahiagrass and small smutgrass ( Sporobolus indicus var. indicus ) at three pH levels in competitive mi xtures at the 50:50 proportion ................................ ................................ ................................ .......... 69
10 Abstract of Di ssertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy BIOLOGY OF SMALL SMUTGRASS ( Sporobolus indicus var. indicus ) AND GIANT SMUTGRASS ( Sp orobolus indicus var. pyramidalis ) AND LONG TERM MANAGEMENT OF GIANT SMUTGRASS By Neha Rana May 2012 Chair: Brent Sellers Cochair: Jason Ferrell Major: Agronomy Smutgrass ( Sporobolus indicus ), a nativ e of tropical Asia, is a perennial weed that affec ts many improved perennial grass pastures in Florida and throughout the southeastern United States. The two varieties of smutgrass predominant in Florida are small smutgrass ( Sporobolus indicus var. indicus ) and giant smutgrass ( Sporobolus indicus var. pyr amidalis ). Seed germination experiments were conducted to determine the impact of environmental conditions on seed germination and emergence of both smutgrass varieties. Temperature, water stress, and depth of burial were the factors that most influenced g ermination, while light and pH had little impact Seeds of both varieties germinated at four simulated Florida temperature fluxes (27/15, 33/24, 29/19 and 22/11 C day/night), though the germination of small smutgrass and giant smutgrass was reduced at 33/ 24 C and 22/11 C, respectively. Small and giant smutgrass germination was inhibited at water potentials below 0.2 Mpa and when small smutgrass seed was placed at any depth below the soil surface. Emergence of giant smutgrass seeds did not occur at depths below 3 cm. Impact of soil pH was tested on competitive ability of bahiagrass with smutgrass varieties. At the recommended soil pH level of 5.5
11 for bahiagrass giant smutgrass was 4 times more competitive than b a hiagrass. Conversely, bahiagrass was 2 times more competitive than small smutgrass at the same pH level T hree field experiments were initiated in 2008 to evaluate the effect of integrated long term management strategies using both cultural and herbicide inputs for giant smutgrass control in bahiagr ass pastures. Burning did not have a significant impact on long term control. In 2011, no differences were observed when hexazinone herbicide was integrated with tillage or hexazinone was combined with supplemental nitrogen compared to sequential or single applications of hexazinone. D ata indicate that a sequential application of hexazinone may be better than introducing tillage N itrogen application after two weeks of hexazinone appears to have greater impact on reducing smutgrass reinfestation than hexazi none alone. Sequential hexazinone applications, when applied at 0.56 kg ha 1 or greater, result ed in increased control as compared to single applications. Collectively, these data suggest that environmental conditions present during summer or fall season f avor growth of both smutgrass varieties, giant smutgrass was more aggressive and competitive than bahiagrass, and sequential applications of hexazinone may have more impac t than implementing tillage practices.
12 CHAPTER 1 INTRODUCTION Forage crops grown in U.S. pastures provide an annual income of $10 billion (USDA 1998), 45% of weeds found in these pastures are non native; causing an economic loss of approximately $1 billion pe r year (Pimentel et al. 2001). Pasture weeds limit the growth of desirable forag es by competing for light and othe r nutrients. In addition, weeds cause reductions in animal performance and stocking rate. Smutgrass [ ( Sporobolus indicus (L.) R. Br. ) ] is a tuft forming invasive perennial grass typically problematic in areas where forage grasses are desired. The name, smutgrass, is given for the dark colored fungus, often found on the seed heads giving it a blackish, smutty appearance (McCaleb and Hodges 1963; Mislevy et al. 2002). The two varieties of smutgrass found in Florida are small smutgrass [ ( Sporobolus indicus (L.) R. Br. var. indicus ) ] and giant smutgrass [ (Sporobolus indicus (L.) R. Br. var. pyramidalis (Beauv.) Veldkamp) ] (Wunderlin and Hansen 2003) In the early 1950s small smutgrass was first noticed as a troublesome pasture weed ( McCaleb et al. 1963 ). By the late 1970s and early 1980s small smutgrass infested about 75% of the improved pastures in cent ral Florida (Mislevy and Martin 1985 ). During the early 1990s giant smutgrass was observed re placing the small smutgrass in sou th Florida pastures (Adjei et al. 2003). By 2000 giant smutgrass had surpassed small smutgrass in infested acreage and become the dominant smutgrass species threatening grazing lands in south Florida. T he reason for this aggressive re pl acement of small sm utgrass by giant smutgrass remains unknown. Bahiagrass ( Paspalum notatum Fluegg ) is the primary pasture grass utilized for grazing in the beef cattle industry in Florida. S mutgrass often invade s bahiagrass
13 pastures, resulting in forage loss, reduced graz ing, and lower calf weaning weight (Burton et al. 1997; Ferrell et al. 2006). Smith et al (1974) reported that as the density of this species increases the production of high quali ty forage is severely reduced. Smutgrass spread to established pastures is slow and may require 3 5 years before becoming dense enough to be recognized as a serious problem, but once established, control of smutgrass is difficult. Since plants in preceding years create large and persistent seed banks, McCaleb et al. (1963) sugg ested that reducing the potential for re infestation provides effective long term control of smutgrass. Smutgrass is an undesirable forage species. Cattle do not preferentially graze smutgrass (McCaleb et al. 1963; Smith and Cole 1972). Smutgrass spread i ncreases as the interspecific competition between smutgrass and desirable forage decreases (Mears et al. 1996). Although mature smutgrass is not palatable to livestock, cattle often consume tender regrowth of smutgrass 1 to 3 week after burning or mowing ( Mullahey 2000 ). This may be attributed to the forage quality of young smutgrass shoots, which is simi lar to bahiagrass (Mullahey 2000 ). Smutgrass growth is reported to decline if smutgrass infested pastures are managed intensively (1 to 3 w ee k rest period) (Ahmad 1979), but it is difficult for ranchers to maintain grazing pressure on smutgrass without overg razing bahiagrass. Weed management of smutgrass has mostly been accomplished through the use of herbicides. In the past, various combinations of mowing, intensive rotational grazing and cultivation were utilized in unsuccessful attempts to reduce smutgrass infestation s (McCaleb and Hodges 1971; Ahmad 1979; Andrade 1979; Mislevy et al. 1980). From the 1950s to the 1980s, 2,2 Dichloropropionic acid ( D alapon ) was the only selective
14 herbicide effective in controlling smutgrass (McCaleb et al. 1963). Mislevy et al. (1980) reported that mowing 5 weeks after dalapon treatment provided effective smutgrass control and increased bahiagrass ground cover. Conversely, M islevy and Currey (1980) determined that bahiagrass ground cover decreased by 50% when dalapon was applied at 3.3 kg ha 1 to achieve 80% smutgrass control. However, dalapon is no longer registered for use in pastures and hexazinone (Velpar) is the only act ive ingredient registered that provides effective control of smutgrass in bahiagrass pastures (Brecke 1981; Mislevy et al. 1999, 2002; Ferrell et al. 2006). Ranchers have been effective in controlling smutgrass by applying 1.12 kg ha 1 hexazinone when the pasture is infested with a smutgrass densi ty of 50% (Ferrell et al. 2006 ). However, re infestation of smutgrass in pastures to initial densities often occurs within 3 to 4 years following the initial chemical treatment. Thus, there was a need to understand long term management practices that can be us ed to suppress smutgrass invasion in establ ished bahiagrass pastures. The overall objective of this research was to study the biology of smutgrass varieties and integrate this information in the development and implement ation of integrated long term manag ement strategies for smutgrass control in bahiagrass pastures. For a successful integrated management system involving both herbicides and cultural inputs there was a need to understand the seed biology of smutgrass. There is some information available on small smutgrass seed biology; however, very little seed biolog y information is available for giant smutgrass. From a management standpoint, it is important to understand how temperature, moisture, pH, light and depth of burial impact the seed germination of both smutgrass varieties Studying these
15 environmental parameters helps provide a framework for developing management practices in pastures. Therefore, the objective of C hapter 3 was to study the effect of environmental factors and soil characteristics on seed germination and emergence of both smutgrass varieties. An effective control of any weed species is only possible if its biological and ecological characteris tics are known (Quinlivan 1972). W eed crop interactions are important to assess non chemica l weed management alternatives (Upadhyaya and Blackshaw 2007). Prior research has reported the management strategies for maintaining optimum bahiagrass production, including an optimum pH of 5.5 and seasonal nitrogen fertilization requirement ( Adjei and Re chcigl 2004; Mackowiak et al. 2008 ; Rechcigl et al. 19 95; Silveira et al. 2007 ; Stephenson and Rechcigl 1991 ). However, t here is no information in the literature regarding the effect of pH on the growth and competitiveness of smutgrass varieties. Studying the impact of bahiagrass smutgrass interactions at different levels of pH would aid in the development of weed management alternatives which can be utilized to control this troublesome pasture weed. Therefore, the objective for C hapter 4 was to examine the relative competitiveness between both smutgrass and bahiagrass. This experiment was co nducted in the greenhouse and provided biological information concerning smutgrass varieties growth and competitiveness in bahiagrass pastures under soil pH levels of 4. 5, 5.5 and 6.5. The third objectiv e of this research was to develop a long term weed management strategy via an integrated approach using both cultural and herbicide inputs to provide effective control and reduce the potential for smutgrass invasion. Thes e experiments
16 were field studies an d centered on present control options, which only includes annual applications of hexazinone applied at 1.12 kg ha 1 This recommendation gives optimum control for 2 3 years, but soon after smutgrass starts re infesting the pasture. We hypothesized that implementing a combination of cultural and herbicide practices would result in better long term smutgrass control. The refore, Chapter 5 has three specific objectives: 1. To characterize long term management of smutgrass usin g cultural practices su ch as burning and tillage in combination with herbicide application. 2. Evaluate the combination of hexazinone and nitrogen fertilization on smutgrass control. 3. Evaluate sequential application s of hexazinone for smutgrass control. In this dissertation, the results through 2011 will be discussed, but the data for these field experiments will be collected for an additional 2 3 years. The results from these experiments will be employed to develop best management practices for long term control of smutgrass in improved bahiagrass pastures.
