The Pennsylvania State University. The Graduate School. College of Agricultural Sciences THE EFFECTS OF TRINEXAPAC-ETHYL AND CULTIVATION ON THE

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1 The Pennsylvania State University The Graduate School College of Agricultural Sciences THE EFFECTS OF TRINEXAPAC-ETHYL AND CULTIVATION ON THE DIVOT RESISTANCE OF KENTUCKY BLUEGRASS CULTIVARS A Thesis in Agronomy by Thomas J. Serensits Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science August 2008

2 The thesis of Thomas J. Serensits was reviewed and approved* by the following: Andrew S. McNitt Associate Professor of Soil Science-Turfgrass Thesis Advisor Jeffrey A. Borger Instructor of Turfgrass Weed Management Peter J. Landschoot Professor of Turfgrass Science Richard N. Arteca Professor of Horticulture Physiology David M. Sylvia Professor of Corp and Soil Science Head of the Department of Crop and Soil Sciences *Signatures are on file in the Graduate School ii

3 ABSTRACT Divots on athletic fields negatively affect playing quality and can result in injuries to athletes. A divot is a piece of turf partially or completely gouged out of surrounding turf by an athlete wearing studded footwear. Divots occur most often in sports that require rapid changes of direction, such as football. Excessive divoting indicates a playing surface with low surface stability. Low surface stability can affect the integrity of the game as players will lose footing frequently. It also affects the aesthetic quality of the surface. Professional and collegiate athletic field rootzones are often constructed with a high percentage of sand, which increases drainage and minimizes the effects of compaction. However, because sand-based rootzones do not have high amounts of silt and clay, soil cohesion is low. As a result, the stability of sand-based rootzones is largely determined by the surface and below-ground biomass of the turfgrass stand. Excessive foot traffic on athletic fields causes a loss of both shoot and root density and, therefore, biomass is reduced. As turfgrass biomass is reduced, the network of tillers, roots, rhizomes, and stolons weakens, reducing surface stability. Applications of the plant growth regulator trinexapac-ethyl (TE) have been reported to increase tiller density and affect the rooting of Kentucky bluegrass (KBG) (Poa pratensis L.). Increased turfgrass density and rooting may increase divot resistance. iii

4 Practitioners have reported increased root and rhizome growth through a combination of vertical mowing and core cultivation. These cultural practices may stimulate an increase in below ground biomass by severing existing roots and rhizomes and creating voids into which roots can grow. This additional growth may increase divot resistance. The objectives of this research were to 1) determine the effects of two TE treatment regimes and cultivation on the divot resistance of nine KBG cultivars under varying levels of simulated traffic and 2) determine if measured turfgrass properties including ground cover, tiller density, below-ground biomass, and shear strength are related to divot resistance. The TE application regimes included applying TE monthly from May-July and May- Oct. The final application for the May-July regime coincided with the beginning of a typical football season and the onset of traffic simulation in this experiment. Plots treated under this regime returned to normal (uninhibited) growth for the duration of the simulated traffic period, which ended in late Oct. Plots managed under the May- Oct. regime continued to receive TE applications throughout the simulated traffic period. The cultivation treatment, which included a combination of vertical mowing and core cultivation, was performed in early May. The vertical mower blades penetrated iv

5 approximately 1.3 cm below the soil surface in order to sever existing roots and rhizomes. Core cultivation was performed immediately after vertical mowing. Following the cultivation treatment, sand topdressing was applied to a depth of 0.95 cm over the entire plot area. Nine cultivars of KBG were established on a sand-based rootzone in The cultivars included Baron, Langara, Limousine, Midnight, Princeton 105 ( P105 ), Penn State A, Penn State B, Rugby II, and Touchdown. Simulated traffic was applied to the test area from late July through late Oct. using the Brinkman traffic simulator. Traffic levels included no (0 passes/week), medium (6 passes/week), and high traffic (12 passes/week). In Nov. of 2006 and 2007, divots were created using a device consisting of a weighted pendulum with a golf club head attached to one end (PENNSWING). The size of each divot was then measured. Small divots indicated high divot resistance. Several KBG cultivars exhibited differences in divot size. Plots of Limousine, Rugby II, and P105 produced the smallest divots (26.6, 29.2, and 30.0 cm, respectively) while plots of Midnight and Penn State B produced the largest divots (35.3 and 35.4 cm, respectively). Other cultivars performed similarly, with differences in divot lengths of less than 2 cm. v

6 Plots treated with TE from May-July were the only plots to produce smaller divots than the control in both 2006 and 2007, and, therefore, had the highest divot resistance of all plots. This increase in divot resistance was consistent at each traffic level. This treatment also increased tiller density and below-ground biomass. In 2007, plots managed under this treatment regime measured highest in shear strength. No treatments affected turfgrass ground cover compared to the control. Divots on plots treated with TE from May-Oct. measured shorter in divot size than the control in 2006, but had no effect in This treatment increased tiller density in both years; however, below-ground biomass, shear strength and turfgrass ground cover ratings did not differ from the control in either year. The cultivation treatment resulted in smaller divots than the control in 2007, but divot size was not different from the control in The improvements in divot resistance were less than improvements observed with TE applied from May-July. Under no traffic, plots receiving the cultivation treatment had similar divot size compared to the untrafficked control; however, the benefits of the cultivation treatment were most evident at the high traffic level. The cultivation treatment did not affect ground cover, tiller density, below-ground biomass, or shear strength. The shear vane device was included in this experiment to measure shear strength as well as to determine if it is a viable option for determining divot resistance. While a vi

7 significant relationship was reported between measurements obtained using the shear vane and PENNSWING, greater differences among applied treatments and cultivars were reported using PENNSWING. Therefore, while the shear vane gave an indication of divot resistance, PENNSWING was better able to detect differences in divot resistance among both applied treatments and cultivars in this experiment. Few research studies have investigated the potential for TE use on athletic fields. The goal of this project was to determine if TE applications improved ground cover and divot resistance. The effects of cultivation practices on divot resistance were also evaluated as well as the divot resistance of various KBG cultivars. Under the conditions of this study, TE applied from May-July was the only applied treatment to improve divot resistance compared to the control in both years. The other applied treatments produced inconsistent divot resistance results, with each treatment improving divot resistance in only one of two years. Field managers with KBG fields that receive little play from May July should consider using TE through July in order to reduce divoting. vii

8 TABLE OF CONTENTS LIST OF FIGURES...x LIST OF TABLES...xi DEFINITION OF TERMS...xix ACKNOWLEDGEMENTS...xx INTRODUCTION...1 LITERATURE REVIEW...4 Playing Quality...4 Properties of Playing Quality...4 Factors Affecting Playing Quality...8 Soil properties...8 Plant factors...9 Traffic...11 Wear...12 Soil Compaction...15 Methods to Measure Surface Stability...19 Shear Strength Testing Devices...19 Divot Creation Testing Devices...22 Effects of Trinexapac-ethyl (TE) on Turfgrass...25 Plant Growth Regulator Background...25 Classification...27 Trinexapac-ethyl Mode of Action...28 Effects of Trinexapac-ethyl Applications on Turfgrass...29 Physiological Responses...30 Tillering...31 Rooting...33 Post-Suppression Growth Surge...36 OBJECTIVES...39 MATERIALS AND METHODS...40 Plot Construction...40 Plot Maintenance...42 Traffic Applications...44 Applied Treatments...45 Evaluation of Turfgrass Ground Cover...47 viii

9 Tiller Density...47 Below-Ground Biomass...48 Shear Strength...50 Divot Resistance...51 Rating Dates and Statistical Analysis...53 RESULTS...54 Turfgrass Ground Cover...54 Shear Strength...74 Divot Size...84 Tiller Density...94 Below-Ground Biomass Correlations DISCUSSION Cultivars Turfgrass Ground Cover Divot Resistance Applied Treatments Turfgrass Ground Cover Divot Resistance Shear Vane SUMMARY AND CONCLUSIONS BIBLIOGRAPHY APPENDIX. ADDITIONAL MATERIALS ix

10 LIST OF FIGURES Figure 1. Cultivar in one replication Figure 2. Applied treatments Figure 3. Traffic levels Figure 4. Brinkman Traffic Simulator Figure 5. Trimmed plug for tiller counting Figure 6. Sample prior to ashing (above) and after ashing (below) Figure 7. Eijkelkamp Type 1B soil shear tester Figure 8. PENNSWING about to create a divot Figure 9. Divots produced by PENNSWING Figure 10.. Scattergram of divot length versus turfgrass ground cover Figure 11. Scattergram of divot length versus tiller density Figure 12. Scattergram of divot length versus below-ground biomass Figure 10. Scattergram of divot length versus shear strength x

