Effects of pre-harvest cultural practices on the divot resistance of thick-cut Kentucky bluegrass sod
Open Access
- Author:
- Mascitti, Evan Christopher
- Graduate Program:
- Agronomy
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- December 11, 2014
- Committee Members:
- Andrew Scott Mc Nitt, Thesis Advisor/Co-Advisor
- Keywords:
- Kentucky bluegrass
sod
divot
nitrogen
turfgrass
topdressing
cutting height
football - Abstract:
- Athletic fields must provide stable and consistent footing to minimize injuries and maximize player performance. Professional athletes in the National Football League (NFL) impart extreme shearing forces to the surface during competition, producing divots. Divots are defined as portions of the turf gouged from the surrounding area by athletes’ cleated footwear. Divoting is the primary mechanism of damage on NFL fields, as opposed to the abrasive wear and soil compaction common on more-frequently but less-intensively trafficked surfaces. During the latter stages of the NFL season, the cumulative removal of above- and below-ground vegetation through divoting can destabilize the playing surface. At this time of year the prevailing atmospheric and edaphic conditions in the northeastern United States are unconducive to turfgrass growth and effectively eliminate turf recovery. To restore stable footing the surface must be replaced with new sod. Aside from ambient environmental conditions at sod farms, the quality of the sod installed at NFL stadia is a direct consequence of management decisions by sod growers prior to the sod harvest and installation. Since the surface will be used for competition immediately after installation, there is little time for athletic field managers to alter the surface characteristics through management practices. While some divoting is inevitable, surface stability will be prolonged if divot sizes and numbers are minimized. It is thus imperative that divot resistance is previously optimized through cultural practices at the sod farm prior to harvest, transport, and installation. The objectives of this research were (1) Maximize divot resistance while maintaining tensile strength of thick-cut Kentucky bluegrass sod through manipulation of pre-harvest cultural practices, and (2) Determine the effects of mowing height, sand topdressing, and nitrogen fertilization on shoot density, thatch accumulation, and below ground biomass and their relationships to divot resistance. Two field experiments were conducted to evaluate the effects of cutting height, sand topdressing, and nitrogen fertilization on the divot resistance of thick-cut sod immediately after harvest. Experiment 1 was spatially replicated at the Joseph Valentine Turfgrass Research Center (University Park, PA) and Tuckahoe Turf Farms (Hammonton, NJ). A four-way blend of Kentucky bluegrass (KBG) (Poa pratensis L.) cultivars was seeded during early fall of 2012. Treatments were initiated in spring of 2013. Cutting height treatments were established as discrete “experiments” maintained at 3.18 and 3.81 cm. Within each cutting height experiment a 2x6 topdressing x nitrogen factorial experiment was conducted. The topdressing treatments were a control receiving no sand and a treatment including three sand applications to total 8.5 kg sand m-2. The six nitrogen treatments ranged from 96-244 kg total N ha-1 yr-1 and were further differentiated by the application timing. Applications supplied 49 kg N ha 1 via granular ammonium sulfate. Plots received either 2, 3, or 4 N applications from March-June and either no additional N or a final N application in September of 2013. Sod was harvested and tested in November 2013 at a standard “thick-cut” profile thickness of 4.45 cm. Divots were produced with Pennswing, a weighted pendulum device. The size of each divot was measured with smaller divots indicating high divot resistance. Other parameters measured in the field included turfgrass color, surface shear strength, and sod strength. To determine the associations among these parameters and potentially related turfgrass morphological characteristics, core samples were collected from each plot and evaluated for shoot density, thatch accumulation, and below-ground biomass. Experiment 2 evaluated the effects of four cutting heights (2.54, 3.18, 3.81, or 4.45 cm) on divot resistance, shear strength, and sod strength. Plots were maintained under identical fertilization and topdressing regimes. Treatment evaluation occurred in November 2013 using the same methods as Experiment 1. All plots in Experiments 1 and 2 were treated with trinexapac-ethyl (TE), a plant-growth regulator which has been shown to increase divot resistance. The use of TE is common among NFL field managers and by sod growers producing turf for the NFL, so TE applications were included as part of the plot maintenance to better simulate a real-world scenario. Cutting height did not significantly affect divot resistance in Experiment 1 or 2. While it is possible that cutting height in fact has no