17 CHAPTER 2 LITERATURE REVIEW Smutgrass [ ( Sporobolus indicus (L.) R. Br.] has long been recognized as a serious weed problem in Florida pastures, and in many established perennial grass pastures of the s outheastern United States. According to a 2007 survey of beef and forage producers s mutgrass is considered the third most troublesome pasture weed after dogfennel [ Eupatorium capillifolium (Lam.) Small] and tropical soda apple ( Solanum viarum Dunal ) in so uth central Florida pastures ( Crawfo rd 2007 ). This is attributed to the fact that smutgrass is adapted to almost all soil types and even the most highly managed pastures are often infested with smutgrass. In addition to being found in improved grass pastur es, it is also found in open areas, roadsides and disturbed waste places from Virginia to Tennessee, Oklahoma, Texas and Florida (Hitchcock 1950). McCaleb et al. (196 3) suggested that smutgrass could become the most dominant plant on the mineral soils of p eninsular Florida and the coastal plains of Georgia if not brought under control Giant smutgrass [ ( Sporobolus indicus (L.) R. Br. var. pyramidalis (Beauv.) Veldkamp) ] in collaboration with giant parramatta grass [ ( Sporobolus indicus (L.) R. Br. var. major (Buse) Baaijens] covers more than 250,000 ha land in eastern Australia (Mears et al. 1992). Giant p arramatta grass is also considered as a serious noxious plant in Australia (Andrews 1995). Research on smutgrass started at the Range Cattle Res earch and Ed ucation Center, Ona, FL in 1955, and continues today. In the 1950s, small smutgrass was first detected in Florida. Most of the studies since the 1950s have been concentrated on small smutgrass; there is little biological information known about giant smutg rass. Giant smutgrass, also known as West Indian dropseed, is taller, more robust and more
18 invasive in comparison t o small smutgrass ( Mislevy et al. 2002 ). It is assumed that giant smutgrass produces similar number of seeds as small smutgrass. Geographica l Range Several species of smutgrass are known to have naturalized throughout the world in Australia, New Zealand, Trinidad, Japan, Philippines, Brazil, Caribbean islands as well as various islands in the Pacific Ocean ( Persad 1980; Andrews 1995 ). In the U nited States, s mall smutgrass [ ( Sporobolus indicus (L.) R. Br. var. indicus ) ] is found in 23 states from New York to Florida and Texas, as well as the west coast ( McCaleb and Hodges 1971 ) while giant smutgrass infestations are found only in Florida and Pu erto Rico (USDA 2007). Smutgrass Problem Statement Smutgrass, an invasive perennial bunch type grass, reduces pasture productivity, out compete s desirable species, and causes significant degradation of native rangeland There are several reasons why smutgr ass varieties are considered problematic; both varieties have low palatability when mature, due to invasive characteristics expeditiously dominate a pasture especially after overgrazing or soil disturbance, and under optimum conditions can produce seeds th roughout the year. The ecological and biological parameters of smutgrass suggest it could be a very successful weed; that is, large number of seeds produced per season, continuity of seed production, persistent seed bank, and a high degree of unpalatabilit y to grazing animals. These factors aid in the ability of smutgrass to out compete desirable forage and displace palatable species. Persad (1976) suggested that after 6 weeks of growth, low digestibility associated with high neutral detergent fiber could b e the primary factor limiting the acceptability of smutgrass. This might be the reason why cattle overgraze
19 the bahiagrass and do not consume neighboring smutgrass plants This further increases smutgrass infestation s and decreases forage quality and forag e production (Mears et al. 1996). Cattle generally avoid mature smutgrass, but consume the tender regrowth of smutgrass 1 3 weeks after mowing or burning before it be comes unpalatable (Mullahey 2000 ). During this immature stage, the forage quality of youn g smutgrass is similar to that of bahiagrass (Mullahey 2000). However, this requires intensive management by the ranchers which may not be economical over an extended period of time. Taxonomy and Morphology Sporobolus R.Br. is a genus of about 160 species found in tr opical and subtropical areas of the w orld ( Simons and Jacobs 1999 ). Taxonomists all over the world have considered the classification of Sporobolus species complex and difficult (Clayton 1965). Some of the species included in this genus are S. i ndicus S. elongatus S. pyramidalis S. diander and S. natalensis. A number of Sporobolus sp. have been reclassified and renamed, creating confusion in identification and recognition. Most taxonomists have regarded giant and small smutgrass plants as var ieties of a single species, largely based on the differences in size and seed head characteristics ( Simons and Jacobs 1999 ). It is widely accepted that smutgrass has three centers of origin in the world: Africa, West Indies and Asia. The two varieties of s mutgrass found in Florida are believed to be native of tropical Asia (Wunderlin and Hansen 2003) Smutgrass, a war m season perennial bunch type grass, is a member of the Poaceae family. The panicle is 15 40 cm long spike like, branched, closely pressed, g lumes obtuse, unequal or half as long as the spikelet (Currey et al. 1973). The two species of smutgrass can be distinguished by the size of their panicle, glumes and plant
20 base (Mislevy et al. 2002). Small smutgrass has appressed panicle branches, a compa ct plant base and the second glume is half the length of spikelet, while giant smutgrass has spreading and ascending panicle branches, a spreading plant base and the second glume less than half the length of the spikelet (Mislevy et al. 2002). Mature seeds are r ed in color and depending on weather and mechanical forces seeds remain attached to the muc ilaginous pericarp ( Currey and Mislevy 1974). Seed Biology Regardless of variety, smutgrass has the potential to produce a large amount of seeds. Small smutgr ass produces more than 1,400 seeds per panicle and nearly 45,000 seeds per plant (Currey et al. 1973); the number of seeds produced by giant smutgrass is expected to be at least similar to this. The favorable time for seed production is from April to Decem ber (Currey et al. 1973). If temperature and soil moisture are suitable smutgrass can produce inflorescences throughout the year. In Florida, inflorescences are present almost anytime during the growing season. Thus, production of seed occurs continuously throughout the growing season with flowering, immature seed, mature seed, and shattering occurring simultaneously on the same plant and on the same inflorescence (Currey et al. 1973). Smutgrass seeds remain dormant in the soil during the winters and can r emain viable over an extended period of time ranging from 2 or more years (McCaleb and Hodges 1971 ; Andrews et al.1996) Research shows the germinatio n percentage of smutgrass ranging between 1 to 9% under field conditions ; however, mechanical or acid scar ification improved the germination percentage up to 94% and 98%, respectively ( Currey et al. 1973 ). This suggest s the presence of a hard seed coat (Currey et al. 1973; Andrews et al. 1996). Although the germination percentage of smutgrass is low, the numbe r of seed produced per panicle is very high,
21 creating large persistent seed banks which encourage germination under favorable conditions. Previous research conducted by Andrew et al. ( 199 6 ) has established that giant parramatta grass i nfestations typically have a seed bank of at least 2500 m 2 which may remain viable up to 10 years. With these types of seed characteristics it is very difficult to control smutgrass varieties without a long term plan Seed Dispersal In addition to wind and water, animals p lay an important role in smutgrass seed dispersal. Animals disperse smutgrass seeds by carrying them on their hair or in the soil on their hooves; they are also able to disperse seeds after ingestion and excretion. Because of its ease of spread management of smutgrass becomes difficult. Andrews (1995) suggested that it takes 3 and 7 days for 50% and 100% respectively, of ingested smutgrass seeds to pass thro ugh the digestive process of a cow. On average, only 19% of the seeds that passed through the digesti ve process remained viable. It was suggested that the possibility of seed dispersal directly by the cattle through excretion is less than the indirect spread of seeds by stick ing to the hairs of livestock. Andrews (1995) also concluded that the dispersion of ingested smutgrass seeds is less likely unless the manure is dispersed right after excretion. Additionally, smutgrass seed can be spread through contaminated hay, soil or seeds stuck on machinery and vehicles. Smutgrass in Forages Bahiagrass, a warm s eason perennial grass, is widely utilized as forage and cover s an estimated 2 .5 million hectares in the state of Florida (Chambliss 1996). Smutgrass is a serious weed problem in bahiagrass pastures. The initial infestation of smutgrass is usually by seed, and the plants are few and fairly inconspicuous in number. As the smutgrass density increases the overall quality of pasture decreases
22 (Smith and Cole 1972 ; Ferrell et al. 2006 ). Smutgrass, once established, is capable of forming dense stands and can exclu de bahiagrass by altering ecological conditions such as light, nutrients and moisture availability. Therefore, smutgrass, if not brought under control during the early stages of infestation, has serious economic impacts on beef cow calf production, thereby causing reduction in the overall productivity. Smutgrass varieties are difficult to distinguish from bahiagrass at seedling stage s h owever, the leaves and stems of older smutgrass plants are noticeably tougher than bahiagrass. In order to maximize bahiag rass forage production it is crucial for a rancher to realize when smutgrass infestation s should be controlled. Ferrell et al. (2006) reported that hexazinone should be applied when the smutgrass density is approximately 35%. However, if the smutgrass dens ity is less than 35%, the cost of control would be higher than the calf gains realized from the net bahiagrass yield. Several grazing trials have reported that cattle show a decided grazing preference for bahiagrass over mature smutgrass (Persad 1976; Val le 1977; Andrade 1979 ; Mullahay 2000 ). The in vitro organic matter digestion (IVOMD), crude protein, and neutral detergent fiber were higher in smutgrass than bahiagrass up to 6 weeks (Currey et al. 1977; Persad 1976). However, after 6 weeks, there was a r apid decline in IVOMD and crude protein of smutgrass and the total fiber content was higher in smutgrass than bahiagrass Management of Smutgrass Traditionally, control of smutgrass has been difficult to achieve. In the past, several cultural and chemical control measures from mowing (McCaleb et al. 1963 ), fertility management, intensive rotational grazing (Mullahey 2000) to extensive use of different herbicides have been used as control option s
23 Cultural Control McCa leb et al. (1963) reported mowing redu ced the basal diameter of the smutgrass plants, but there was no reduction in plant density. Mor eover, plants reverted to former densities wh en mowing ceased. M owing also contributed to seed spread (McCaleb et al. 1963; McCaleb and Hodges 1971). Mislevy et al. (2002) reported that mowing reduced the total nonstructural carbohydrates (TNC) in giant smutgrass, suggesting a possible increase in control with herbicide due to a weekend plant However, mowing giant smutgrass prior to hexazinone application provid ed no additional control when compared to nonmowed plants (Mislevy et al. 2002, Ferrell et al. 2006). Cultivating pastures heavily infest ed with smutgrass resulted in unsa tisfactory and variable control; smutgrass plants were not destroyed by cultivation a nd new plants grew from seed already present in the soil (McCa leb et al. 1963). Mullahey (2000 ) conducted e xperiments on the use of stocking methods (continuous and rotational) in combinations with hexazinone for the control of giant smutgrass and the reco very of bahiagrass. Giant smutgrass groundcover increased (18 22%) under continuous stocking whereas under rotational stocking smutgrass groundcover decreased (14 11%) and bahiagrass groundcover increased under rotational stocking (83 88%) and remained co nstant under continuous stocking (77 75%) (Mullahey 2000 ). Grazing pressure had more impact on small smutgrass density and biomass than grazing frequency, but cattle lost weight with poor body condition (Valle 1977; Andrade 1979). Ferrell et al. (2006) stu died the bahiagrass response to low, medium and high densities of giant smutgrass. It was reported that under < 20% giant smutgrass ground cover, bahiagrass yielded 1164 kg ha 1 mo 1 As the giant smutgrass densities increased to medium (20 to 70%) and hig h (> 70%) levels bahiagrass yield was reduced 51 and
24 87%, respectively, compare d with low giant smutgrass densities. It was concluded that bahiagrass was competing with giant smutgrass at low densities and as smutgrass densities increases above 35%, the co st of control will be less than the calf gains realized from the net bahiagrass yield. B urning has also been practiced to remove old growth on smutgrass plants. Vogler et al. (1998) has reported that ex posure of 125 C for 15 seconds reduced smutgrass seed viability to zero. Most smutgrass seed (60 90%) are present on the soil surface in the top 5 10 mm of soi l (Andrews 1995); destruction of these see ds can reduce the seed bank considerably. Chemical Control Many chemicals have been evaluated to determi ne their effect on smutgrass, but dalapon and hexazinone were found to be the most effective (Meyer and Baur 1979; Mislevy and Currey 1980; Mislevy et al. 1980, 1985, 1999 2002; Smith 1982; Brecke 1981 ; Ferrell et al 2006 ). These studies have also reporte d dalapon phytotoxicty to bahiagrass and i ncreasing dalapon from 1.7 to 5.0 kg ha 1 increased smutgrass control to 80%. Mislevy et al. (1980) reported that mowing or roller chopping plus fertilization enhanced bahiagrass recovery after treatment with dala pon. However, dalapon, one of the most effective control options, is no longer registered for use in pastures (Mislevy et al. 1999). The only labeled herbicide for selective smutgrass control in pastures is hexazinone under the trade name of Velpar (Mislev y et al. 2002). Hexazinone has been shown to provide acceptable control of small and giant smutgrass at rates of 0.56, 0.84, and 1.12 kg ha 1 (Mislevy et al. 1999, 2002; Ferrell et al. 2006). However, a pplication rates of 0.56 kg ha 1 provide d inconsistent giant smutgrass control, ranging between 65 to 98% (Mislev y et al. 2002; Ferrell et al. 2006) The application rate of 1.12 kg ha 1 hexazinone pr ovides the most consistent long term control of giant smutgrass (Mislevy
25 et al. 2002; Ferrell et al. 2006). He xazinone has been reported to be phytotoxic to bahiagrass at 1.40 and 1.68 kg ha 1 (Mi slevy et al. 2002). Wilder (20 1 1 ) reported that although giant and small smutgra ss vary greatly in size, there was no significant difference observed in the treatment rat es of hexazinone with respect to control He also indicated that hexazinone efficacy was more dependent on application timing than surfactant usage because no differences were observed in hexazinone treatments with or without surfactant. Spraying hexazino ne at the recommended use rate of 1.12 kg ha 1 modera tely injures bahiagrass, but recovers from temporary burn and yellowing within 40 days of application (Sellers et al. 2009). However, dry matter accumulation of bahiagrass was 9 to 38% lower relative to the untreated at 1.12 kg ha 1 ( Sellers et al. 2008). Previous grazing restrictions limited grazing to 60 days after applying hexazinone but that restriction has been significantly red uced as of March 2011. Currently pastures treated with hexazinone at 1.26 kg ha 1 can be grazed im mediately and treated hay may be cut 38 days after application. Hexazinone Usage and Mode of Action Hexazinone is a me mber of the s triazine family of herbicides It is sold under the trade names Velpar 1 and Velo s sa 2 and is reg istered for smutgrass control in established stands of bahiagrass and bermudagrass [ Cynodon dactylon (L.) Pers.] In addition, it is also registered for pineapple and sugarcane dormant or semi dormant alfalfa, prior to bud break in first year Christmas tr ees, and forestry site preparation ( Senseman 2007 ). 1 DuPo nt Velpar E. I. duPont de Nemour s and Company, 1007 Market S t, Wilmington, DE 19898. 2 Velossa Helena Chemical Company, 225 Schilling Blvd, Suite 300, Collierville, TN 38017
26 In non agricultural sites, hexazinone is registered for industrial sites, railroads, rig ht of ways, and storage areas. Hexazino ne provides contact and residual control against many annual and perennial br oadleaf and grass weed s including brush species. Hexazinone should only b e applied when there is sufficient soil moisture for uptake and translocation of the herbici de. For best results it should be applied when at least 1 cm of rainfall occurs wi thin two weeks of application, humidity is high, and air temperatures are above 27 C. Hexazinone is a soil active and xylem mobile herbicide ( Senseman 2007). It is readily absorbed by the roots. Hexazinone is also absorbed when foliar applied, but there is little translocation because it lacks phloem mobility. On entering a plant, h exazinone inhibits photosynthesis at the photosystem II complex The mechanism of action of hexazinone is as follows : It binds to the Qb binding niche on the D1 protein in photosystem II complex in chloroplast thylakoid membrane. Herbicide binding at this site blocks the electron flow from Qa to Qb ( Senseman 2007 ). This in turn blocks CO 2 fixation and formation of ATP and NADPH 2 which are all needed for plant growth However, plant death does not generally occur due to inhibition of these reactions Since electron flow is blocked the build up of electrons is passed to chlorophyll molecule s forming triplet state chlorophyll which then reacts with the oxygen (O 2 ) in th e cell to form radical oxygen (O 2 ) ( Senseman 2007) B oth triplet chlorophyll and radical oxygen collectively react with lipid membranes to produce lipid radicals. These lipid radicals in turn react to oxidiz e other lipids and proteins c ausing cell membrane disruption, cellular leakage loss of chlorophyll and carotenoids, and ultimately death of
27 the plant ( Senseman 2007). Typical symptoms shown by plants tr eated with hexazinone include f oliar chlorosis followed by necrosis. Summary and Objectives Single management programs have often failed to provide effective smutgrass control, therefore, an integrated management approach is necessary. Historically the control of smutgrass has been focused on high rates of fertilizer application, herbicide treatments and maintenance of improved pastures. These methods initially reduced smutgrass infestation s, but not over long period s of time. This is pre dominantly due to the fact that smutgrass re infestation occurs within 3 to 4 years after the initial herbicide treatment. Therefore this res earch is focus ed on studying the biology and long term management of giant and small smutgrass in bahiagrass pastures. To achieve this goal several experiments were conducted including laboratory seed germination experiments, greenhouse competition experim ents and field experiments. These experiments will provide valuable information to assist in devising a best manage ment plan for smutgrass control.