11 LIST OF TABLES Table 1. Physical properties of root-zone mix Table 2. Cultivar, applied treatment, and traffic level main effects and interactions for turfgrass ground cover in Table 3. Cultivar, applied treatment, and traffic level main effects and interactions for turfgrass ground cover in Table 4. Mean turfgrass ground cover values for cultivars under medium traffic in Table 5. Mean turfgrass ground cover values for cultivars under medium traffic in Table 6. Mean turfgrass ground cover values for cultivars under high traffic in Table 7. Mean turfgrass ground cover values for cultivars under high traffic in Table 8. Mean turfgrass ground cover values for applied treatments under medium traffic in Table 9. Mean turfgrass ground cover values for applied treatments under medium traffic in Table 10. Mean turfgrass ground cover values for applied treatments under high traffic in Table 11. Mean turfgrass ground cover values for applied treatments under high traffic in Table 12. Mean turfgrass ground cover values for the traffic level main effect in Table 13. Mean turfgrass ground cover values for the traffic levels main effect in Table Oct mean turfgrass ground cover values for the cultivar by traffic level interaction xi

12 Table Oct mean turfgrass ground cover values for the cultivar by traffic level interaction Table Oct mean turfgrass ground cover values for the applied treatment by traffic level interaction Table 17. Cultivar, applied treatment, and traffic level main effects and interactions for shear strength in 2006 and Table 18. Cultivar, applied treatment, and traffic level main effects and interactions for shear strength pooled over 2006 and Table 19. Mean shear strength values for the cultivar main effect pooled over 2006 and Table 20. Mean shear strength values for the applied treatment main effect in 2006 and Table 21. Mean shear strength values for the traffic level main effect in 2006 and Table 22. Mean shear strength values for the cultivar by traffic level interaction pooled over 2006 and Table 23. Mean shear strength values for the applied treatment by traffic level interaction pooled over 2006 and Table 24. Cultivar, applied treatment, and traffic level main effects and interactions for divot size in 2006 and Table 25. Cultivar, treatment, and traffic level main effects and interactions for divot size pooled over 2006 and Table 26. Mean divot length, width, and depth values for the cultivar main effect pooled over 2006 and Table 27. Mean divot length, width, and depth values for the applied treatment main effect in 2006 and Table 28. Mean divot length, width, and depth values for the traffic level main effect pooled over 2006 and Table 29. Mean divot length values for the cultivar by traffic level interaction pooled over 2006 and xii

13 Table 30. Cultivar, applied treatment, and traffic level main effects and interactions for tiller density in 2006 and Table 31. Cultivar, applied treatment, and traffic level main effects and interactions for tiller density pooled over 2006 and Table 32. Mean tiller density values for the cultivar main effect in 2006 and Table 33. Mean tiller density values for the applied treatment main effect pooled over 2006 and Table 34. Mean tiller density values for the traffic level main effect pooled over 2006 and Table 35. Cultivar, applied treatment, and traffic level main effects and interactions for below-ground biomass in 2006 and Table 36. Cultivar, applied treatment, and traffic level main effects and interactions for below-ground biomass pooled over 2006 and Table 37. Mean below-ground biomass values for cultivar main effect in 2006 and Table 38. Mean below-ground biomass values for the applied treatment main effect pooled over 2006 and Table 39. Mean below-ground biomass values for the traffic level main effect pooled over 2006 and Table 40. Mean below-ground biomass values for the cultivar by applied treatment interaction pooled over 2006 and Table 41. Mean below-ground biomass values for the applied treatment by traffic level interaction pooled over 2006 and Table 42. Spearman correlation coefficients (n=288) between measured plot characteristics in 2006 and Table Aug mean turfgrass ground cover values for the cultivar by traffic level interaction Table Aug mean turfgrass ground cover values for the cultivar by traffic level interaction xiii

14 Table Aug mean turfgrass ground cover values for the cultivar by traffic level interaction Table Sept mean turfgrass ground cover values for the cultivar by traffic level interaction Table Sept mean turfgrass ground cover values for the cultivar by traffic level interaction Table Sept mean turfgrass ground cover values for the cultivar by traffic level interaction Table Sept mean turfgrass ground cover values for the cultivar by traffic level interaction Table Oct mean turfgrass ground cover values for the cultivar by traffic level interaction Table Oct mean turfgrass ground cover values for the cultivar by traffic level interaction Table Oct mean turfgrass ground cover values for the cultivar by traffic level interaction Table Aug mean turfgrass ground cover values for the cultivar by traffic level interaction Table Aug mean turfgrass ground cover values for the cultivar by traffic level interaction Table Aug mean turfgrass ground cover values for the cultivar by traffic level interaction Table Sept mean turfgrass ground cover values for the cultivar by traffic level interaction Table Sept mean turfgrass ground cover values for the cultivar by traffic level interaction Table Sept mean turfgrass ground cover values for the cultivar by traffic level interaction Table Sept mean turfgrass ground cover values for the cultivar by traffic level interaction xiv

15 Table Oct mean turfgrass ground cover values for the cultivar by traffic level interaction Table Oct mean turfgrass ground cover values for the cultivar by traffic level interaction Table Oct mean turfgrass ground cover values for the cultivar by traffic level interaction Table Oct mean turfgrass ground cover values for the cultivar by traffic level interaction Table Aug mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Aug mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Aug mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Sept mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Sept mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Sept mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Sept mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Oct mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Oct mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Oct mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Oct mean turfgrass ground cover values for the applied treatment by traffic level interaction xv

16 Table Aug mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Aug mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Aug mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Sept mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Sept mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Sept mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Sept mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Oct mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Oct mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Oct mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Oct mean turfgrass ground cover values for the applied treatment by traffic level interaction Table Aug mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Aug mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Aug mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Sept mean turfgrass ground cover values for the cultivar by applied treatment interaction xvi

17 Table Sept mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Sept mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Sept mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Oct mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Oct mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Oct mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Oct mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Aug mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Aug mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Aug mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Sept mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Sept mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Sept mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Sept mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Oct mean turfgrass ground cover values for the cultivar by applied treatment interaction xvii

18 Table Oct mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Oct mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Oct mean turfgrass ground cover values for the cultivar by applied treatment interaction Table Oct mean turfgrass ground cover values for the cultivar by applied treatment interaction Table 109. Mean shear strength values for the applied treatment by cultivar interaction pooled over 2006 and Table 110. Mean shear strength values for the cultivar by applied treatment by traffic level interaction pooled over 2006 and Table 111. Mean divot length values for the cultivar by applied treatment interaction pooled over 2006 and Table 112. Mean divot length values for the applied treatment by traffic level interaction pooled over 2006 and Table 113. Mean divot length values for the cultivar by applied treatment by traffic level interaction pooled over 2006 and Table 114. Mean tiller density values for the applied treatments by cultivar interaction pooled over 2006 and Table 115. Mean tiller density values for the cultivar by traffic level interaction in 2006 and Table 116. Mean tiller density values for the applied treatment by traffic level interaction in 2006 and Table 117. Mean tiller density values for the cultivar by treatment by traffic level interaction pooled over 2006 and Table 118. Mean below-ground biomass values for cultivar by traffic level interaction pooled over 2006 and Table 119. Mean below-ground biomass values for the cultivar by applied treatment by traffic level interaction pooled over 2006 and xviii

19 DEFINITION OF TERMS Below-ground biomass the vegetative matter consisting of the crown, rhizomes, and roots below the soil surface. Crown a highly compressed stem with a succession of nodes separated by short internodes, located at the base of the leaves. Divot a piece of turf partially or completely gouged out of surrounding turf by a golf club head, horse shoe, or studded footwear. Gibberellins a large group of chemically related plant hormones associated with promotion of stem growth, seed germination, and many other functions. Newton-meter (Nm) the SI unit of rotational force, 1 Nm = 1 kgm 2 /s 2. Plant growth regulator (PGR) an organic compound, which when present or applied in small amounts, results in a change in turf growth and development. Rhizome an underground, elongated stem (or shoot) with scale leaves and adventitious roots arising from the nodes. Studded footwear a type of shoe containing posts on the sole of the shoe that penetrate into the soil surface at footstrike; commonly worn by athletes in order to improve traction during athletic maneuvers. Thatch a loose, intermingled organic later of dead and living shoots, stems, and roots that develops between the zone of green vegetation and the soil surface. Tiller a lateral shoot, usually erect, that develops intravaginally from an axillary bud. Torque rotational force, the product of force x lever arm (moment of force), usually presented in Nm (Newton meter). Turf a cover of mowed vegetation; usually comprised of turfgrasses, growing intimately within an upper soil stratum of intermingled roots and stems. Turfgrass a species or cultivar of grass, usually of spreading habit, which is maintained as mowed turf. xix

20 ACKNOWLEDGEMENTS I would like to thank all of the people who made this project possible. It is not easy to come back to school after spending five years working in the real world and I relied heavily on those around me. First, I would like to thank my wife, Tamela, for her support and encouragement during this project. Thanks also to my parents who instilled in me a strong work ethic and provided me with support during this process. I would also like to thank my committee of advisors, Dr. Andrew McNitt, Dr. Peter Landschoot, Mr. Jeffrey Borger, and Dr. Richard Arteca for their help and guidance. It is truly an honor to work with such devoted scientists. A special thanks must also go to Dr. James Brosnan for his camaraderie, answering my many questions, and helping me transition back into academia. You all serve as role models to me. I also need to thank Dianne Petrunak and Dave Livingston. Dianne helped me on countless occasions and I sincerely thank her for help and advice. Dave s help at the research facility was also much appreciated and his suggestions helped this project run smoothly. I do not think that I could have completed this project without the help of both of these individuals. xx