influence on divot resistance, the lack of a cutting height effect in this project may be attributable to other factors in the experiments masking such an effect. The blend of cultivars was uniform across all experimental units. KBG cultivars are known to respond differently to various cutting heights. The individual cultivars therefore may have responded in opposing manners, masking the cutting height effect. In addition, the use of TE regulates plant morphology in a fashion similar to close defoliation (producing compact shoots and increased plant density). TE was regularly applied to all plots in Experiments 1 and 2, and may have helped mask any cutting height effect which would otherwise have occurred. Lower cutting heights tended to produce higher shear strength at VRC (both Experiments 1` and 2), although TTF showed a weak trend in the opposite direction. Increased shear strength under closer cutting may be partially attributed to higher shoot density. Shear strength and density were positively, correlated, though the relationship was relatively weak (r=0.26). The low correlation coefficient indicates other turfgrass properties also influence shear strength. Cutting height had no effect on sod strength in Experiment 1, but greater sod strength was obtained in Experiment 2 with higher cutting heights. However the benefit was minor and not realized until cutting height reached 4.45 cm, which is likely too high for sod intended for the NFL. As acceptable sod strength was obtained under all cutting heights, it is recommended that sod growers producing thick-cut sod for in-season replacements maintain the production field close to 3.18 cm. This cutting height is comparable to those used at NFL stadia and would not require the stadium manager to severely reduce the cutting height prior to play. Divot resistance was negatively impacted by high N rates. Small divots indicated higher divot resistance. At the VRC location divot lengths were 37% larger under the 4-1 N treatment than the 3-0 treatment, which was most effective. Smaller yet analogous increases in divot width and depth were also noted at each location under the 4-1 N treatment. The 4-1 N treatment (244 kg ha-1 total N) was the treatment most similar to the actual fertilization schedule used by TTF in 2013 during production of thick-cut sod for NFL stadia. This research project suggests N rates below the current standard may be advantageous with regard to divot resistance. High shoot density and darker green color were associated with greater N rates, but on NFL surfaces these traits are of minimal importance compared to divot resistance. Anecdotal concerns that close mowing and/or topdressing would render the turf unharvestable at 4.45 cm sod thickness were not substantiated in this project. All treatment combinations produced sod strength greater than 100 kg, which in these experiments was estimated as the minimum acceptable strength for thick-cut, big-roll sod. Significant correlations among response variables occurred, but the correlation coefficients were relatively weak. These relationships indicate divot resistance is affected by multiple properties rather than a single turfgrass characteristic. Divot length was the strongest measure of divot resistance and was negatively related to shear strength (r= -0.47) and sod strength (r= -0.38). Surprisingly, divot length was not significantly correlated to below-ground biomass. Thatch thickness, however was significantly related to divot length (r= -0.26). The relationship was negative, indicating plots with thicker thatch layers also had shorter divots. Such a relationship is contrary to reports from practitioners, most of whom opine divot resistance to be compromised by thatch buildup. The relationship between thatch and divot resistance is complex and may depend on factors not accounted for in this study such as depth of cleat penetration, impact angle, and the physiochemical properties of the thatch layer (e.g. water status, lignin content, state of decomposition). In the future, a research tool should be developed which more accurately simulates the divot-producing mechanisms of NFL athletes. Little research has investigated production of thick-cut sod with regard to in-season replacement of American football fields. The goal of this project was to optimize the cultural practices used by sod growers when producing thick-cut Kentucky bluegrass sod for NFL football stadia. All treatments produced harvestable sod at 4.45 cm thickness despite prior industry concern toward the imposition of close clipping as well as applications of TE and sand topdressing. Under the conditions of this study, divot resistance was improved by reducing nitrogen inputs from the current standard. Further research is needed to understand the effects of more-refined nitrogen regimes as well as the influence of thatch on divot resistance. The results of this project will aid sod growers and turfgrass managers in producing divot-resistant natural grass playing fields.