28 CHAPTER 3 EFFECTS OF ENVIRONME NTAL F ACTORS ON SEED GERMI NATION AND EMERGENCE OF S MALL SMUTGRASS ( Sporob olus indicus var. indicus ) AND GIANT SMUTGRASS ( Sporobolus indicus var. pyramidalis) Smutgrass [ Sporobolus indicus (L.) R. Br. ] an aggressive non native weed, has become a serious threat in many perennial grass pastures throughout the southeastern United States, particularly in the sandy soils of central and south Florida (McCaleb and Hodges 1971). The name, smutgrass, is given for the dark colored fungus ( Bipolaris spp.) that often infects the inflorescence of the plant and gives it a black, sooty appeara nce (McCaleb and Hodges 1963; Mislevy et al. 2002) The two varieties of smutgrass found in Florida, small smutgrass [ Sporobolus indicus (L.) R. Br. var. indicus ] and giant smutgrass [ Sporobolus indicus (L.) R. Br. var. pyramidalis (P. Beauv.) Veldkamp.], are believed to be native of tropical Asia ( Wunderlin and Hansen 2003) Small smutgrass is found in 23 states from New York to Florida and Texas, as well as the pacific coast, while giant smutgrass infestations are found only in Florida and Puerto Rico (US DA 2007). In addition to being found in improved grass pastures, this perennial bunch type grass is also found along roadsides and disturbed waste places (Hitchcock 1950). Taxonomists all over the world have considered the classification of Sporobolus spec ies comple x and difficult ( Simon and Jacobs 1999) A number of Sporobolus sp. have been reclassified and renamed, creating confusion in identification and recognition. The two varieties of smutgrass in Florida can be distinguished by its size and seed head characteristics (Mislevy et al. 2002). Small smutgrass has appressed panicle branches, a compact plant base and grows up to a height of 1 m, whereas giant smutgrass has spreading and ascending panicle branches, a spreading plant base and
29 is about 1.5 2 m in height (Mislevy et al. 2002). Giant smutgrass, also known as West Indian dropseed, is taller, more robust and more invasive in comparison to small smutgrass (Fe rrell et al. 2006; Mislevy et al. 2002 ). Smutgrass represents an important agronomic and e nvironmental concern in pastures throughout the southern and western United States. Crawford (2007 ) listed smutgrass as the third most troublesome pasture weed after dogfennel [ Eupatorium capillifolium (Lam.) Small] and tropical soda apple ( Solanum viarum Dunal ) in south Florida pastures. Small smutgrass was first reported as a troublesome pasture weed in Florida in the early 1950s (Hitchcock 1950). By the late 1970s and early 1980s, it had become the dominant and most serious pasture weed in central Florid a infesting about 75% of the improved pastures in central Florida (Mislevy and Martin 1985). In the early 1990s, giant smutgrass was observed displacing small smutgrass in south Florida pastures (Adjei et al. 2003), and giant smutgrass had become the domin ant smutgrass variety observed in grazing lands of south Florida. Currently, giant smutgrass can be found throughout Florida and is progressing northward into Georgia (B. Sellers, personal observations). The reason for this aggressive displacement of small smutgrass remains unknown. Regardless of variety, smutgrass has the potential to produce a large amount of seeds. However, seed production and limited germination experiments have been only conducted with small smutgrass. Previous research has shown that small smutgrass produces more than 1,400 seeds per panicle and nearly 45,000 seeds per plant (Currey et al. 1973); the number of seeds produced by giant smutgrass is expected to be similar. Seed production occurs continuously from April to December (McCal eb and
30 Hodges 1971) with flowering, immature seed, mature seed, and shattering occurring simultaneously on the same plant and on the same inflorescence (Currey et al. 1973). S mutgrass seeds are thought to remain viable for at least two years (McCaleb and H odges 1971); this is suggested to be due to the presence of a hard seed coat (Andrews et al. 1996; Currey et al. 1973). Small smutgrass germination has been shown to range between 1 to 9% under field conditions; however, mechanical scarifica tion improved g ermination from 9 to 94% (Currey et al. 1973). From these data it had been hypothesized that giant smutgrass shares similar seed characteristics as small smutgrass, however, no studies have been performed comparing the two varieties. To date, no research has specifically investigated environmental effects on germination of small and giant smutgrass. Considering the aggressive spread of giant smutgrass in south Florida pastures over the last decade, these experiments might help explain if giant smutgrass ha s a competitive advantage over small smutgrass with respect to germination requirements. Furthermore, a better understanding of how smutgrass seeds respond to these environmental conditions may help predict differences in growth and habit of small and gian t smutgrass, which may ultimately help in devising an integrated management plan for this troublesome weed. Therefore, germination experiments were conducted to determine if seed germination of small and giant smutgrass is influenced by dormancy, temperatu re, light, pH, osmotic potential, and depth of burial. Materials and Methods Seed Sources. Small and giant smutgrass seed were harveste d over several days from several bahiagrass ( Paspalum notatum Fluegg) pastures near Gainesville
31 and Ona, FL in May 2008 and July 2009 respecti vely Seeds were stripped from the inflorescence and cleaned using sieves prior to separation into heavy and light fractions using differential airflow. The heavy fraction possessed the fully developed seed which were later used in t he germination tests, and the light fraction consisted of under developed immature seed, which were discarded. For each smutgrass variety, individual samples were combined to create a single collection after preliminary results (data not shown) indicated n o differences among samples with regard to germination. Clean seeds were bulked, placed in a plastic container, and s tored in the laboratory at 232 C for the duration of the experiment. Germination Protocol. Fifty seeds of small or giant smutgrass were p laced evenly in a 9 cm diameter Petri dish 1 lined with two pieces of filter paper. The fil ter papers were soaked with 5 m L of deionized water or the appropriate test solution and sealed with parafilm to avoid moisture loss. In all studies, each treatment h ad four replications and each experiment was conducted twice. Unless otherwise stated, seed were placed in controlled environment growth chambers maintained with a daily alternating thermoperiod (30/20 C) and photoperiod (16 h light/8 h dark). Germ ination was recorded every 2 d for a period of 14 d A seed was considered germinated when the emerged radicle measured at least 2 mm. Baseline Germination. Prior research has shown that smutgrass seed, unless scarified, have a low rate of germination (Currey et a l. 1973). The objective of baseline germination was to assess potential dormancy in both varieties of smutgrass. Smutgrass seed were g rown in growth chamber at 30/20 C day/n ight temperature. 1 Fisherbrand Petri dishes, Fisher Scientific, 2000 Park LaneDrive, Pittsburgh, PA 15275.
32 After 14 d remaining ungerminated seed were tested for viabilit y according to the procedures described in the Handbook on Tetrazolium Testing (Moore 1985). Seeds were immersed in 0.25 % tetrazolium solution for 3 hours under dark conditions. The seed were then bisected and observed at 20X magnification and recorded a s viable if the embryo stained red or pink. Light. To determine the effect of light on germination, small and giant smutgrass seed were placed in Petri dishes under green light wavelength (monochromatic 545 nm) The dishes were then immediately wrapped wi th 2 layers of aluminum foil to ensure no light penetration. In the control treatment, Petri dishes remained unwrapped and seed were exposed to environmental conditions as described in the germination protocol. Temperature. The effect of constant temperatu re was evaluated by placing Petri dishes in growth chambers at 10, 15, 20, 25, 30, 35, and 40 C under constant light (200 mol m 2 s 1 photosynthetic photon flux density [PPFD]) with no day/night fluctuations. A separate germination study was conducted to determine the effect of alternating temperature regimes on smutgrass seed germination. Four alternating temperature regimes were chosen to simulate seasonal variations common in Florida. The regimes, 27/15 33 /24, 29/19 and 22/11 C, correspond to average air temperature conditions in Florida during spring, summer, fall and winter, respectively (Prez et al. 2009). Seed were incubated at these four temperature fluxes in one of four separate growth chambers. Seed placed in the growth chamber were exposed to 12 h light period. There were five replications for each temperature regime and the experiment was repeated three times.
33 pH. To test the effects of pH on smutgrass germination, seed were germinated in buffer solutions at pH levels of 4, 6, 8, and 10. Deio nized water was used as a control. Buffer solutions were prepared using 100mM Citric Acid NaOH (pH 4), 25mM MES (potassium hydrogen phthalate, 2 [4 morpholino] ethanesulfonic acid) (pH 6), 50mM HEPES (N 2(2 hydroxyethyl) piperazine 2 ethanesulfonic acid ) (pH 8), and 100mM Glycine NaOH (pH 10). Test solutions were adjust ed using HCl or NaOH. All environmental conditions were maintained the same as previously stated in the germination protocol. Water Stress. To evaluate the effect of water stress on smutgr ass seed germination, seed were placed in aqueous solutions of polyethylene glycol to obtain osmotic potentials of 0.2, 0.4, 0.6, 0.8, 1.0 MPa. Se ed placed in deionized water were used as a control. The water potentials were confirmed using a vapor pr essure osmometer 2 calibrated with aqueous solution s of sodium chloride. All environmental conditions were maintained as previously described in the germination protocol. Depth of Burial. Eight seeds of small or giant smutgrass were planted separately in 3.5 cm by 18 cm conetainers 3 at six planting depths of 0 (soil surface), 3, 6, 9, 12, and 15 cm, with each planting depth in a different conetainer. The soil used in this experiment was top soil and sand in 2:1 v/v ratio. The growth chamber was set at fluc tuating day/night temperatures of 30 /20 C in 16 h light/8 h dark, respectively. The conetainers were watered as needed to maintain optimum moisture for seed 2 Wescor vapor pressure osmometer, Model 5500, Wesco r, Inc., 459S. Main Street, Logan, UT 84321. 3 3.5 by 18 cm conetainers, Stuewe & Sons, Inc. 2290 SE Kiger Island Drive, Corvallis, OR 97333.