21 INTRODUCTION Regardless of intensity of use, athletic fields are expected to provide a safe, stable playing surface. As the focus on player safety increases and the popularity of infilled-synthetic turf systems rises, increased expectations have been placed on the performance of natural turfgrass fields. This high level of scrutiny, as well as the continuing desire to provide a safer, more playable surface, has led to increased efforts to improve management practices for athletic fields. On high profile fields, athletic field rootzones are often built with a high percentage of sand to ensure good drainage and minimize the effects of compaction. A highsand soil lacks soil cohesion and is typically less stable than soils with greater amounts of silt and clay. The surface stability of these high-sand rootzone fields is greatly influenced by the amount of above and below-ground biomass (Rogers et al., 1988). As turfgrass density is reduced by traffic during field use, the surface stability is reduced and may result in participants slipping because of excessive divoting. Recognizing that high-sand rootzones are prone to divoting, synthetic reinforcements (artificial root systems) have been mixed into the sand to increase divot resistance (McNitt and Landschoot, 2001; McNitt and Landschoot, 2005). However, most of the synthetic products must be incorporated into the rootzone at the time of field 1

22 construction, therefore limiting their use to new field installations. The associated costs can also be high. Plant growth regulators (PGRs) have been used on turfgrass since the 1950s. In addition to reducing shoot growth, PGRs affect other aspects of plant growth. Trinexapac-ethyl (TE), a commonly used PGR, has been shown to increase both tiller density and rooting (Ervin and Koski, 1998; McCullough, 2006a). Increases in these growth characteristics may increase surface stability, and, therefore increase divot resistance. Cultivation practices may also influence divot resistance. By removing soil cores from the rootzone, core cultivation creates voids in soil, increasing oxygen levels and providing space into which roots can grow. The practice of vertical mowing removes thatch from the surface, but when blades are set to penetrate into soil, it may stimulate root and rhizome growth by severing existing roots and rhizomes (McCarty and Miller, 2002). Sherratt et al. (2005) showed that vertical mowing once per month increased divot resistance. A combination of these cultivation practices may result in increased divot resistance. Much is known about the effects of TE on golf course turfgrass quality. However, the effects of TE on turf used for athletic fields are relatively unknown. In order to better understand the implications of TE applications on athletic field quality, the 2

23 effects on turfgrass growth subjected to the stresses commonly found on athletic fields must be characterized. One goal of this research was to document the effects of TE applications and cultivation on divot resistance of turfgrass grown on highsand rootzones subjected to simulated traffic. The other goal of this research was to determine the divot resistance of various Kentucky bluegrass (KBG) (Poa pratensis L.) cultivars. 3

24 LITERATURE REVIEW This literature review is divided into two sections. The first section focuses on how the playing quality of athletic fields is related to shear and divot resistance. The second section summarizes research on the plant growth regulator trinexapac-ethyl on turfgrass. Playing Quality Properties of Playing Quality A major function of an athletic field is to provide a safe, consistent playing surface that maintains adequate playing quality and turfgrass cover regardless of weather conditions (Henderson et al., 2003). Bell et al. (1985) divided the components of playing quality into both ball-to-surface and player-to-surface interactions. Ball-tosurface interactions include ball rebound and ball roll, while player-to-surface interactions include hardness and traction (friction). The ball-to-surface interaction describes the behavior of the ball as it comes into contact with the playing surface, indicating the manner in which the ball rolls, bounces, or spins (Bell et al., 1985). This interaction is an important playing quality 4

25 component for sports like cricket and soccer where the ball is in frequent contact with the playing surface. The interaction between the player and the surface is critical for sports in which players are running and changing direction as they do in football and soccer. This interaction refers to the forces acting on the players when they are in contact with the playing surface. The player-to-surface interaction can be further broken down into two components: 1) stiffness and resilience 2) traction and friction (Bell et al., 1985). The stiffness of a playing surface is the ratio of applied force to the amount of deflection of the surface. Stiffness is often considered as the degree of hardness or softness of a surface (Bell et al., 1985). Resiliency is the ratio of the energy returned to the player after contacting the surface as a proportion to the energy exerted (Bell et al., 1985). As the stiffness of a surface is increased, less energy is absorbed, resulting in greater resiliency because more energy is being transferred back to the athlete. As more energy is transferred to the athlete, more stress is placed on joints and muscles. Traction and friction are properties that allow an athlete s shoe to grip the surface without which the athlete would slip and possibly fall when making body movements necessary for the sporting activity (Canaway and Bell, 1986). Traction generally refers to footwear with studs, spikes, or cleats while friction refers to 5

26 footwear with smooth soles or shallow treads. In most sports, including football, athletes primarily wear studded footwear because the studs penetrate the playing surface resulting in greater grip. Traction can be further broken down into translational and rotational forces. Translational traction forces refer to the movement of an athlete s foot in a straightforward or backwards motion along its longest axis (Middour, 1992). A measure of translational traction determines the degree to which the studs of a shoe grip the surface (Shorten and Himmelsbach, 2002). A rotational traction force is generated when an athlete s foot changes direction and is rotated about an axis (Middour, 1992). Rotational traction measurements determine the tendency for foot fixation during changes of direction (Shorten and Himmelsbach, 2002). Both translational and rotational traction can be static or dynamic. Static forces resist sliding or pivoting when there is no movement between the shoe and the surface. Dynamic traction is the resistance that occurs during sliding or pivoting motions (Shorten and Himmelsbach, 2002). The ability of a playing surface to withstand the forces of athlete foot strike is influenced by the shear strength of the surface. Shear strength is the maximum resistance of a playing surface to shearing stresses (Beard and Beard, 2005). A surface with high shear strength will be able to resist shearing as forces are placed 6

27 upon it better than a surface with low shear strength. Shear strength is critical for maximum playing quality and player safety since too little shear strength can result in athlete slippage and possibly injured muscles while too high shear strength can result in injuries such as turf toe (Stier and Rogers, 2001). Therefore, an athletic field s shear strength is a key component of playing quality. In certain instances when the forces imposed on the playing surface are greater than the shear strength of the surface, a divot is created. A divot on an athletic field can be described as a piece of turf partially or completely gouged out of surrounding turf by studded footwear (McNitt, 2000). The divot resistance of a playing surface is the surface s ability to resist divot creation or limit divot size. Divot creation represents an overwhelming force from footstrike or an extreme case of a lack of shear strength. Therefore, the shear strength of the surface gives an indication of divot resistance. Excessive divoting results in surface unevenness. An uneven surface lowers the playing quality of the field, causes athletes to slip, and increases the risk of ankle and knee injuries (Powell and Schootman, 1993; Waddington and McNitt, 1995). For example, in the National Football League (NFL), poor field conditions, including excessive divots, have been blamed for causing players to slip and multiple injuries. As a result, the NFL Players Association has argued for the establishment of playing surface standards in order to improve field conditions and increase player safety (Garber, 2001). 7

28 Factors Affecting Playing Quality Playing quality is primarily affected by two playing surface characteristics: soil properties and plant factors (Canaway and Baker, 1993). The interactions of these characteristics with the stresses of traffic along with dynamic external factors such as environmental conditions, management practices, pests, and disease, largely determine the playing quality of a surface (Canaway and Baker, 1993). The following sections will discuss how playing quality is related to soil properties, plants factors, and traffic effects. Soil properties. The rootzone composition of an athletic field influences several soil properties. These properties include infiltration, moisture retention, and aeration. Additionally, the composition of the rootzone affects the degree to which the field can become compacted. Soil compaction reduces porosity which, in turn, affects infiltration, moisture retention, and aeration. As a result, when economically feasible, rootzones are constructed by placing a primarily sand mix over a gravel layer and drainage system. Sand is the predominant component because of its free-draining properties and because its single-grained structure minimizes the effects of compaction. Therefore, sand-based rootzones maintain adequate porosity for drainage and an aerated growth medium for plant roots (Bingaman and Konke, 1970). 8

29 While sand-based rootzones offer several benefits, the main disadvantage of sand use in rootzones is reduced surface stability. Sand s single-grain structure does not allow for cohesion between particles, and, as a result, surface stability (shear strength) is low. Canaway (1983) reported that the shear strength of eight turfgrass species tested on sand-based rootzones was approximately half the shear strength of the species grown on a native soil, which contained more silt and clay particles. One way to increase surface stability is by mixing limited amounts of soil and/or peat with the sand. This practice has been shown to increase the shear strength of sand (Whitmeyer and Blake, 1989; Henderson 2000), thus positively affecting field playing quality. However, as the amount of sand and/or peat increases, drainage is reduced. Plant factors. Plant factors affecting playing quality are primarily influenced by the turfgrass species grown on the athletic field. Growth characteristics of the species subsequently affect above and below-ground biomass. The ability of a species to withstand the stresses of play and maintain above and below-ground biomass is critical in order to maximize playing quality. As a result, species are selected for use on athletic fields based on growth characteristics. For example, when KBG is grown under optimum conditions, it produces a dense cover and sufficient biomass to provide a strong playing surface (Carrow and Wiecko, 1989). 9

30 Plant factors contribute significantly to the shear strength of the playing surface. Shear strength is influenced by the presence of roots and rhizomes and has been shown to increase with increasing amounts of below-ground biomass (Adams, 1981; Zebarth and Sheard, 1985; Rogers et al., 1988). Rogers et al. (1988) suggested the amount of vegetation appears to be the most important factor affecting shear strength. The vegetation provides above-ground biomass, which protects the rootzone from compaction and reduces surface hardness. More importantly, the root system provided by the vegetation offers necessary shear strength, especially on high-sand rootzones. Gibbs et al. (1989) found that while the amount of aboveground plant material moderately increased surface traction on high-sand rootzones, the amount of root material was more important to traction. Other researchers have also reported that plant roots significantly contribute to shear strength. Adams and Jones (1979) reported that the roots of 100-day old perennial ryegrass (Lolium perenne L.) increased the shear strength of fine sand by almost threefold. In addition, Adams and Jones (1979) presented data indicating that the effects of roots on shear strength appeared to be almost independent of the fine material content in a rootzone. Similarly, Van Wijk (1980) compared root free soil to field soil containing roots and found an increase of approximately 300% in shear strength measurements in soil containing roots. 10

31 Increasing roots and rhizomes is perhaps the most critical component to creating a divot resistant surface. Traffic reduces vegetation and thus below-ground biomass. Researchers have sought to mimic the effects of below-ground biomass by mixing synthetic materials into the rootzone (Baker, 1997). McNitt and Landschoot (2005) studied the effects of various soil reinforcing materials on a number of athletic field playing quality parameters including divot resistance. The researchers reported that soil reinforcing materials significantly reduced divot size compared to soil without reinforcements. Beard and Sifers (1990) and Adams (1997) also found that soil reinforcing materials increased divot resistance. Traffic. The response of the turfgrass stand to traffic affects all components of playing quality, including divot resistance. All athletic fields are subjected to foot traffic during use. Traffic is the product of two separate stresses: 1) wear on turfgrass plants and 2) soil compaction, and is considered the most frequent and damaging stress to turfgrass plants on athletic fields (Minner et al., 1993). The following section will discuss the effects of various agronomic factors on both the wear and soil compaction components of traffic. 11

32 Wear - Researchers have defined wear as an injury to plant tissues from pressure, tearing, or scuffing (Carrow and Petrovic, 1992). Beard (1973) described wear as the immediate result of crushing, tearing, or shearing actions of foot and vehicular traffic. In a conceptual model of playing quality developed by Canaway and Baker (1993), wear was considered to be the most important factor affecting the quality of the surface. Canaway and Baker (1993) stated that a trial may be conducted without wear and show a particular response to the experimental treatments; however, if wear is applied, the response may disappear, be reversed or largely magnified. Therefore, according to Canaway and Baker (1993), athletic field research conducted without the application of wear treatments has restricted value. The nature of the sport being played and the athletes participating in that sport can influence the wear injury of turfgrass. More specifically, the size of athletes and the type of footwear they choose greatly influence wear injury. For example, the studded footwear that is preferred by certain athletes causes turf disruption and shearing. For non-studded or flat-soled footwear, turfgrass damage mainly results from the frictional abrasion between the sole of the shoe and the turf (Canaway, 1975). Turfgrass species differ in their ability to tolerate wear stress. Shearman and Beard (1975a) investigated the wear tolerance of several cool-season grasses. The trial compared the foot-like and vehicular wear tolerance of seven turfgrass species that 12

33 included Manhattan perennial ryegrass, Merion KBG, Kentucky 31 tall fescue (Festuca arundinacea Schreb.), Italian ryegrass (Lolium multiflorum Lam.), Pennlawn red fescue (Festuca rubra L.), Cascade chewings fescue (Festuca rubra commutate L.), and rough bluegrass (Poa trivialis L.). Four methods were used to determine wear tolerance: 1) visual rating of wear injury, 2) percent total cell wall content, 3) percent verdure, and 4) percent chlorophyll per unit area remaining after wear treatment. The researchers found perennial ryegrass to be the most wear tolerant species evaluated under vehicular wear. KBG ranked second in vehicular wear tolerance, but was found to be most tolerant to wear caused by foot traffic. Chewings fescue and rough bluegrass were the least wear tolerant species under both types of wear. Since Shearman and Beard s study (1975a), many researchers have continued to investigate wear tolerance among species (Canaway, 1981; Cockerham et al., 1990; Henry et al., 1995; Bonos et al., 2001). While clear wear tolerance differences are evident among species, cultivar differences within species also exist. In order to understand the differences among cultivars of a particular species, researchers have studied anatomical and morphological differences. Trenholm et al. (2000) examined wear tolerance differences among several seashore paspalum (Paspalum vaginatum Swartz.) cultivars and multiple hybrid bermudagrass cultivars. The researchers found that reduced leaf total cell wall content (TCW) was the most important mechanism related to enhanced wear tolerance of seashore paspalum. Other important factors 13

34 found to enhance wear tolerance included lower leaf strength, lower stem TCW, greater leaf moisture, greater shoot density, and higher potassium shoot tissue concentration. Trenholm et al. s (2000) conclusion that reduced leaf TCW contributed to enhanced wear tolerance directly contradicts results reported by Shearman and Beard (1975b). Shearman and Beard (1975b) reported that increases in cell wall constituents led to greater wear tolerance because of increased leaf strength. Trenholm et al. (2000) explained their findings by stating that the reduced TCW resulted in a decrease in leaf tensile strength, allowing for greater leaf elasticity, which enables the plant to better tolerate wear. Trenholm et al. (2000) reported that the factors affecting hybrid bermudagrass wear tolerance differed from those affecting seashore paspalum wear tolerance. Whereas wear tolerance of seashore paspalum strongly correlated with TCW, the researchers found that high stem moisture and reduced stem cellulose content were the most important factors in determining hybrid bermudagrass wear tolerance. Other important factors enhancing the wear tolerance of hybrid bermudagrass were greater leaf moisture, shoot density, leaf lignin, and stem and leaf lignocellulose. Trenholm et al. (2000), noting the differences in factors affecting wear tolerance between species, suggested that species-specific wear tolerance screening should be developed. 14

35 Because KBG is one of the most commonly used cool-season turfgrass species on athletic fields (Puhalla et al., 1999), Brosnan et al. (2005) conducted a study in order to identify anatomical and morphological characteristics that contribute to KBG wear tolerance. Wear treatments were applied to the 2000 National Turfgrass Evaluation Program (NTEP) KBG field plots with a differential slip-wear apparatus, which scuffs the turf without producing high levels of soil compaction (Canaway, 1976). Following the wear application, the researchers selected the ten most wear tolerant and intolerant cultivars for further study. Brosnan and colleagues evaluated fieldgrown and greenhouse-grown plants to determine if any significant morphological or anatomical differences existed between wear tolerant and intolerant cultivars. Wear tolerant cultivars exhibited a more vertical leaf angle, greater TCW and lignocellulose content, and a lower shoot moisture content and leaf turgidity. Among the differences, leaf angle proved to be the most important factor separating wear tolerant from intolerant cultivars. Soil Compaction - Soil compaction is defined as the pressing together of soil particles, adversely altering soil physical properties for optimum turfgrass growth and culture (Carrow and Wiecko, 1989). Soil compaction negatively affects turfgrass growth for two main reasons: 1) a lack of oxygen in the rootzone and 2) increased resistance to root penetration (Puhalla et al., 1999). Soil compaction reduces oxygen levels in the rootzone by reducing total pore space and, in particular, air-filled macropores (Carrow and Petrovic, 1992). Because the number of 15

36 macropores is reduced, oxygen diffusion rates are lowered and, in extreme cases, can fall below rates critical for plant growth (O Neil and Carrow, 1983; Agnew and Carrow, 1985). In addition to providing oxygen to the rootzone, macropores also serve as channels for root and rhizome penetration and growth (Carrow and Petrovic, 1992). As the number of channels is reduced, penetration resistance increases, thereby inhibiting root and rhizome growth. Shearman and Watkins (1985) studied rootzone compaction effects on the lateral spread of KBG rhizomes. The researchers reported reduced lateral spread in compacted compared to non-compacted rootzones, indicating rhizomes had difficulty penetrating the surrounding rootzone. Agnew and Carrow (1985) reported an increase in surface rooting (0-5 cm) coupled with a decrease in deep rooting (10-20 cm) in KBG grown on a compacted rootzone. The researchers attributed this root distribution to roots encountering greater resistance. Sills and Carrow (1983) found rooting to decrease on compacted soils at all rootzone depths measured (0 to 5 cm, 5 to 15 cm, and 15 to 25 cm). Along with negatively influencing turfgrass growth, soil compaction also affects athletic field playing surface characteristics. Increases in soil bulk density resulting from soil compaction affect the hardness of the playing surface. In a study investigating the impact absorption of surfaces used for thoroughbred horse racing, Zebarth and Sheard (1985) reported a positive correlation between soil bulk density 16