34 germination. Seedlings were considered emerged when cotyledons were visible and seedlin g emergence was recorded 14 d after planting. Statistical Analysis. All experiments, unless otherwise specified, were conducted twice using a completely randomized design with four replications. There were no significant (P < 0.05) trial by treatment interactions; th erefore, data were combined over experimental runs. Residuals of pooled data were plotted for visual inspection to confirm homogeneity of variance. Data were subjected to analysis of variance (ANOVA) and ignificant differences were 0.05). Nonlinear regression analysis was used to determine how constant temperature and pH affected germination. Results and Discussion Baseline Germination. Previous seed research indicated a high level of seed dormancy for small smutgrass as germinati on was only 9% (Currey et al. 1973). The seed husks were not removed in the previous experiment; however, mechanical scarification increased germination from 9% to at l east 94% (Currey et al. 1973). In the current study, s ince seed husks were removed from seed of both smutgrass varieties, it was expected th at seed germination would be maximized The average germination rate for both varieties was 88% and no differences were observed between the varieties (data not shown). Tetr azolium testing revealed that n on germinated seeds were not viable. These results indicate that smutgrass seed had no seed dormancy, as in many other grass species (Moreno and McCarty 1994; Teuton et al. 2004). Seed of both the varieties were readily viable and germinated when exposed t o suitable environmental conditions. Mature seed of giant rats tail grass ( Sporobolus pyramidalis P. Beauv), a
35 serious agricultural pest in tropical and sub tropical areas of Australia, was also found to be highly viable (Vogler and Bahnisch 2006). Conside ring that seed dormancy was not observed with either smutgrass variety, management practices that target emerging or emerged weed seedlings can be utilized. This preliminary testing also demonstrated that germination was consistent for the following experi ments. Light. Both smutgrass varieties germinate d under light and dark conditions Germination of both smutgrass varietie s was highest (88%) under 30/20 C light/dark conditions as compared to dark conditions when germination was 69% and 40% lower for smal l and giant s mutgrass, respectively (Figure 3 1 ). Several weed species, including tropical signalgrass [ Urochloa subquadripara (Trin.) R.D. Webster ; Teuton et al 2004] natalgrass [ Melinis repens (Willd.) Zizka; Stokes et al. 2011], gian t paramatta grass [ Sporobolus indicus (L.) R. Br. v ar. major (Buse) Baaijens ; Andrews et al. 1997] have no light requirement for germination. Although germination for both small and giant smutgrass decreased in dark, giant smutgrass germination was 65% greater than small sm utgrass. These results suggest that the smutgrass seed are capable of germination regardless of light conditions, allowing these varieties to germinate when shaded by litter or the crop canopy (Trenholm 2000). This is important because the presence of bahi agrass the predominant forage in Florida, would not limit smutgrass seedling emergence from shading These data could indicate one reason why smutgrass has so successfully invaded 50 70% of the bahiagrass pastures in south and central Florida. Temperatu re. When exposed to constant temperature, both varieties of smutgrass germinated over a temperature range of 20 to 40 C ( Fig ure 3 2 ). No germination was observed for either variety at 10 or 15 C. This was expected as low
36 germination at these temperatures i s commonly observed in tropical or subtropical species (Akanda et al. 1996; Burke et al. 2003; MacDonald et al. 1992; Stokes et al. 2011; Teuton et al. 2004). Based on regression analysis, the optimum temperature for small and giant smutgrass seed g erminat ion ranged from 25 to 35 C and 30 to 40 C, respectively. Germination of small smutgrass was significantly higher (P < 0.05) than gia nt smutgrass from 20 to 30 C. Small smutgrass seed germination decreased at 40 C, whereas, giant smutgrass germination incr eased by 28% as temperature increased from 30 to 35 C. Although germination of gi ant smutgrass was 3% less at 40 C, it was not s ignificantly different from 35 C. These data suggest that giant smutgrass is capable of enduring high temperatures more effectiv ely than small smutgrass. Germination temperatures for smutgrass varieties are similar to those found in broadleaf signalgrass [ Urochloa platyphylla (Nash) R.D. Webster ; Burke et al. 2003] It is interesting to note that small and giant smutgrass germinati on was at least 23 and 43% lower, respectively, than when germ ination was tested at the 30/20 C temperature flux. These results are consistent with previous research conducted by Andrews et al. (1997) in which g ermination of giant parramatta grass was high est at 30/15 C, while germination was reduced under constant temperature. Germination of both smutgrass varieties was higher when exposed to the four alternating temperature regimes when compared to constant temperature. The preference for alternating tem peratures could be associated with the requirement of diurnal temperature flux that is commonly observed in a large number of species (Thompson et al. 1977). For example, germination of southern crabgrass [ Digitaria ciliaris (Retz.) Koel.], India crabgrass [ Digitaria longiflora (Retz.) Pers.], and goosegrass
37 [ Eleusine indica (L.) Gaertn.] was greatest at alternating light/dark temperature regimes (Chauhan and Johnson 2008 a ; Nishimoto and McCarty 1997). Small and giant smutgrass both germinated equally well (85 to 89%) under spring (27/15 C) and fall (29/19 C) conditions. Considering the average temperatures in Florida, these data infer that temperature conditions are optimum for smutgrass growth and proliferation throughout the year. However, differences in germination (P < 0.05) between varieties were observed when see d were exposed to winter (22/11 C) or summer (33/24 C) temperatures (Fig ure 3 3). The germination response of small smutgrass (86 0.75%) was significantly higher than giant smutgrass (53 4. 2%) at 22/11 C; whereas, giant smutgrass germination (82 3.7%) was significantly higher when compared to small smutgrass (59 2.3%) at 33/24 C. This is consistent with the constant temperature data in that the temperature requirements were slightly high er for giant smutgrass than small smutgrass. These results suggest that during late winter or early spring when temperatures are relatively low, small smutgrass germination is higher, whereas during mid summer or fall when the temperatures are relatively h igh, giant smutgrass has a higher rate of germination. This may explain why small smutgrass is being displaced by giant smutgrass in south Florida pastures. pH. Seed s of b oth smutgrass varieties germinate d over a pH range of 4 to 10. Maximum germination f or both smutgrass varieties was observed between pH 6 and 8 ( Fig ure 3 4); however, small smutgrass germination was 25% greater than giant smutgrass. Seed germination of s mall and giant smutgrass at pH 4 was 55 5.2% and 53 3.1%, respectively ; this may e xplain why smutgrass is found in bahiagrass pastures where soil pH often ranges from 4 to 6. Germination was greatly reduced as
38 pH increased from 8 to 10. Typically, the optimum pH range for seed germination lies between pH 5 and 8; germination is usually pH conditions (Susko and Hussein 2008). Both smutgrass varieties germinated over a broad pH range indicating an ability to adapt to a wide range of soil conditions. This is of importance in Florida because bahi agrass pastures are maintained at a soil pH between 5.5 and 6.5, which is ideal for both bahiagrass productivity and smutgrass seed germination. Therefore, it is unlikely that altering soil pH would reduce smutgrass seed germination in most soil types foun d in Florida. Water Stress. Smutgrass seed germination was greatly influenced by decreasing water potential. Germination of both varieties was highest when seed s were placed in deionized water and 0.2 Mpa. At 0.2 Mpa, there was a significant difference between varieties; the average germination rate of small and giant smutgrass was 91 1.4% and 86 1.7%, respectively (data not shown). No germination was observed for either variety at water potentials less than 0.2 MPa, suggesting that both varieties a re highly sensitive to water stress and favor a moist environment for germination. This level of sensitivity to osmotic stress has also been observed in tropical signalgrass (Teuton et al. 2004), natalgrass (Stokes et al. 2011) and several other sp ecies (C hauhan and Johnson 2008b ; MacDonald et al. 1992; Pahlevani et al. 2008). March through May is the driest time of the year in south Florida (Kalmbacher and Linda 1994); therefore, small and giant smutgrass are unlikely to germinate until early to mid June w hen summer rains begin. Since seeds of both smutgrass varieties are likely to remain ungerminated in times of drought stress, a possible management option for ranchers could be to burn or cultivate before the summer rain showers. This would allow for seed
39 burial, destruction by heat, or allow seeds to germinate and emerge after rainfall, which can be subsequently treated with a labeled herbicide to decrease the soil seed bank. Depth of Burial. Depth of burial greatly affected seedling emergence of both smu tgrass varieties. Seedling emergence for both varieties was greatest (39% for small smutgrass and 64% for giant smutgrass) when seed was placed on the soil surface (data not shown). Small smutgrass did not emerge from any depth below the soil surface. Only 6% of giant smutgrass seedlings emerged from a depth of 3 cm, and no emergence was recorded at depths greater than 3 cm. Although at a greatly reduced rate, giant smutgrass was able to emerge from deeper in the soil profile than small smutgrass. In this g ermination test, seedling emergence was lower for both varieties on the soil surface than germination in the Petri dishes, this could be due to the poor contact between the soil and the seed, or lower level of water potential close to the seed (Ghorbani et al 1999). Out of 20 weed species tested by Benvenuti et al. (2001), all the species showed a slight decrease in emergence at depth of 2 cm and as the burial depth increased the decrease was exponential; none of the weed species germinated at depths great er than 12 cm. Light penetration is an important factor that limits buried seed germination (Wesson and Wareing 1969; Woolley and Stoller 1978) however, this research indicates that smutgrass seeds do not require light for germination. Several researchers have suggested that seedling emergence may be limited by an induction of seed dormancy due to hypoxia, production of anaerobic metabolites (Benvenuti and Macchia 1995; Boyd and Acker 2004; Holm 1972), and reduction in diurnal temperature fluctuations as pl anting depth increases (Thompson and Grime 1983). Furthermore, carbohydrate reserves in small seeds, similar to those of smutgrass, may not be
40 sufficient to support seedling emergence as planting depth increases (Bewley and Black 1994). This may explain wh y greater seedling emergence for both smutgrass varieties was observed on the soil surface, compared to larger seeds which often have greater energy reserves to emerge from deeper locations in the soil profile. Currently, pasture renovation costs are appr oximately $800 1,0 00 per hectare and higher seedling emergence from seeds on the soil surface indicates that management practices which achieve burial of weed seeds would be an effective strategy to control this problematic pasture weed. Practices that result in less than 6 cm burial promoted greater emergence of smutgrass seedlings (McCaleb et al. 1963). Therefore, for this control strategy to be effective the seeds must remain adequately covered as future disking or tillage could bring the buried seed back to the soil surface. Light and pH have little impact on smutgrass seed germination, whereas, temperature, water stress and depth of burial are factors that influence germination. Smutgrass seed germination was higher under a diurnal temperature flux than at constant temperatures, suggesting it requires diurnal temperature fluctuations for maximum germination. Giant smutgrass germination was higher than small smutgrass under high constant temperature and under the high temperature flux. This coupled wi th the advent of the rainfall from June through September in south Florida is conducive to giant smutgrass germ ination. Furthermore, giant smutgrass germination is approximately 50% higher under dark conditions as compared to small smutgrass, and can emerg e from a depth of 3 cm. This suggests that giant smutgrass has the ability to rapidly dominate a pasture even under a dense canopy or excessive plant litter. These experiments also revealed that small smutgrass does not have an inherently poor rate
41 of germ ination as both varieties germinated equally well at diurnal temperature fluctuations. This suggests that viable seed only remains ungerminated in the soil seed bank due to unfavorable conditions, rather than any inherent dormancy mechanisms. Based on the se results, if temperature and soil moisture are favorable, both varieties germinate early in the season, possibly before the desired forage resumes growth in the spring. From a management standpoint these results are important because control practices w ill have the most impact when implemented from early June to late September, as these are the times seedling emergence and new growth is most likely. Additionally, fertilizer applications during the summer could increase the competitive ability of desirabl e forage species, which in turn would reduce smutgrass seedling emergence in the following fall. Future research needs to be conducted to study the seed germination rate of small and giant smutgrass under field conditions in north and south Florida.
42 Figure 3 1. Effect of light on germination of small smutgrass ( Sporobolus indicus var. indicus ) and gia nt smutgrass ( Sporobolus indicus var. pyramidalis ) incubated at 30/20 C in continuous darkness or a 16h light/8h dark period. Values represent the mean of eight observations and the error bars are SE of the mean. The asterisk sign indicate s significant di fferences between the two smutgrass varieties under dark conditions LSD P 0.05
43 Figure 3 2. Effect of constant temperature on small smutgrass ( Sporobolus indicus var. indicus ) and giant smutgrass ( Sporobolus indicus var. pyramidalis ) seed germination. A single data point represents the mean value of ten observations with standard error of mean.
44 Figure 3 3. Effec t of diurnal temperature flux ( 27/15, 33/24, 29/19, and 22/11 C) on germination of small smutgrass ( Sporobolus indicus var. indicus ) and giant smutgrass ( Sporobolus indicus var. pyramidalis ) in a 12 h photoperiod The temperatures are gro uped according to the seasons. Values represent the mean of ten observations with standard error. The asterisk sign indicate s significant differences between the two smutgrass varieties at 22/11 C and 33/24 C accordi P 0.05.