37 and increased peak deceleration (surface hardness) values. McNitt and Landschoot (2003) also found a strong positive relationship between soil bulk density and surface hardness in a study evaluating the effects of soil reinforcements on athletic field rootzones. High levels of surface hardness can be dangerous to athletes when they fall and impact the surface (Rogers III and Waddington, 1990). Soil compaction has also been found to affect shear resistance. Shear resistance is the force necessary to cause failure over a shear plane and may be considered an index of soil strength (Zebarth and Sheard, 1985). High levels of shear resistance often indicate high surface stability. Zebarth and Sheard (1985) found increases in soil bulk density (resulting from soil compaction) to increase shear resistance on bare soil surfaces; however, no relationship between soil bulk density and shear resistance was found when turfgrass cover was present. Other researchers have reported that high soil bulk density values decrease shear resistance on bare soil, attributing the decrease in shear resistance to a lack of turfgrass cover (Rogers III et al., 1998; Gibbs et al., 1989). Wear and soil compaction occur at the same time on an athletic field, but normally one stress causes more damage than the other (Carrow and Wiecko, 1989). On athletic fields constructed with high-sand rootzones, wear is the dominant stress because high-sand rootzones maintain adequate macroporosity under high levels of compaction. On athletic fields grown on sites with a finer soil texture (more silt and 17

38 clay), soil compaction may be the dominant stress negatively affecting the quality of the playing surface. Also, the dynamics of traffic stress change throughout the season. For example, the effect of repeated use throughout the season may be cumulative, leading to higher levels of soil compaction at the end of the season compared to at the beginning of the season. As a result, soil compaction plays a more important role late in the season compared to the beginning of the season when wear is the dominant stress. Researchers have not always separated the effects of traffic into wear and soil compaction. According to Carrow and Wiecko (1989), the term wear has sometimes been used to refer to situations that include both wear and soil compaction. Several studies have analyzed the effects of wear without soil compaction (Bonos et al, 2001; Brosnan et al., 2005). These tests applied wear to turf over a short period of time, thus negating the long-term effects of soil compaction. However, the majority of tests apply wear over a period of weeks or months and include some degree of soil compaction. As a result, such trials evaluate both the wear tolerance and the soil compaction tolerance of the turfgrass. Canaway and Wiecko (1989) suggested researchers carefully define the nature of the wear treatments, and more accurately describe the treatments as traffic treatments, when they induce both wear and soil compaction components. 18

39 Methods to Measure Surface Stability Shear Strength Testing Devices. The following is a description of devices that have been used to determine the shear strength of athletic field playing surfaces. Investigators used the described devices to measure traction; however, it is important to note that many of the described procedures are also measuring shear strength by determining the point at which the turf or ground breaks loose, indicating a complete loss of traction. Canaway (1975) developed a device that used a steel disc, 15 cm in diameter, with several types of athletic footwear studs attached to it. A 32.5 cm shaft was threaded into the center of the disc and was loaded with circular weights to give a total weight of 47.8 kg. To ensure stud penetration, the apparatus was dropped onto the turfgrass surface from a height of several centimeters. Researchers used an industrial torque wrench to measure the force (Nm) required to rotate the studs through the turf. Canaway (1975) used this number to represent the traction of a particular turf. Winterbottom (1985) modified the Canaway (1975) device for use on synthetic turf. The weights were replaced by a spring compressed to a standard length by the operator s weight. The spring was very sensitive to changes in spring length. 19

40 Therefore, this device was restricted to use on level ground and was found to be unsatisfactory for use on natural turf. Canaway and Bell (1986) also modified the original Canaway (1975) apparatus. In order to increase ease of use, the researchers increased the overall height of the device to 90 cm. This modification created a more convenient torque wrench height for the operator. Additional modifications included a two-handled torque wrench and lifting handles. For ease of transport and to standardize the height of drop at 6 cm, a trolley was developed to transport the device between testing locations. Shildrick and Peel (1984) created a device that included a Geonor inspection vane borer fitted with a medium size (standard) vane (4.0 cm deep, 2.0 cm diameter of sheared soil cylinder). The vane was inserted into the soil so that the top of the vane lay level with the surface and rotated. Shear strength values were recorded as kpa required for vane rotation. Zebarth and Sheard (1985) investigated the shear strength of turfgrass racing surfaces for thoroughbred horses using a device developed to test impact resistance and resistance to shear using a simulated horse hoof. An 8-cm wide steel vane was inserted vertically into the surface to a depth of 6 cm and was connected to the bottom of a rotating arm. Torque was applied on the upper end of the rotating arm by a rope and a winch. To measure tension in the rope, a strain gauge device was 20

41 inserted between the rotating arm and the rope. Shear strength was measured as the peak force required to rotate the vane out of the testing surface. A field shear test apparatus, known as the shear vane (Type 1B, Eijkelkamp Equipment, Giesbeek, The Netherlands) has been used to measure the shear strength of athletic fields (Middour, 2002; Stier et al., 1999; Stier and Rogers, 2001; Rogers et al., 1988). Shear strength values were obtained by pressing the vanes into the ground to a depth of 1.6 cm and then rotating the device to measure the maximum torque necessary to cause shearing of the turf and soil. The device consisted of 12 fins, 1.0 to 2.0 cm wide, alternatively welded to a circular disc that was connected to the handle by a straight shaft. A torque wrench equipped with a scale (Nm) attached to the handle provided torque values. A standard range of shear vane values classifying a field as acceptable has not been established. Stier and Rogers (2001) suggest a value of 10 Nm as the minimum acceptable value since the turf is easily torn at values less than 10 Nm. The researchers suggest values between 10 and 15 Nm to be considered fair; values 15 to 20 Nm to be considered good; and values greater than 20 Nm to be exceptional. Henderson et al. (2003) and Sherratt et al. (2005) measured shear strength with the turf shear tester, (Braden Clegg PTY LTD, Model No. CCB1A). The turf shear tester consisted of a 50 mm wide shearing tip which could be set to multiple depths, 21

42 10, 20, 30, and 40 mm. The apparatus caused surface displacement and generated an index of shear strength at the surface in the horizontal direction. A digital readout displayed the shear strength in Nm. Divot Creation Testing Devices. Testing procedures measuring shear strength also give an indication of divot resistance because divoting may be thought of as the complete shearing or removal of the turf/root system from the remainder of the rootzone (McNitt and Landschoot, 2005). The following describes procedures used in experiments to induce divots in order to determine divot recovery or resistance to divoting. Research evaluating divot recovery and closure rates utilized tools designed to remove divots of the same size. Calhoun (1996) used a device that was constructed with a cutting blade attached to a wooden handle that pivoted on an axle above the ground. The blade cut a consistent arc and subsequent measurements were made over time to determine divot closure rates. Walker et al. (2003) used a similar device to measure divot closure rates. The apparatus, known as a radial divot extraction device, consisted of a plywood base and a rotating wooden arm. The wooden arm made contact with the ground through 22

43 an opening cut in the plywood base and produced a divot. Divot closure rates were then measured. Beard and Sifers (1990) developed a divot simulation apparatus to measure divot resistance by measuring divot size. The researchers also measured divot recovery. The device consisted of an adjustable horizontal swinging pivot bar positioned above the soil surface by a metal frame. Attached to the center pivot was a 140-cm-long bar, 25 mm in diameter. Attached to the lower end of the free swinging bar was a nine iron golf club. The free swinging device was dropped from a set height of 200 cm above the soil surface, producing a divot. Divot size was measured to determine divot resistance. Adams (1997) used a similar device in a divot resistance experiment. McNitt and Landschoot (2001) and McNitt and Landschoot (2005) measured divot resistance with a similar apparatus to the devices used by Beard and Sifers (1990) and Adams (1997). The apparatus consisted of the head of a golf club pitching wedge attached at the end of a v-shaped, weighted pendulum. The pitching wedge and pendulum were fastened to the three-point hitch of a tractor. The height of the head, relative to the testing surface was controlled with an adjustable metal pad. The pad could be set at different heights and when the three-point hitch was lowered the pads rested on the soil surface. To make a divot, the pendulum was released from a horizontal position. The pendulum was weighted with a 76-kg weight consisting of a 23

44 steel cylinder filled with lead. Divot size was measured to determine divot resistance. 24

45 Effects of Trinexapac-ethyl on Turfgrass Plant Growth Regulator Background PGRs have been traditionally used on golf courses and other turfgrass areas where a reduced turfgrass growth rate is desired. Athletic field managers have been hesitant to apply PGRs to athletic fields, fearing a slower growth rate could reduce turfgrass recovery. However, in addition to reducing plant growth rate, PGRs such as trinexapac-ethyl [4-(cyclopropyl-α-hydroxy-methylene)-3,5-dioxocyclohexanecarboxylic acid ethyl ester] affect other aspects of plant growth such as tillering and rooting. The PGR-induced effects on tillering and rooting may improve traffic tolerance and increase surface stability. A PGR is an organic compound, natural or synthetic, which when present or applied in small amounts, results in a change in turf growth and development (DiPaola, 1988). PGRs have traditionally been used on turfgrass to control seedhead production and to reduce mowing frequency by suppressing vertical shoot growth. As the first PGRs were introduced in the 1950s, reduced plant growth was coupled with a reduction in turfgrass cover and quality (Watschke and DiPaola, 1995). However, as researchers continued to develop and test PGRs, negative plant responses were reduced or eliminated while desirable plant effects increased (Watschke and DiPaola, 1995). As a result, many turfgrass managers include PGR 25

46 applications as a part of their turfgrass maintenance program (Christians, 2001; Ervin and Zhang, 2004; Huang, 2007). PGRs influence plant growth by targeting the actions of plant hormones. Gibberellins, auxins, cytokinins, ethylene, abscissic acid, and brassisnosteroids are naturally occurring plant hormones (Arteca, 1996). Each hormone affects various aspects of plant growth. For example, cytokinins promote cell division and retard senescence (Taiz and Zeiger, 2006). While plant hormones are produced and regulated endogenously, exogenous applications of synthetic PGRs can alter plant growth characteristics via stimulatory or inhibitory hormonal action. PGRs retard plant growth by either inhibiting cell division or altering gibberellin biosynthesis. The inhibition of cell division directly reduces plant growth by reducing the rate of cell division. Conversely, alterations in the gibberellin biosynthetic pathway do not affect the rate of cell division, but rather the size of plant cells. In addition to affecting cell size, altered gibberellin biosynthesis affects root growth, flowering, senescence, and nucleic acid synthesis (Danneberger and Street, 1990). 26

47 Classification In the 1980s, PGRs were classified as either Type I or Type II (Kaufmann, 1986; Watschke et al., 1992). Type I PGRs inhibit or suppress growth by rapidly stopping cell division and differentiation in the meristematic regions of the plant and are primarily absorbed by the foliage. Type II PGRs are primarily root-absorbed and suppress plant growth via the interference of gibberellin biosynthesis, resulting in a reduction of cell elongation but not cell division. A more specific five-category system, based on mode of action, has replaced the two category classification system of the 1980s (Ervin and Zhang, 2007a; Watschke and DiPaola, 1995). Class A materials interfere with gibberellin production late in the biosynthetic pathway and are commonly used on moderately to intensively managed turf. An example of a Class A PGR is TE. Class B compounds interfere with the production of gibberellins early in the biosynthetic pathway and can be used on moderately to intensively managed turfgrass but are limited by the degree of turf injury that often accompanies application. Flurprimidol and paclobutrazol are examples of Class B PGRs. Class C PGRs are mitotic inhibitors that prevent cell division and offer excellent seedhead control; however, applications often hinder turfgrass recuperative potential. Class C PGRs include maleic hydrazide and mefluidide. Class D materials, generally considered herbicides, are phytotoxic but have growth regulating characteristics when applied at very low rates. Glyphosate is 27

48 an example of a Class D material. Class E PGRs affect plant growth by releasing the hormone ethylene which inhibits seedhead production. An example of a Class E PGR is ethephon. Trinexapac-ethyl Mode of Action As a Class A PGR, TE affects gibberellin production late in the biosynthetic pathway. Biosynthesis of gibberellins occurs via the mevalonic acid pathway (Arteca, 1996). The main hormonal functions of gibberellins are the promotion of longitudinal growth, the induction of hydrolytic enzymes in germinating seeds, and the induction of bolting in plants (Rademacher, 2000). While 136 naturally occurring gibberellins have been identified (MacMillan, 2002), only two are involved in TE-induced growth suppression. According to Adams et al (1992), GA 1 is the gibberellin primarily responsible for shoot elongation. GA 1 is produced by the conversion of GA 20 via the 3-β-hydroxylase enzyme. Adams et al. (1992) found that TE drastically lowered the level of extractable GA 1 while GA 20 levels were elevated in an experiment conducted on barley. Based on the concentrations of gibberellins, Adams et al. (1992) suggested that TE targets the enzyme GA 20-3-β-hydroxylase, thus inhibiting the conversion of GA 20 to GA 1 and allowing for reduced leaf cell elongation but normal cell division. 28

49 TE is a foliar-applied PGR that is absorbed primarily through the crown and leaf blade of the plant. Fagerness et al. (1998) reported that 24 hours after application, 50% of the absorbed TE remained in the foliage, 33% was translocated into the crown, and less than 5% of TE was translocated to roots or rhizomes of KBG. As a result of this finding, Fagerness et al. (1998) suggested that TE does not limit lateral growth of rhizomatous and stoloniferous turfgrass species. In fact, PGR application may actually stimulate lateral growth. For example, Calhoun (1996) observed divot closure rates of TE treated creeping bentgrass (Agrostis stolonifera L.) to be equal to or greater than untreated plants, suggesting possible enhanced lateral growth. Effects of Trinexapac-ethyl Applications on Turfgrass Applications of TE produce several desirable effects on turfgrass. For example, TE consistently reduces turf-clipping yields (McCullough et al., 2006b; Ervin and Koski, 2001b; Richie et al., 2001). TE has also been shown to enhance overall turfgrass quality (Steinke and Stier, 2003; Goss et al., 2002; Lickfeldt et al., 2001) and promote resistance to dollar spot disease (Sclerotinia homoeocarpa) (Lickfeldt et al., 2001; Zhang and Schmidt, 2000; Golembiewski and Danneberger, 1998). The effects of TE applications can be divided into physiological responses, tiller growth effects, and root growth effects. Each will be discussed in the following section. 29

50 Physiological Responses. In addition to visible effects observed in the field such as reduced plant height, TE affects the cellular growth and biochemical composition of turfgrass plants. Ervin and Koski (2001a) reported treatment with TE yielded smaller, more numerous photosynthetic mesophyll cells and also increased chlorophyll-b concentration. As a result, photosynthetic rates may be influenced by applications of TE. Stier et al. (1997) reported that TE does not reduce photosynthesis in KBG. In fact, it can be concluded that based on an increase in the metabolic products of photosynthesis (nonstructural carbohydrates), TE-treated plants exhibit an increase in photosynthetic activities. TE applications to bermudagrass have been shown to increase total nonstructural carbohydrates (TNC). TNCs can be stored by the plant and act as an energy reservoir for stress tolerance, dormancy recovery, and regrowth after leaf tissue removal (Davies, 1965). Waltz Jr. and Whitwell (2005) conducted a study in order to determine the effects of repeated TE applications on root and shoot tissue carbohydrate assimilation in Tifway hybrid bermudagrass [Cynodon dactylon (L.) Pers. X C. transvaalensis Burtt-Davey]. The researchers found that root tissue TNC in TE-treated bermudagrass increased at least 38 percent compared to non-treated plants. TE-treated shoot tissue TNC increased up to 32 percent. The increased TNC content in both roots and shoots was evident for 4 to 6 weeks after application. 30

51 In addition to increasing TNC levels, TE may also increase structural carbohydrate levels in cell walls, possibly resulting in higher wear tolerance (Heckman et al., 2005). Structural carbohydrates, such as cellulose, hemicellulose, and lignin, have been shown to be important factors in leaf tensile strength and turfgrass wear tolerance (Shearman et al., 1975b, 1975c; Brosnan et al., 2005). Heckman et al. (2005) studied leaf cell density and elongation in tall fescue (Festuca arundinacea Schreb.) and St. Augustinegrass [Stenotaphrum secundatum S. (Walt.) Kuntze]. The researchers reported increased cell density and greater cell wall content for TEtreated plants. As a result, Heckman et al. (2005) proposed that TE may increase wear tolerance and may be used as a tool by turfgrass managers during periods of high wear. Tillering. Tillering is the process by which aerial shoots emerge intravaginally from axillary buds and leads to an increase in the number of new shoots occurring adjacent to the parent shoot (Turgeon, 1996). The number of tillers contributes to the total biomass and strength of the turfgrass stand. Increases in tiller density have been positively correlated to turfgrass shear resistance (Shildrick and Peel, 1984) and traction (McNitt and Landschoot, 2005). Tiller density has been shown to be influenced by leaf elongation rates. Jones et al. (1979) reported high rates of leaf elongation in tall 31