45 Figure 3 4. Seed germination of smutgrass varieties at different levels of pH incubated at 30/20 C alternating day/night temperature in 16h light/8 h dark. Each data point represents the mean ( SE) of eight observations. Data adapted from Wilder ( 2009)
46 CHAPTER 4 IMPACT OF SOIL p H ON BAHIAGRASS COMP ETITION WITH SMALL SMUTGRASS ( S porobolus indicus var. indicus ) AND GIANT SMUTGRASS ( S porobolus indicus var. pyramidalis ) Smutgrass [ Sporobolus indicus (L.) R. Br.], an invasive perennial bunch t ype grass is commonly found in many improved perennial grass pastures throughout the southeastern United States, particularly in the sandy soils of Florida (Radford et al. 1968). The infestation of smutgrass is not rapid; it is first noticed in disturbed places of a pasture, and after 3 5 years becomes dense enough to be recognized as a serious problem (McCaleb et al. 1963) According to Wunderlin and Hansen (2003), two varieties of smutgrass exist in Florida, small smutgrass [ Sporobolus indicus (L.) R. Br. var. indicus] and giant smutgrass [ Sporobolus indicus (L.) R. Br. var. pyramidalis (P. Beauv.) Veldkamp.]. These two varieties can be distinguished by their size and seed head characteristics (Mislevy et al. 2002 ). Giant smutgrass also known as West In dian dropseed has been considered to be more robust and invasive when compared to small sm utgrass (Mullahey 2000 ; Mislevy et al. 2002 ). Smutgrass, a native to tropical Asia, is a serious weed problem in forages, especially bahiagrass ( Paspalum notatum Flu egg). Bahiagrass, a warm season perennial grass, is an important forage; it is estimated to cover at least 1 million ha in Florida and nearly 2.5 million ha throughout the southeaste rn United States (Chambliss 1996 ). Presently, the only option for selecti ve smutgrass control in bahiagrass pastures is hexazinone (Mislevy et al. 2002). Weeds can compete with forage crops to reduce their nutritional quality and yield. Several factors determine the intensity of weed crop competition including environmental co nditions such as temperature, soil pH, photoperiod, availability of
47 resources, and biological factors including weed species, density as well as spatial distribution affects weed crop interactions (Patterson 1995; Zimdahl 2004). Soil pH has a pronounced ef fect on the growth and distribution of crops and weeds, and greatly affects the relative competitive ability between the two (Weaver and Hamill 1985). Information on the competitive ability of bahiagrass with that of smutgrass varieties is limited. Previo us research conducted by Ferrell et al. (2006) indicated that giant smutgrass competition with bahiagrass at medium and high densities reduced bahiagrass yield by 30 to 70% respectively, under field conditions. Bahiagrass does not tolerate shading; theref ore, dense stands of smutgrass can nearly eliminate bahiagrass underneath its canopy (Trenholm 2000; Mislevy et al. 2002). Furthermore, cattle do not preferentially graze smutgrass, and the overall productivity and quality of the pasture decreases as the s mutgrass density increases (McCaleb et al. 1963; Smith et al. 1974). In Florida, bahiagrass is widely grown on native flatwoods soils which are naturally acidic, with pH around 4.5 (Adjei and Rechcigl 2004; Silveira et al. 2007). Although bahiagrass is ad apted to grow at a wide range of pH from 4.5 6.5, the recommended soil pH for optimum bahiagrass growth and nutrient u ptake is 5.5 (Hanlon et al. 2009 ; Mackowiak et al. 2008; Silveira et al. 2007). Prior research has indicated that maintaining the soil p H > 5.0 increased bahiagrass productivity and reduced weed populations (Adjei and Rechcigl 2004; Rechcigl et al. 1995; Stephenson and Rechcigl 1991). Since soil pH levels of 5.5 6.5 have been reported to be favorable for rapid growth of bahiagrass, it wa s hypothesized that correcting the soil pH of native Florida Spodosols from 4.5 to 5.5 6.5 may increase bahiagrass competitiveness, resulting in lower smutgrass growth at higher pH levels.
48 Much research has been conducted to study the response of bahiagr ass to various levels of soil pH, but there has been no research conducted to study the effects of soil pH on competitive abilities of bahiagrass and smutgrass varieties. The present research was designed to compare the relative competitive ability and bio mass accumulation of bahiagrass and the two varieties of smutgrass at different levels of soil pH and densities. Since chemical control options are limited, this information can be an important component of integrated weed management which may help to impr ove the strategies used to control this troublesome weed in bahiagrass pastures. Materials and Methods Replacement series experiments (Rejmanek et al. 1989 ; Radosevich 1987) were conducted in a controlled environment in 2010 and 2011 to compare the compet itive ability of bahiagrass with each of the two varieties of smutgrass at three levels of soil pH (4.5, 5.5 and 6.5) and at two densities; 4 (low) and 8 (high) plants pot 1 The two varieties of smutgrass were grown in two separate experiments with bahiag rass at planting ratios of 100:0, 75:25, 50:50, 25:75, and 0:100. Bahiagrass and each smutgrass variety was planted at ratios of 4:0, 3:1, 2:2, 1:3 and 4:0 for 4 plants pot 1 and planting ratios of 8:0, 6:2, 4:4, 2:6 and 0:8 for 8 plants pot 1 at the thre e levels of soil pH. The experiment was a completely randomized design with three replications. Smyrna sand (sandy, siliceous, hyperthermic Aeric Alaquod) having a pH of 4.5 0.2 and 0.08% organic matter content was collected from a pasture at the Range C attle to remov e any extraneous plant material Before initiating the experiment, a soil neutralization curve (Kellogg et al. 1957) was obtained by incubating the soil with lime as described by Tran & Lierop (1981), with
49 some modifications. Air dried soil (144 g) wa s placed in plastic cups (128 m L ) and mixed with hydrated lime 1 at rates equivalent to 1, 2, 3, 4, 5, 6, 7, and 8 Mg ha 1 Four replicates were used for each treat ment. Soil samples were kept at room temperature and brought to 60% of the maximum water retention capacity by adding distilled water to a predetermined weight. Soils samples were incubated for 28 days and soil pH was determined on 1:2 soil:water slurry (1 :2, v/v ). A quadratic model, Equation 4 1 was fitted where y is the target soil pH levels of 5.5 or 6.5, and x is the amount of lime to be added to reach the desired pH. y = ax 2 + bx + c ( 4 1) Based on this regression model (Figure 4 1), the amo unt of lime required to raise the soil pH levels to 5.5 and 6.5 was calculated and mixed with native soil using a cement mixer 2 The soil was brought to field capacity using distilled water and placed into with 22 cm 2 diameter 4L pots. Soil in each pot was thoroughly mixed by hand and each pot was weighed so that the appropriate amount of distilled water was added to maintain soil water content at f ield capacity every third day. After 5 weeks, all pH measurements were within 0.4 of the target pH, and remai ned constant throughout the experiment. Seed of both smutgrass varieties were collected from various pastures near Gainesville and Ona. Clean seed were bulked and stored in the refrigerator at 5 C prior to the experiment. Bahiagrass and smutgrass seed wer e planted in separate flats. 1 Hi yield Horticultural Hydrated Lime (Ca(OH) 2 ), Voluntary Purchasing Groups, Inc. Box 460, Bonham, TX 75418 2 Mini cement mixer, model# CM125, Northern Industrial Tools, 2800 Southcross Dr W., Burnsville, MN 55306
50 cm, the seedlings were transplanted into the pots. Pots were surface watered with distilled water and fertilized 3 weekly throughout the experimen t. Pots were placed in a greenhouse under 27/21 5 C day/night temperatures with a 16/8 hr photoperiod. Pots were rotated every fourth day to avoid possible microenvironmental differences within the greenhouse. Pots were maintained weed free by hand weedi ng. Above ground biomass of bahiagrass and smutgrass varieties was clipped at the soil surface six weeks after transplanting. For each pot, total shoot biomass was collected for each species, placed in individual paper bags and oven dried at 60 C for 7 day s. Shoot dry weights were recorded and treatment means calculated for each species. Data analysis. Statistical analysis indicated that there were no significant (P < 0.05) trial by experimental run interactions; therefore, data were pooled over the two ru ns. Residuals of pooled data were plotted for visual inspection to confirm homogeneity of variance. Obvious outliers were removed from the data and no data transformations were performed. Dry shoot biomass of bahiagrass and smutgrass varieties were convert ed to relative yield (RY ) (Cousens 1991; Harper 1977; Weigelt and Jolliffe 2003) to produce relative yield diagrams (or replacement series diagrams) in order to determine the competitiveness of bahiagrass and each smutgrass variety in mixture compared with monoculture at each of the three pH levels using the following Equation 4 2 and Equation 4 3 : Relative yield of bahiagrass (RY B ) = Yield of bahiagrass in bahiagrass:smutgrass mixture/Yield of bahiagrass in monoculture ( 4 2) 3 Miracle Gro ( 24 8 16 ), Scotts Company LLC, 14111 Scot tslawn Rd, Marysville, OH 43041
5 1 Relative yield of smutgrass (RY s ) = Yield of smutgrass in bahiagrass:smutgrass mixture/Yield of smutgrass in monoculture ( 4 3) Weed aggressivity index (AI) values were determined as an additional measure to characterize relative competitiveness of spec ies when grown in mixture (McGilchrist and Trenbath 1971; Roush and Radosevich 1985). Weed AI values < 0 indicate that smutgrass is less competitive than bahiagrass and AI values > 0 indicate that smutgrass is more competitive than bahiagrass. AI values fo r shoot biomass accumulation were calculated for bahiagrass and smutgrass varieties at e ach pH and density level using E quation 4 4 : AI = RY S RY B ( 4 4) In addition to these competition indices, data were also analyzed using PROC MIXED in SAS (SAS 2010 ) with mean shoot weight plant 1 pot 1 as the response variable and pH, de nsity, ratio, species, and interactions as fixed effects. Means were Results and Discussion Shoot biomass of Bahiagrass:Gi ant Smutgrass in the competitive mixtures Soil pH had a significant effect on the growth of both bahiagrass and giant smutgrass. When grown in monoculture, both bahiagrass and giant smutgrass growth was significantly higher at soil pH 5.5 than at the othe r two levels. Bahiagrass biomass grown at soil pH 4.5 was 65 and 52% lower at the low and high density, respectively, compared to plants grown at soil pH 5.5 (Table 4 1). Similarly, bahiagrass biomass at soil pH 6.5 was 40 and 45% lower at the low and high density, respectively, compared to plants grown at soil pH 5.5. Bahiagrass biomass when grown at soil pH 4.5 and 6.5 was not significantly different. Giant smutgrass growth was significantly different at all
52 soil pH levels. Biomass of giant smutgrass at s oil pH 4.5 was 88 and 70% lower at the low and high density, respectively, compared to plants grown at soil pH 5.5 Increasing the soil pH to 6.5 resulted in less biomass reduction compared to soil pH 4.5 At this pH plant biomass was no greater than 37% l ow er at both planting densities compared to plants grown at soil pH 5.5. These results indicate that soil pH 5.5 provided favorable conditions for growth of both bahiagrass and giant smutgrass. This was expected for bahiagrass because its productivity is c onsidered to be optimum at soil pH of 5.5 ( Mylavarapu et al 1997). Previous research conducted by Stephenson and Rechcigl (1991) has indicated that the optimum growth for most weed species occurred at pH 5.5. Similar results have been reported for large c rabgrass [ Digitaria sanguinalis (L.) Scop. ] where growth was found to be higher at soil pH 5.5 than at other pH levels (Buchanan et al. 1975; Johnson and Burns 1985 ). Soil pH significantly affected the competitiveness of bahiagrass and giant smutgrass. Gi ant smutgrass was more competitive than bahiagrass across all pH levels and densities. The RY diagrams indicate that at 4 plants pot 1 the relative competitive ability of giant smutgrass was greater than bahiagrass at equal proportions (50:50) or higher gi ant smutgrass proportions (Figure 4 2a, 4 3a and 4 4a). The magnitude of this competition was high except at pH 6.5 where giant smutgrass was only slightly more competitive than bahiagrass when compared to other pH levels. However, at 8 plants pot 1 gian t smutgrass was more competitive than bahiagrass across all pH levels (Figure 4 2b, 4 3b, and 4 4b). Similar results were obtained in a competition study of corn with 4 weed species at three levels of pH, where corn yields were reduced by weed competition at all pH levels (Weaver and Hamill 1985). Giant smutgrass