52 fescue were associated with low tiller numbers. This raises the question: if leaf elongation is suppressed, does tiller density increase? Researchers have reported that PGRs induce tillering on various turfgrass species. Dernoeden (1984) reported an increase in tiller density on a PGR-treated KBG/red fescue (Festuca rubra ssp. rubra) turfgrass stand. Ervin and Koski (1998) conducted a growth chamber and greenhouse study to determine the effects of repeated TE applications on the tiller production of developing perennial ryegrass plants. TE treated plants exhibited a tiller number increase compared to the control in both the growth chamber and the greenhouse. Ervin and Koski (2001b) studied KBG responses to TE, traffic, and nitrogen and found some evidence of increased tillering resulting from TE application. TE treatments increased tiller density in one of the three years the trial was conducted. The increase in turfgrass tillering resulting from TE applications may be the result of altered carbohydrate partitioning. Ervin and Zhang (2004) suggested that because the process of leaf elongation is not functioning as a normal energy sink in TEregulated shoots, the next energy sink would most likely be the crown. If greater carbohydrates are moved to the crown, more energy may be available for tillering. Researchers have also reported that applications of TE affect levels of cytokinins, the plant hormone controlling cell division. Ervin and Zhang (2007b) found that 32

53 applications of TE resulted in increased cytokinin content in KBG, creeping bentgrass, and hybrid bermudagrass. The researchers suggested that increased levels of cytokinins in the crown increase cell division and allow for the formation of additional tillers. The researchers also stated that the antisenescence properties of cytokinins may reduce overall leaf senescence, and, thus, result in greater tiller density. While several studies have reported that TE applications increase tiller density, other studies have reported no increase in tiller density on TE-treated KBG. Lickfeldt et al. (2001) reported no increase in tiller number resulting from repeated TE applications on KBG. Stier and Rogers III (2001) reported an increase tiller number for TE treated Supina bluegrass (Poa supina) but not for KBG grown under low light conditions. Rooting. TE applications may stimulate root growth. Researchers have theorized that PGR applications could shift photoassimilate and energy normally used for shoot growth to root growth (Kaufmann et al., 1983). McCullough (2006a) studied the effects of TE applications on dwarf bermudagrass (Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davey). TE was applied at 0, 0.25, 0.05, and kg ha -1 every three weeks on Tifeagle bermudagrass for 12 weeks. A significant positive 33

54 linear relationship between root mass and TE rate was reported. The researchers found TE to increase root mass by 11 37%. In addition, TE applications were also found to increase root length. Similarly, TE applications have been shown to increase the root mass of zoysiagrass [Zoysia matrella (L.) Merr.] grown under shaded conditions (Qian and Engelke, 1999). Researchers have also shown that TE affects KBG rooting. Beasley et al. (2005) analyzed TE effects on KBG root architecture in a hydroponic system. The study included a single TE application (0.27 kg ha -1 ) and plant response evaluations for seven weeks. After a TE-induced initial total root length and surface area reduction of 48 and 46%, respectively (one week after treatment), total root length and surface area increased to a greater extent than that of the control. This increase prompted Beasley et al. (2005) to state that increases in both root length and surface area could indicate an increase in rooting for TE-treated plants. Beasley et al. (2005) also reported that TE application increased tiller number; however, TE induced tillering resulted in reduced total root length and surface area per tiller. Goss et al. (2002) found a similar response with increases in tiller number of 40 to 52% coupled with reductions in root mass per tiller for TE-treated creeping bentgrass. These findings indicate that TE applications result in more numerous but possibly weaker tillers because of a reduction in rooting. A possible explanation for this phenomenon may be that as tiller density increases, less growing space is 34

55 available for each tiller, resulting in a decrease of individual tiller size and root mass. Although the roots of the tillers may be weaker on a per tiller basis, the larger total number of tillers and roots results in increased shear strength. Shildrick and Peel (1984) found a relationship between tiller size and shear strength. The researchers reported that tiller size was negatively correlated (r = -0.29) with shear strength, indicating that large tillers are associated with lower tiller densities and less shear strength than small tillers found in highly dense turfgrass stands. Several studies have reported that TE applications neither increase nor decrease rooting of various turfgrass species. Ervin and Koski (2001b) found TE applications to have no effect on the root mass of trafficked KBG. Other researchers found that TE applications did not affect rooting of creeping bentgrass (Fagerness and Yelverton, 2001). Inconsistent findings on the influence of TE applications on tillers and roots may be the result of study design, plot maintenance, and/or environmental conditions. For example, results reported on tiller and root effects of TE-treated KBG may be a function of whether or not simulated traffic treatments were applied in the experiment, as this has been found to influence experimental results (Canaway and Baker, 1993). Additionally, other variables affecting tiller development and root growth such as soil type, fertility, mowing height, and cultivation practices varied 35

56 among experiments. Temperature, rainfall, and day length during the studies may also have influenced experimental results. Post-Suppression Growth Surge Applied at the label rate appropriate for the turf species, TE regulates plant growth for a period of about twenty-eight days. Following the normal four week growth regulation period, a post-suppression growth surge is often observed during which plant growth rates can be 120 to 160% of untreated turf (Lickfeldt et al., 2001). Lickfeldt et al. (2001) compared TE applied at 0.17, 0.23, or 0.29 kg a.i. ha -1 every four weeks and 0.23, 0.29, or 0.34 kg a.i. ha -1 every six weeks for three growing seasons. The researchers reported a four week interval was necessary to keep KBG growth regulated and to eliminate the growth surge observed during the six week application schedule. This post-suppression growth surge can be utilized to allow the turfgrass to grow rapidly when it is beneficial to do so. Lickfeldt et al. (2001) suggested that turfgrass growth can not only be reduced but managed with TE applications by utilizing the post-suppression growth surge to achieve better turf performance on athletic fields. For example, season-long growth suppression could be ended during a time of maximum turf use (Sept. for football) to achieve an increased growth rate and recovery, and thus, better turf performance. Tiller number has also been shown to 36

57 increase more rapidly for TE treated plants during the post-suppression growth surge (Beasley et al, 2005), resulting in more plant material and possibly increased shear and divot resistance. Allowing turfgrass to return to normal, unregulated growth during periods of traffic appears to be an important factor in turfgrass traffic tolerance. For example, Ervin and Koski (2001b) found traffic, coupled with TE application, resulted in large clipping reductions. The researchers postulated that a practical implication of such a severe reduction in clippings under traffic and TE may be a reduction in quality and, possibly, traffic tolerance. However, Ervin and Koski (2001b) also stated that greater tiller density resulting from TE applications may partially offset concerns of reduced traffic tolerance. Research has supported the theory presented by Ervin and Koski (2001b) that TE applied during traffic stress reduces turfgrass traffic tolerance. Marshall (2007) reported that bermudagrass under TE-induced growth regulation had reduced traffic tolerance compared to untreated bermudagrass. Marshall (2007) applied TE to bermudagrass beginning in Sept., coinciding with the initiation of traffic applications. Because the bermudagrass was not growing at its normal rate, the turf was unable to recover from traffic injury, leading to reduced traffic tolerance. The investigator also found that even when perennial ryegrass was overseeded into dormant bermudagrass and became the dominant species, TE applications resulted in 37

58 a decrease in ground cover. In accordance with suggestion made by Lickfeldt et al. (2001), regulating turfgrass growth until the period of usage and then allowing the growth to surge should be the most effective way to utilize TE on an athletic field. 38

59 OBJECTIVES The playing quality of an athletic field is partially dependent upon the divot resistance of the turfgrass stand under the stresses of traffic. Without adequate turfgrass coverage and rooting, athletic fields are prone to divoting, which can compromise the playing quality and safety of the playing surface. Few studies have focused on techniques to improve the divot resistance of athletic fields. Applications of the PGR TE have been reported to increase tiller density and affect rooting of KBG and, as a result, may increase the divot resistance of athletic fields. Also, a combination of vertical mowing and core cultivation may increase divot resistance by severing existing roots and rhizomes, thus stimulating additional root and rhizome growth. The objectives of this research project were as follows: 1. Determine the effects of two TE treatment regimes and cultivation on the divot resistance of nine Kentucky bluegrass cultivars under varying levels of simulated traffic. 2. Determine if measured turfgrass properties including ground cover, tiller density, below-ground biomass, and shear strength are related to divot resistance. 39

60 MATERIALS AND METHODS Field studies were conducted in 2006 and 2007 on plots located at the Joseph Valentine Turfgrass Research Center at The Pennsylvania State University. Plot Construction The plot area was established in 2001 and consisted of an under-drained gravel layer, approximately 15 cm deep. Not more than 10% of the gravel particles were greater than 12.7 mm and more than 65% of the particles were between 6.35 mm and 9.5 mm. Not more than 10% of the particles were less than 2 mm. A 6.5 cm intermediate layer consisting of 82% fine gravel and coarse sand ( mm) was installed on top of the gravel. A 25.4 cm layer of a 80% sand: 20% peat (v:v) root-zone mix was placed over the intermediate layer. The particle size distribution and physical properties of this rootzone mix upon installation are listed in Table 1. 40