53 aggressivity index values (AI) at soil pH levels 4.5, 5.5, and 6.5 confirm the relative competitive ability of giant smutgrass (Table 4 2). However, giant smutgrass AI values at pH 4.5 were 84% hig her than pH 6.5 when grown at equal proportions (50:50), indicating that at low pH the aggressivity of giant smutgrass was higher. At both density levels, planting ratios of 50:50 and 25:75, giant smutgrass was more aggressive than bahiagrass (Table 4 2). In general, as the giant smutgrass proportion in the mixture increased, gian t smutgrass was more aggressive; as the proportion of giant smutgrass decreased, aggressivity of bahiagrass increased (75:25). Analysis of mean shoot weight data of bahiagrass and giant smutgrass when grown in equal proportions indicated that giant smutgrass had at least 4 times and 2 times higher biomass accumulation than bahiagrass at pH 5.5 and pH 6.5, respectively (Figure 4 5 a & b). The mean shoot weight analyses corroborate t he results of the competitive indices demonstrating that the relative competitive ability of giant smutgrass was higher than bahiagrass at pH 5.5. Results from both the RY and AI indicate that giant smutgrass is both more competitive and aggressive than ba hiagrass at the recommended soil pH level for optimum bahiagrass production. Therefore, managing soil pH alone will not likely be an alternative management strategy for giant smutgrass. Shoot biomass of Bahiagrass:Small Smutgrass in the competitive mixtur es Soil pH affects the growth and competitive ability of bahiagrass and small smutgrass. Biomass of both bahiagrass and small smutgrass grown in monoculture was significantly higher at soil pH 5.5 than when grown at soil pH 4.5 and 6.5. Bahiagrass biomass was 67 and 73% lower at the low and high planting density, respectively, when grown at soil pH 4.5 compared to those grown at soil pH 5.5 (Table 4 3) Increasing the soil pH to 6.5 resulted in less of a reduction as bahiagrass biomass was 50% less
54 compared to plants grown at soil pH 5.5, regardless of planting density. A similar trend was observed in small smutgrass biomass. Biomass of small smutgrass grown at soil pH 4.5 was reduced by 88% compared to plants grown at pH 5.5, regardless of planting density. Increasing the soil pH to 6.5, resulted in only a 39% reduction in small smutgrass biomass compared to those grown at soil pH 5.5, regardless of plant density (Table 4 3) These results indicate that the growth of both bahiagrass and small smutgrass is op timum at soil pH 5.5. In contrast to giant smutgrass, the replacement series diagrams of bahiagrass vs. small smutgrass suggest that bahiagrass had a greater relative competitive ability than small smutgrass at both planting densities across all pH levels except at pH 6.5. At 4 plants pot 1 bahiagrass was more competitive than small smutgrass at equal planting ratios or at ratios with higher proportions of bahiagrass (Figure 4 6a and 4 7a). However, small smutgrass was more competitive than bahiagrass at s oil pH 6.5 (Figure 4 8a). Similarly, at 8 plants pot 1 small smutgrass was more competitive than bahiagrass at pH 6.5; whereas, bahiagrass was more competitive than small smutgrass at pH 4.5 and 5.5 (Figure 4 6b, 4 7b, and 4 8b). The AI values indicated t hat bahiagrass was more aggressive than s mall smutgrass at all pH levels. An excep tion was observed at soil pH 6.5 at both density levels when planted at 50:50 and 25:75 proportions (Table 4 4 ). The AI values at the 50:50 proportions increased from 0.38 t o 0.26 at 4 plants pot 1 and from 0.23 to 0.11 at 8 plants pot 1 as soi l pH increased from 4.5 to 6.5. This indicated that the competitive ability of small smutgrass was improved by higher soil pH. Mean shoot weight data of bahiagrass and small smutgrass indicated that biomass accumulation of bahiagrass was 2 times higher than small smutgrass at pH 5.5 at both
55 planting densities. Conversely, small smutgrass biomass accumulation was 2 times greater than bahiagrass at pH 6.5 (Figure 4 9a) at the low planting density, but there was no significant difference in biomass accumulation at the high density level (Figure 4 9b). The mean shoot weight analysis supports the results of competitive indices suggestin g that bahiagrass has a better relative competitive abili ty than small smutgrass at pH 4.5 and 5.5. These data infer that small smutgrass is not as aggressive as giant smutgrass when compared to bahiagrass at low soil pH. The results obtained from this study indicate that both smutgrass varieties had a differen tial response to soil pH; giant smutgrass was more competitive than bahiagrass at all pH levels, whereas, small smutgrass was more competitive than bahiagrass only at pH 6.5. This differential response could be due to the biological differences observed be tween the two smutgrass varieties. We hypothesized that the raising the pH of acid soils might increase the competitiveness of bahiagrass over smutgrass varieties. However, the results demonstrated that increasing the soil pH made the environment conducive for growth of both bahiagrass and smutgrass varieties. Smutgrass seed germination experiments indicated that both smutgrass varieties germinate over a wide range of pH (Rana et al. 2012). Furthermore, in these studies there is some evidence to suggest th at management practices that favor bahiagrass growth are also likely to favor growth of smutgrass varieties. As for giant smutgrass, these results in dicate that amending soil pH may not be a likely option to increase the growth and competitive ability of b ahiagrass over giant smutgrass. However, for small smutgrass, it is likely to increase the aggressivity of bahiagrass in bahiagrass vs. small smutgrass mixture, unless the soil pH is raised above 5.5. The results from this
56 experiment suggest that giant smu tgrass might be expected to become a stronger competitor to bahiagrass, but future research needs to be conducted to compare the competitive abilities of giant and small smutgrass. Information gained in this study will be used to inform ranchers of the imp ortance of pH in terms of bahiagrass productivity and smutgrass management.
57 Table 4 1. Effect of pH on bahiagrass and giant smutgrass ( Sporobolus indicus var. pyramidalis ) shoot biomass when grown in monoculture at low (4 plants pot 1 ) and high (8 plants pot 1 ) density levels. Bahiagrass Giant Smutgrass pH Low a,b High Low High -----------------mean weight plant 1 ( g) -------------------------4.5 0.95 (0.06) b 0.98 (0.17) b 0.82 (0.12) c 1.19 (0.21) c 5.5 2.71 (0.28) a 2.00 ( 0.32) a 6.72 (0.63) a 3.86 (0.20) a 6.5 1.49 (0.08) b 1.20 (0.19) b 4.24 (0.48) b 2.68 (0.23) b a Means within a density level followed by the same letter are not significantly different 0.05. b Values in parentheses represent the standard error of the mean.
58 Table 4 2. Effect of pH on aggressivity index values as determined from bahiagrass and giant smutgrass ( Sporobolus indicus var. pyramidalis ) relative shoot weight data at low (4 plants pot 1 ) and high (8 plants pot 1 ) density levels of mixtures. Bahiagrass:Giant Smutgrass Low density a,b,c High density pH 75:25 50:50 25:75 75:25 50:50 25:75 4.5 0.29 (0.09) A b 0.31 (0.06) A a 0.52 (0.09) A a 0.25 (0.18) A b 0.30 (0.08) A a 0.68 (0.10) A a 5.5 0.40 (0.07) A c 0.18 (0.05) AB b 0.54 (0.06) A a 0.17 (0.07) A c 0.36 (0.08) A b 0.63 (0.08) A a 6.5 0.44 (0.05) A c 0.05 (0.03) B b 0.61 (0.07) A a 0.32 (0.12) A c 0.20 (0.09) A b 0.69 (0.06) A a a Means within a b Means within a pH level followed by the same Protected LSD P c Values in parentheses represent standard error of the mean.
59 Table 4 3. Effect of pH on bahiagrass and small smutgrass ( Sporobolus indicus var. indicus ) shoot biomass when grown in monoculture at low (4 plants pot 1 ) and high (8 plants pot 1 ) den sity levels. Bahiagrass Small Smutgrass pH Low a,b High Low High -------------------mean weight plant 1 (g) ----------------------4.5 1.10 (0.15) c 0.90 (0.19) c 0.42 (0.13) c 0.33 (0.10) c 5.5 4.58 (0.52) a 3.25 (0.28) a 3.83 (0.42) a 2.71 ( 0.15) a 6.5 2.30 (0.25) b 1.70 (0.16) b 2.33 (0.24) b 1.68 (0.13) b a Means within a density level followed by the same letter are not significantly different 0.05. b Values in parentheses represent the standard error of the mean.
60 Table 4 4 Effect of pH on aggressivity index values as determined from bahiagrass and small smutgrass ( Sporobolus indicus var. indicus ) relative shoot weight data at low (4 plant s pot 1 ) and high (8 plants pot 1 ) density levels of mixtures. Bahiagrass:Small Smutgrass Low density a,b,c High density pH 75:25 50:50 25:75 75:25 50:50 25:75 4.5 0.54 (0.08) A b 0.38 (0.11) B b 0.62 (0.14) A a 0.63 (0.07) A c 0.23 (0.06) B b 0.63 (0.10) A a 5.5 0.64 (0.20) A c 0.09 (0.05) B b 0.46 (0.07) A a 0.63 (0.10) A b 0.28 (0.08) B b 0.62 (0.15) A a 6.5 0.59 (0.10) A b 0.26 (0.12) A a 0.57 (0.14) A a 0.52 (0.09) A b 0.11 (0.12) A a 0.66 (0.13) A a a Means within a ratio fol b Means within a pH level followed by the same c V alues in parentheses represent standard error of the mean.
61 Figure 4 1. Influence of hydrated lime on the pH of a Smyrna Sand. Each data point represents the mean ( SE) of four observations.
62 Figur e 4 2 Relative shoot weight of bahiagrass and giant smutgrass ( Sporobolus indicus var. pyramidalis ) at pH 4.5 in competitive mixtures for (A) the low planting density (4 plants pot 1 ), and (B) the high planting density (8 plants pot 1 ). Each data point represents the mean of six observations with standard error of mean.
63 Figure 4 3 Relative shoot weight of bahiagrass and giant smutgrass ( Sporobolus indicus var. pyramidalis ) at pH 5.5 in competitive mixtures for (A) the low planting density (4 plants pot 1 ), and (B) the high planting density (8 plants po t 1 ). Each data point represents the mean of six observations and the error bars are standard error of the mean.
64 Figure 4 4 Relative shoot weight of bahiagrass and giant smutgrass ( Sporobolus indicus var. pyramidalis ) at pH 6.5 in competitive mixtur es for (A) the low planting density (4 plants pot 1 ), and (B) the high planting density (8 plants pot 1 ). Each data point represen ts the mean of six observations and the error bars are standard error of the mean.
65 Figure 4 5. Mean shoot weight of bahi agrass and giant smutgrass ( Sporobolus indicus var. pyramidalis ) at three pH levels in competitive mixtures at the 50:50 proportion for (A) the low planting density (4 plants pot 1 ), and (B) the high planting density (8 plants pot 1 ). Values represen t the mean of six observations and the error bars are Standard error of the mean Means with in a pH level followed by the same letter are not significantly different
66 Figure 4 6. Relative shoot weight of bahiagr ass and small smutgrass ( Sporobolus indicus var. indicus ) at pH 4.5 in competitive mixtures for (A) the low planting density (4 plants pot 1 ), and (B) the high planting density (8 plants pot 1 ). Each data point represents the mean of six observations and t he error bars are standard error of the mean.
67 Figure 4 7. Relative shoot weight of bahiagrass and small smutgrass ( Sporobolus indicus var. indicus ) at pH 5.5 in competitive mixtures for (A) the low planting density (4 plants pot 1 ), and (B) the high planting density (8 plants pot 1 ). Each data point represents the mean of six observations and the error bars are standard error of the mean.
68 Figure 4 8. Relative shoo t weight of bahiagrass and small smutgrass ( Sporobolus indicus var. indicus ) at pH 6.5 in competitive mixtures for (A) the low planting density (4 plants pot 1 ), and (B) the high planting density (8 plants pot 1 ). Each data point represents the mean of six observations and the error bars are standard error of the mean.
69 Figure 4 9. Mean shoot weight of bahiagrass and small smutgrass ( Sporobolus indicus var. indicus ) at three pH levels in competitive mixtur es at the 50:50 proportion for ( A) the low planting density (4 plants pot 1 ), and ( B) the high planting density (8 plants pot 1 ). Values represent the mean of six observations and the error bars are standard error of the mean Means with in a pH level followed by the Protect
70 CHAPTER 5 INTEGRATED MANAGEMEN T TECHNIQUES FOR LON G TERM CONTROL OF GIANT SMUTGRASS ( S porobolus indicus var. pyramidalis ) IN BAHIAGRASS PASTURES Bahiagrass ( Paspalum notatum Fluegg), a predominant pasture grass utilized by the beef cattle industry in Florida, is estimated to cover at least 1 million h ectares in the state and over 2.5 million hectares throughout the southeastern United States (Burton et al. 1997; Chambliss 1999). In the United States, Florida is the third largest beef cattl e producing state east of the Mississippi River and is ranked 10th in the nation. With over 1.1 million head of beef cattle, bahiagrass plays an important role in maintaining the productivity of the beef cattle industry, but smutgrass has been a major obst acle over the la st 60 years (McCaleb et al. 1963 ; McCaleb and Hodges 1971). Smutgrass [ Sporobolus indicus (L.) R. Br. ] a perennial bunch type grass, has been a serious invader in established bahiagrass pastures for many years in Florida. According to a 2 007 survey of beef and forage practices in south Florida smutgrass ( Sporobolus indicus (L.) R. Br.) is listed as the third most troublesome pasture weed after dogfennel [ Eupatorium capillifolium (Lam.) Small] and tropical soda apple ( Solanum viarum Dunal ) (Crawford 2007). Two varieties of smutgrass have been reported in Florida: small smutgrass [ Sporobolus indicus (L.) R. Br. var. indicus ] and giant smutgrass [ Sporobolus indicus (L.) R. Br. var. pyramidalis (P. Beauv.) Veldkamp.] ( Wunderlin and Hansen 2003 ). Small smutgrass was the predominant smutgrass variety in the 1970s and 1980s (Mislevy and Curry 1980); however, by the early 1990s giant smutgrass was first reported in south Florida pastures where it rapidly proliferated and presently is the dominant s mutgrass variety throughout central and south Florida (Sellers et al. 2009).