61 Table 1. Physical properties of root-zone mix 1 Size Fraction Percent Physical properties >2.0 mm 1.0% Bulk density 1.68 g/cm mm 6.8% Total porosity 36.5% mm 33.0% Aeration porosity 40 cm mm 34.0% Capillary porosity 40 cm mm 17.5% Saturated Hydraulic mm 1.9% conductivity 73 cm/hr silt 3.0.% Organic matter (LOI) 0.6% clay 2.8% 1 ASTM F A grid consisting of twenty-seven 3.05 m x 4.57 m plots was laid over the surface and arranged in three rows of nine plots each. In 2001, each plot was seeded with a different KBG cultivar and organized as a randomized complete block. Seeding rate was 97 kg ha -1. The nine cultivars were Baron, Rugby II, Princeton 105 ( P105 ), Touchdown, Limousine, Midnight, Langara, and two experimental cultivars ( Penn State A and Penn State B ). 41

62 Plot Maintenance From the planting date in 2001 until the initiation of the experiment in 2006, turf on the plots was cut with a rotary mower set at 3.8 cm. The plot area received an average of 244 kg N ha -1 per year. Pest control was applied on an as needed basis. In this experiment, the turf was cut three times per week using a rotary mower set to a 3.2 cm cutting height. Clippings were returned to the site. The plots were irrigated to prevent drought stress. Two applications of azoxystrobin fungicide were made in the spring of 2007 to help control summer patch disease (Magnaporthe poae). One application of carbaryl insecticide was made in the summer of 2007 to control hunting billbug larvae (Sphenophorus venatus vestitus Chittenden.). Soil tests revealed no nutrient deficiencies. The plots were fertilized four times per year. The 2006 fertilizer applications were: 28 March, 50.4 kg N ha -1 ( ); 13 April, 50.4 kg N ha -1 ( ); 04 May, 75.7 kg N ha -1 ( ); and 30 Nov., 45.4 kg N ha -1 ( ). The 2007 applications were: 26 March, 50.4 kg N ha -1 ( ); 25 April, 50.4 kg N ha -1 ( ); 10 May 75.7 kg N ha -1 ( ). Cultivar plots were split by treatments including two regimes of TE (Primo Maxx 1 MEC) applications, one aggressive cultivation treatment, and an untreated control. The overall experimental design was a strip-split plot design with three replications. 42

63 Simulated traffic was the strip and applied treatments acted as the split plot. The subplots for the control and TE treatments measured 1.02 m by 0.91 m. The cultivation treatment subplots were 1.02 m by 1.83 m in order to accommodate the size of the cultivation unit. There were 324 subplots total. Figure 1. Cultivar in one replication (yellow box) Figure 2. Applied treatments (between red lines) Figure 3. Traffic levels (between blue lines) 43

64 Traffic Applications Simulated traffic was applied using a Brinkman Traffic Simulator (Cockerham and Brinkman, 1989) (Fig. 1). The Brinkman Traffic Simulator consists of two cleated rollers connected by a chain and sprockets, in a frame, which is pulled by a small tractor. The device weighs 410 kg. The device produces wear, compaction, and lateral shear, which is produced by the different sprocket sizes turning the rollers at unequal speeds. According to Cockerham and Brinkman (1989), two passes with the traffic simulator creates the equivalent number of cleats indentations produced between the hash marks at the 40 yard line during one NFL game. A Ventrac tractor (Model 4200VXD, Ventrac Inc., Orrville, OH) equipped with a dual wheel turf package was used to pull the traffic simulator. In 2006, simulated traffic applications began on 25 July and ended on 31 Oct. In 2007, traffic began on 30 July and ended on 30 Oct. Traffic was applied during this time of the year in order to simulate the traffic imposed on a field during a typical football season. Blocks were divided into three levels of traffic: no traffic (0 passes), medium traffic (two passes, three times per week), and high traffic (4 passes, three times per week). Typically, traffic was applied regardless of weather conditions or soil water content. Occasionally, because of heavy precipitation or schedule conflicts, traffic was not applied on the scheduled day. In these cases, traffic was applied on the following day. 44

65 Figure 4. Brinkman Traffic Simulator Applied Treatments TE was applied at approximately 28-day intervals at a rate of 0.17 kg a.i. ha -1. Two separate TE treatments were applied with one beginning in May and ending in July, consisting of three applications (TE A) and the other beginning in May and ending in Oct., consisting of six applications (TE B). TE was applied with a CO 2 backpack sprayer at a pressure of 345 kpa and a spray volume of 383 L ha -1. A single nozzle boom equipped with a flat fan nozzle #11004E was used. TE application dates were: 25 May, 21 June, 24 July, 17 Aug. (TE B only), 16 Sept. (TE B only), and 10 Oct. 45

66 (TE B only) in 2006; and 24 May, 22 June, 26 July, 23 Aug. (TE B only), 18 Sept. (TE B only), and 16 Oct. (TE B only) in The cultivation treatment was performed on 03 May in 2006 and 08 May in Vertical mowing was performed using a Ryan Mataway (Cushman Inc., Lincoln, NE) walk behind vertical mowing unit equipped with vertical blades spaced 2.5 cm apart. The unit was set to penetrate approximately 1.3 cm below the soil surface. Core cultivation immediately followed vertical mowing and was accomplished using a Toro Greens Aerator (Toro Company, Bloomington, MN) equipped with 1.3 cm diameter hollow tines with a spacing of 6.4 cm. Cores were manually removed. Following the core cultivation treatment, the entire experimental area received an application of topdressing sand applied to a depth of 0.95 cm. 46

67 Treatment Evaluation Turfgrass Ground Cover Weekly turfgrass ground cover was rated visually as the percent of plot area covered with turfgrass. Tiller Density Four cultivars, Limousine, Rugby II, P105, and Midnight, were selected for evaluation based on the cultivars varying degrees of divot resistance. Two cores (2.5 cm diameter x 6.7 cm deep) per plot were randomly extracted on 27 Nov. in 2006 and 29 Nov. in 2007 (Fig. 2). Cores were refrigerated at 5 C until analyzed. Tiller number was determined for each plug by cutting the plant shoots to soil level. Rubber bands were placed across the plug, thus dividing the plug into quarters for ease of counting. The average tiller numbers for the two sub-sample cores were used to represent the tiller density of the subplot. 47

68 Figure 5. Trimmed plug for tiller counting Below-Ground Biomass The same cores used for tiller density evaluation were used for evaluation of belowground biomass. After determining the number of tillers on each core sample, the thatch layers were removed and the cores were cut to a 2.54 cm depth. Cores were placed into a sieve with 0.15 mm diameter openings and submersed into a tub filled with water in order to remove soil from the below-ground biomass. After most of the soil was separated from the below-ground biomass, the samples were oven-dried at 60 C for 24 hours. Oven-dried samples were weighed then ashed in a furnace at 440 C for 16 hours (ASTM F , Method A). The ash sample s weight and 48

69 this loss on ignition were used to represent the amount of below-ground biomass (Fig. 3). Figure 6. Sample prior to ashing (above) and after ashing (below) 49

70 Shear Strength Turf shear strength was tested using a shear vane, Type 1B (Eijkelkamp Equipment, Giesbeek, The Netherlands) (Fig. 4). The device consists of 12 fins, 1.0 to 2.0 cm wide, alternatively welded to a circular disc that is connected to the handle by a straight shaft. A torque wrench equipped with a scale in Newton meters (Nm) was attached to the handle and provided torque values. Shear strength values were obtained by pressing the vanes into the ground and then rotating to measure the torque necessary to cause shearing of the turf and/or soil. Two random measurements per plot were performed and averaged together in order to obtain one value per subplot. Figure 7. Eijkelkamp Type 1B soil shear tester 50

71 Divot Resistance Divots were produced using the PENNSWING device (Fig. 5-6). PENNSWING consists of the head of a golf club pitching wedge fastened to the end of a weighted pendulum. The pendulum is weighted with a lead-filled steel cylinder weighing 76 kg. Two adjustable metal pads allow for height adjustment of the machine relative to the ground. The machine was adjusted to penetrate the soil to a 15mm depth. The device was attached to the three-point hitch of a tractor. Six 11.3 kg weight plates were used to prevent the unit from lifting off the ground upon impact with the soil. A divot was created by releasing the pendulum from a horizontal position, resulting in the head of the pitching wedge impacting and cutting through the soil surface. Measurements were taken to determine the maximum length, width, and depth of each divot. Two divots per plot were created and measurements were averaged in order to represent one length, width, and depth measurement per plot. Because the device is set to penetrate the surface to a pre-determined depth (15mm) and divot width is controlled by the width the of club head (6.6 cm), divot length was used to quantify divot resistance. At the same time that divot resistance was evaluated, volumetric soil water content values were obtained using a Theta Probe (Type ML2x, Delta-T Devices, Cambridge, England). During both ratings dates in 2006 and 2007, the volumetric water content was 24%. 51

72 Figure 8. PENNSWING about to create a divot. Figure 9. Divots produced by PENNSWING. 52