71 Forage losses due to lack of smutgrass control is a major problem in bahiagrass pastures (Ferrell et al. 2006; Mislevy et. al 2002). Smutgrass is considered one of the most common and problematic pasture weed in Florida because of its ability to out compete desirable species (Ferrell et al. 2006), high rate of seed germination (Rana et al. 2012), prolific seed production (Currey et. al 1973), and limited herbicide options (Brecke 1 981; Mislevy et al. 1999). Additionally, cattle generally avoid mature smutgrass, but they readily consume tender re growth of smutgrass within 1 to 3 weeks of burning or mowing (Mullahey 2000). This may be because the forage quality of young smutgrass is similar to bahiagrass (Mullahey 2000). Managing smutgrass intensively during this resting period of 1 to 3 weeks has been reported to reduce growth and vigor of smutgrass (Mullahey 2000). However, it is difficult for commercial producers to employ such lab or intensive management strategies through rotational grazing. Smutgrass is slow to establish, but once established, control of smutgrass becomes difficult. In the past, cultural practices when employed alone, did not provide a consistent level of control on a long term basis. Mowing alone or in combination with hexazinone did not influence giant smutgrass density (Ferrell and Mullahey 2006); rather, it increased seed dispersal (McCaleb et al. 1963). Complete renovation of pastures heavily infested with smu tgrass resulted in unsatisfactory and variable control (McCaleb et al. 1963). Since plants in preceding years create large and persistent seed banks, McCaleb et al. (1963) suggested that reducing the potential for re infestation may provide effective long term control of smutgrass. Burning is an inexpensive method to remove old growth on smutgrass plants, however, there has not been any research done on burning followed by herbicide application for smutgrass control. An initial burn
72 in the management progra m may reduce the existing seed bank, stimulate seed germination, and make seedlings available for control by follow up herbicide applications (Ditomaso et al. 2006). Considering lack of smutgrass control with cultural practices, management of smutgrass ha s mostly been accomplished through the use of herbicides. Dalapon (2,2 Dichloropropionic acid), the first herbicide widely used for smutgrass control, was substantially injurious to bahiagrass at rates which provided > 80% smutgrass control (Brecke 1981; M islevy and Currey 1980). In 1989, hexazinone under the trade name of Velpar was registered for smutgrass control in bahiagrass pastures (Mislevy et. al 2002). There are some discrepancies in the literature concerning application timing and rate of hexazino ne, but it has consistently provided optimum control of both smutgrass varieties at 1.12 kg ha 1 when applied during mid summer and early fall (Mislevy et al. 2002, Ferrell et al. 2006; Wilder 2011). Control of giant smutgrass is excellent first year after application, but re infestation of giant smutgrass to its initial densities often occurs within 2 3 years following hexazinone application (J.J Mullahey, personal communication). Additionally, hexazinone is one of the most expensive pasture herbicides, costing approximately $20 per liter ($90 per kg of active ingredient). Considering that hexazinone may have to be applied at full application rates every two to three years in highly infested bahiagrass pastures, management of smutgrass is very cost intens ive. Previous research has indicated that techniques for management of smutgrass have been focused on high rates of fertilizer application, herbicide treatments, or maintenance of improved pastures. These methods reduced smutgrass infestations, but
73 not ov er a long period of time indicating that single management programs have often failed to provide effective smutgrass control. Therefore, integrated management strategies for smutgrass are necessary. The objective of this research was to develop a comprehe nsive giant smutgrass management plan by integrating cultural and chemical inputs into a long term management program. The results from these experiments may help to develop best management practices for giant smutgrass control in improved perennial grass pastures. Materials and Methods Site Descriptions. In 2008, three field experiments were initiated simultaneously in establ ished bahiagrass pastures at diff erent locations. Experiments 1 and 2 were Lorida, FL xperiment 3 was conducted near Ona, FL nd The soil type found in Zolfo springs and Ona was Pomona fine sand (sandy, si liceous, hyperthermic Ultic Alaquod), and in Lorida was Immokalee fine sand (sandy, siliceous, hyperthermic Arenic Alaquod). Soil pH at all sites was 5.0 (0.1). Giant smutgrass was the predominant variety at all locations and in the text from hereafter it is mentioned as smutgrass. Unless otherwise specified, the treatments in all experiments were applied in July, and herbicides were applied by a tractor sprayer at a carrier v olume of 281 liters per hectare. Experiment 1 The objective of this experiment w as to determine if a n integrated approach of burning, tillage and herbicide application can provide smutgrass suppression The experimental design was a split plot with b urn and no burn as the whole plot factor and
74 treatments as the sub plot factor. Burnin g was performed in the early spring of 2008, a typical timing for burning operations in south Florida. Sub plot treatments consisted of hexazinone, complete renovation, and fall roller chopping. H exazinone was applied at 1.12 kg ha 1 during July as descri bed above, complete renovation was performed by applying glyphosate at 4.48 kg ha 1 in early June, followed by seed bed preparation and planting of bahiagrass seed at 22 kg ha 1 and fall roller chopping carried out in October. Subplots measured 30 x 30 m a nd treatments were arranged in a randomized block design with four replications. In 2009, hexazinone was applied to the entire study site at 0.56 kg ha 1 The Lorida site was abandoned in 2009 as the experimental site was mistakenly destroyed by the produc er. Experiment 2 The o bjective of this experiment was to reduce the competition from smutgrass through the use of hexazinone and promote the growth of bahiagrass by application of nitrogen. The experimental design was a randomized complete block design wi th a 2X2 factorial of fertiliz ation and herbicide application, respectively. In 2008, hexazinone at 1.12 kg ha 1 was applied to the entire experimental area. In 2009, hexazinone was applied at 0 or 0.56 kg ha 1 followed by nitrogen application at 0 or 56 kg ha 1 two wee ks later. The plot size was 30 X 30 m with each treatment replicated three times. Experiment 3 The objective of this experiment was to determine if sequential hexazinone applications will provide extended smutgrass control. The experimental design was a randomized complete block design with treatment s arranged in a strip plot In 2008, hexazinone was applied at three different rates of 0.56, 0.84, and 1.12 kg ha 1 In 2009, second application of hexazinone was superimposed with three rates of hexazinone at
75 0.28, 0.56 and 0.84 kg ha 1 An untreated check was included both years The plot size was 6 X 6 m with three replications and smutgrass plants were counted in every plot. Treatments were applied using an ATV mounted sprayer e quipped using f lat fan nozzles calibrated to deliver 281 liters per hectare. In experiment s 1 and 2, annual smutgrass density was evaluated using 1 m 2 quadrats (5 plot 1 ); smutgrass plants per quadrat were counted 12 months after treatment (MAT) from 2009 to 2011. Additi onally, pre treatments counts were conducted for experiment 1 and 2 in 2008 to examine differences in smutgrass density before and after applying treatments. Statistical Analysis For all experiments normality and equal variance assumptions were examined a nd data were transformed when necessary. Data were analyzed using PROC MIXED in SAS (SAS 2010) and mean separation was appropriate. However, non transformed means are presented in the text and figures. Data were presented separat ely by year except experiment 3. I nteracti ons not mentioned in the text w ere not significant (P 0.05). Number of plants m 2 from 2008 to 2011 (36 months after first treatment application), was the respon se variable in experiments 1 and 2. In experiment 1 analysis, burn/no burn (w hole plot), treatments (sub plot), and year were considered as fixed effects. Year was considered fixed because year effects and interactions of year with burn/no burn and treatme nts were of interest. Years were analyzed as repeated measures and replicates and their interaction as random effects. For experiment 2, a rank transformation was performed. Nitrogen and hexazinone rates were considered fixed effects, with site, replicates and their interactions as random effects. For experiment 3, a log transformation was used to
76 normalize the data. Smutgrass counts in 2010 and 2011, 24 months after the first and second hexazinone application, respectively, were the response variable. Hexa zinone rates app lied in 2008 and 2009, and interactions were considered fixed effects, with site, replicates, and interaction with hexazinone rates as random effects. Results and Discussion Experiment 1 Burning had no significant (P = 0.360) effect on sm utgrass control. Smutgrass density in the subplots were significantly different (P = 0.0001) as a result of the cultural and herbicide treatments applied, and year by treatment interaction was also significant (P = 0.0008). Initial counts of 2008 indicate that smutgrass density averaged 3 plants per m 2 throughout the plots (Table 5 1). In 2009 and 2010, 12 and 24 months after the first and second hexazinone application, smutgrass density significantly decreased by 83 and 95%, respectively, compared to the p re treatment counts of 2008. This was expected because research conducted by Wilder (2011) indicates that hexazinone when applied at rates of 1.12 kg ha 1 provided > 90% control 12 MAT. Smutgrass density in 2011, 24 months after the second hexazinone appli cation, was reduced by 93% when compared to 2008. No significant differences were observed in the number of smutgrass plants from 12 months after first hexazinone application (2009) to 24 months after second hexazinone application (2011), indicating that h exazinone provided excellent control of giant smutgrass during this evaluation period. Control of smutgrass was lowest in the completely renovated plots compared to hexazinone and fall roller chopping 12 months after the first treatment (Table 5 1). Smutg rass density in 2009 was 91 and 94% lower in plots treated with hexazinone and fall roller chopping, respectively, than in plots that were completely renovated. This may
77 be because soil disturbance caused by the tillage operations resurfaced the seeds pres ent deep in the soil profile, and stimulated germination under optimum conditions. Moreover, giant smutgrass seed can germinate at or less than 3 cm depths (Rana et al. 2012). Thus, these data infer that pasture renovation did not destroy all smutgrass pla nts, and new seedlings emerged from seed in the soil. In contrast, 0.56 kg ha 1 of hexazinone in 2009 which succeeded the renovation treatment in 2008 significantly reduced the smutgrass density by 92% in 2010 (12 months after second treatment application) The reason for achieving an excellent control even with a low rate (0.56 kg ha 1 ) was may be due to the sufficient soil moisture at the time of hexazinone application or may be the young plants are more susceptible to hexazinone at this rate tha n the old er plant s (Ferrell and Mullahey 2006; Wilder 2011) (Table 5 2). Since the present cost of pasture renovation is approximately $ 1 000 per hectare, it may be unpractical to use pasture renovation, unless smutgrass infestation is 5 0%. Furthermore, if pasture renovation is the method of choice due to a high smutgrass infestation, a hexazinone application will likely be necessary to achieve optimum control. Fall roller chopping significantly reduced smutgrass ground cover by 89 a nd 99% in 2009 and 2010, respectively, compared to 2008, before applying any treatments (Table 5 1). In 2009, the control provided by fall roller chopping was similar to that provided by hexazinone (1.12 kg ha 1 ). Rainfall, as compared to the last 30 years was substantially lower from November 2008 April 2009 (Table 5 2). Because of low moisture availability, the destroyed smutgrass plants were not able to re root and conditions were not favorable for new seedlings to germinate until the following growin g
78 season. In 2010, 12 months after applying hexazinone (0.56 kg ha 1 ) in 2009, smutgrass density was reduced by 91% compared to 2009. However, 24 months (2011) after the second treatment application, smutgrass density began to increase. Similar results wer e reported when roller chopping was followed by dalapon application for smutgrass control (Mislevy et a l. 1980). Weed seeds remain stratified under redu ced tillage (Hoffman et al. 1998 ), including pastures or rangeland where tillage operations are often no t performed. Soil disturbance exposes these stratified seeds to favorable conditions and germination often occurs in b are spots left after tillage and herbicide application. Moreover, weed seeds buried by tillage operations retain viability longer than tho se near the soil surface (Roberts 1964); therefore, it is important to ensure that the seeds remain adequately covered as future disking or tillage can carry the seeds back to the soil surface. Similar results were reported by McCaleb et al. (1963 ) indicat ing that cultivation provided variable and unsatisfactory control of smutgrass, and a follow up treatment of herbicides was necessar y to achieve complete control. Cultivation was often used as a complement to herbicides when fuel and farm labor were less expensive, but currently it is not very cost effective. In 2011, although there is no significant difference between cultural treatments that succeeded hexazinone application and those which received two sequential application s of hexazinone, the results i ndicate that smutgrass density may continue to rise at a faster rate in plots that were renovated or roller chopped as compared to hexazinone application without tillage. It might be too early to determine the best management plan, but it is likely that an y tillage operation may result in an increase in smutgrass density compared to sequential applications of hexazinone.
79 Experiment 2 There were no significant differences in smutgrass density as a result of treatments applied in 2008 and 2009. The initial sm utgrass counts before applying any treatments were 2 plants m 2 (Table 5 3). As expected, the density of smutgrass in 2009 was reduced by at least 91% by the initial hexazinone application in 2008. There were no differences among treatments in 2010, althou gh no smutgrass was recorded in plots that received additional hexazinone (0.56 kg ha 1 ) and nitrogen. Smutgrass was observed in all plots in 2011, but there were no significant differences among treatments. However, there is a trend for reduced smutgrass density in plots that received both hexazinone and nitrogen in 2009. A follow up hexazinone treatment likely controlled smutgrass plants which were missed during the first application or new seedlings that emerged from the seed bank. Additionally, a fertil izer application 2 weeks after hexazinone application may have aided bahiagrass growth. It is likely that nitrogen fertilization helped to fill in the bare spots left by dead smutgrass thereby giving bahiagrass a competitive advantage over smutgrass. There was also a trend for increased smutgrass density in plots only receiving nitrogen in 2009. In 2011, 24 months after second treatment application, smutgrass had started re infesting the treated plots, but the hexazinone and fertilizer treatment appears to have the best impact on suppressing smutgrass. These results indicate that although treatments super imposed over the initial hexazinone application did not result in any significant differences; however there is a trend for increased smutgrass density in plots that did not receive any further herbicide or fertilizer applications.
80 Experiment 3 In 2010, the hexazinone rates applied in 2008 and 2009 were significant (P = 0.0065 and P = 0.0038), but there was no interaction between the rates applied over 2 ye ars. However, in 2011, 24 months after the second treatment application, interaction between hexazinone rates applied in 2008 and 2009 were significant (P = 0.04). This interaction is important because it indicates the rates which provide the best control of hexazinone if we split the application across two years. The plots which received 2 applications of hexazinone, except for the sequential application of 0.28 kg ha 1 resulted in acceptable levels of control. The density of smutgrass was reduced by 80 a nd 87% in plots that received 0.84 kg ha 1 and 1.12 kg ha 1 respectively, as compared to the untreated check (Table 5 4 ). The single application treatment of 0.56 kg ha 1 did not decrease smutgrass density as compared to the untreated check. These data su pport the findings of Wilder (2011), indicating that variability in the level of gi ant smutgrass control increase d at hexazinone rates less than 0.56 kg ha 1 Sequential applications of hexazinone at 0.56 followed by 0.56 kg ha 1 0.56 followed by 0.84 kg ha 1 0.84 followed by 0.56 kg ha 1 and 1.12 followed 0.84 kg ha 1 decreased smutgrass density by at least 90% as compared to the untreated. This level of control was not significantly different from control by hexazinone applied at 0.84 followed by 0.84 kg ha 1 1.12 followed by 0.28 kg ha 1 and 1.12 followed by 0.56 kg ha 1 but these treatments provided 60% lower smutgrass control than the untreated checks. The rainfall in both years of application was similar (Table 5 2), and the reasons for differenc es among these treatments remain unexplained. However, rainfall within one week of application at each site was not recorded, which may have had a significant impact on hexazinone efficacy.
81 Hexazinone is a fairly expensive herbicide, if we consider the co st of these rates, it may be more beneficial to apply 0.56 kg ha 1 in first year followed by 0.56 kg ha 1 the next year to allow decreased costs and potentially treat more acreage For example, if a rancher has an annual budget of $5,000 for weed control a nd applied the current recommended use rate of hexazinone at 1.12 kg ha 1 at a cost of $100 per hectare, only 50 hectares will be able to be treated. However, if the rancher were to treat the smutgrass infestation with 0.56 kg ha 1 at a cost of $50 per hec tare, 100 hectares will be able to be treated. In addition to being able to treat additional smutgrass infested pastures, a follow up application of 0.56 kg ha 1 may provide control of any smutgrass plants which were missed the previous year or new seedlin gs that emerge from the seed bank. The original objective of this research was to determine an integrated management strategy that provides consistent long term smutgrass control in bahiagrass pastures. Since it takes three to four years for smutgrass dens ity to return to pre treatment levels, it is simply too early to determine if we have found a method that accomplishes this objective. However, the data trends from experiment one indicate that hexazinone alone may, in fact, be the best option rather than introducing tillage to remove large smutgrass plants prior to applying hexazinone. Additionally, applying nitrogen two weeks after hexazinone appears to have greater impact on reducing smutgrass reinfestation than hexazinone alone, indicating that promotin g bahiagrass growth quickly after hexazinone application may increase the competitive ability of bahiagrass over giant smutgrass. Lastly, sequential hexazinone applications, when applied at 0.56 kg ha 1 or greater in the third experiment, tended to result in increased
82 control as compared to single applications. Overall, it appears that sequential applications of hexa zinone may have more impact than implementing tillage practices. However, it is too early to deduce a definitive impact of any of the practices examined on long term giant sm utgrass control.
83 Table 5 1 E ffect of hexazinone herbicide renovation, and fall roller chopping on smutgrass density from 2009 to 2011. Treatments Year a 2008 2009 2008 b 2009 2010 2011 ----------------No. of plant s m 2 ----------------Hexazinone Hexazinone 2.80 a 0.48 bc 0.13 cd 0.18 bc Renovation Hexazinone 2.88 a 5.53 a 0.23 bc 0.60 b Fall roller chopping Hexazinone 2.93 a 0.33 bc 0.03 d 0.39 b a Means followed by the same letter within columns and rows are not significantly b Sub plot treatments: Hexazinone applied at 1.12 kg ha 1 in 2008 followed by hexazinone at 0.56 kg ha 1 in 2009 R enovation performed by applying glyphosate at 4.48 kg ha 1 in June followed by seed bed preparation in 2008, followed by hexazinone at 0.56 kg ha 1 in 2009 F all roller chopping in October 2008, followed by hexazinone at 0.56 kg ha 1 in 2009 c Initial smutgrass counts in 2008 before applying any treatments.
84 Table 5 2. Summary of rainfall by months at Ona (south Florida) during the duration of all experiments. Rainfall data was collected by the Florida Automated Weather Network. Month 2008 2009 2010 2011 30 yr Avg ---------------------------(cm) ------------------------Jan 2.4 3.0 5.0 6.3 5.3 Feb 4.0 0.9 6.1 0.9 6.6 Mar 5.7 2.6 15.0 14.8 7.9 Apr 0.8 1.2 7.2 2.0 6.6 May 7.1 16.8 16.5 3.4 9.4 Jun 25.3 17.1 10.9 12.8 22.1 July 19.8 19.5 14.5 18.8 21.3 Aug 25.4 31.4 30.0 28.8 21.1 Sep 14.3 8 .5 9.0 30.7 18.5 Oct 4.2 0.7 0.0 10.0 7.6 Nov 2.0 2.9 6.8 0.4 4.8 Dec 3.2 12.2 2.1 0.3 5.1 Total 114.2 116.8 123.1 129.2 136.4 Table 5 3. Effect of hexazinone and nitrogen applica tion on smutgrass density from 2009 to 2011. Treatments a Year 2008 2009 2008 b 2009 2010 2011 Hexazinone rate (kg a.i. ha 1 ) Hexazinone rate (kg a.i. ha 1 ) Nitrogen rate (kg ha 1 ) ----------No. of plants m 2 --------1.12 0.00 0.00 1.83 a 0.17 a 0.17 a 0.63 a 1.12 0.00 56.00 1.88 a 0.12 a 0.18 a 0.74 a 1.12 0.56 0.00 2.23 a 0.13 a 0.10 a 0.67 a 1.12 0.56 56.00 2.13 a 0.10 a 0.00 a 0.30 a a Means within a year followed by the same letter within columns and rows are not represent mean of 6 observations. b Initial smutgrass counts before applying any treatments.
85 Table 5 4. Effect of sequential application of hexazinone on smutg rass density 24 MAT. Each data point represent s mean of six observations Treatments a 2008 Hexazinone rate (kg a.i. ha 1 ) 2009 Hexazinone rate (kg a.i. ha 1 ) No. of plants plot 1 (24 MAT b ) Annual Cost in 2008 ha 1 ($) Annual Cost in 2009 ha 1 ($) Total cost in 2 years ha 1 ($) 0.00 0.00 11.7 a 0.00 0.00 0.00 0.00 0.28 17.2 ab 0.00 25.00 25.00 0.00 0.56 12.7 abcdef 0.00 50.00 50.00 0.00 0.84 5.0 efg 0.00 76.00 76.00 0.56 0.00 13.0 abcd 50.00 0.00 50.00 0.56 0.28 11.7 abc 50.00 25.00 75.00 0.56 0.56 1.0 fg 50.00 50.00 100.00 0.56 0.84 1.3 fg 50.00 76.00 126.00 0.84 0.00 2.5 cdefg 76.00 0.00 76.00 0.84 0.28 7.7 abcde 76.00 25.00 101.00 0.84 0.56 0.8 g 76.00 50.00 126.00 0.84 0.84 4.7 g 76.00 76.00 152.00 1.12 0.00 1.5 g 10 0.00 0.00 100.00 1.12 0.28 5.7 bcdefg 90.00 25.00 125.00 1.12 0.56 4.8 bcdefg 90.00 50.00 150.00 1.12 0.84 1.2 defg 90.00 76.00 176.00 a Means within a year followed by the same letter are not significantly different according b Abbreviati ons: MAT, month after treatment.
86 CHAPTER 6 CONCLUSION Both smutgrass varieties germinated over a broad range of environmental conditions, indicating their capability of year round ger mination. These results indicate that both species have the ability to germinate over a wide range of environmental conditions, but that germination is inhibited by moisture stress and depth of burial. However, germination is only likely to occur under field conditions during the summer or fall growing season when rainfall is prevalent. Considering that giant smutgrass prefers higher temperatures than small smutgrass, the advent of rainfall from June through September is conducive for germ ination. Practices that focus on the germination pattern of smutgrass, including timing of hexazinone application or months best suited f or tillage operations may lead to better long term management of smutgrass in Florida. The results from the competiti on study indicated that giant smutgrass is more competitive than bahiag rass at soil pH levels 4.5, 5.5, and 6.5, whereas bahiagrass is more competitive than small smutgrass except at soil pH 6.5. Amending soil pH may not be a likely option to increase the growth and competitive ability of bahiagrass over giant smutgrass. However, for small smutgrass, it is likely to increase the aggressivity of bahiagrass in bahiagrass vs. small smutgrass mixtures, unless the soil pH is greater than 5.5. Cultural treatments did not provide satisfactory giant smutgrass control; however, cultural treatments succeeded by a hexazinone application the following year seems to provide a good control for 12 months after treatment. Complete renovation with glyphosate is not a likely option for smutgrass control because smutgrass density 12 months after treatment was at least 12 times higher than hexazinone or fall roller
87 chopping. Renovation followed by hexazinone application reduced smutgrass density, but re infestation potential in renovated plots tends to be higher than roller chopping and hexazinone treatment. Roller chopping provided good control 12 months after treatment and a follow up treatment of hexazinone further reduced smutgrass ground cover. From these results it appears that smutgrass can quickly dominate a pasture especially after soil disturbance. The hexazinone fertilizer management study indicates that smutgrass density tends to be higher in plots that did not receive any further herbicide or fertilizer applications, consequently, adding nutrients can increase forage production and allow the bahiagrass to quickly fill in the voids created by dying smutgrass. Currently, he xazinone at the rate of 1.12 kg ha 1 costs $100 per hectare. Data from the sequential hexazinone experiments indicated that several rates where sequential application of hexazinone provided optimum control as compared to single hexazinone applications. Howe ver, hexazinone rate of 0.56 kg ha 1 in two applicatio ns over a period of 2 years appears to be most cost effective because it provided simi lar level of control as 1.12 kg ha 1 If the commercial producers have a budget, they will be able to cover more area if they split the application into 2 years. In conclusion, there is a differential response am ong the two varieties of smutgrass. Giant smutgrass performs favorably in higher temperatures, greater burial depths, and low and high pH soils levels indicating that it is more vigorous and aggressive than small smutgrass; this is why it has been such a p roblem in pastures and grazing lands in south Florida. Correcting soil pH may not affect bahiagrass competitive ability against giant smutgrass. Since, germination of both these varieties is most likely under summer or fall, the cultural or
88 herbicide appli cations should be conducted during this time of the year. Roller chopping succeeded by hexazinone application, hexazinone followed by a combination of hexazinone and fertilizer application and sequential applications of hexazinone over a period of 2 years hold promise for the future. Therefore, further data needs to be collected for these field experiments to give a conclusive best management plan for long term management of smutgrass.
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97 BIOGRAPHICAL SKETCH Neha Rana was born in 1983 to Harish and Neelam Rana. The younger of two children, she comes from the state of Him achal Pradesh, a captivating region of the Indian Himalayas. She attended Panja b University, Chandigarh and obtained a Bachelor s of Science degree in 2005 and Masters of Science degree in 2007 in b otany Upon completion of she worked at the Ins titute of Microbial technology, Chandigarh In fall 2008 she attended the University of Florida and obtained a doctor of philosophy degree in agronomy in spring 2012