SportsTurf

18 SportsTurf | May 2016 www.sportsturfonline.com FIELD SCIENCE Not surprisingly, the grass had more visual appeal when higher N rates were applied; however, these rates also reduced root mass and divot resistance. The goal of NFL field managers and sod growers isn't to maximize color or density. Their objective is to produce stable turf with as many roots and rhizomes as possible. Let's examine plant biology to propose why applying less fertilizer produced more resilient turf. Turfgrass plants are "shoot prioritizers." This means when N is readily available, the plants use most of their carbohydrates to produce new leaf tissue. The grass may look dark green and vigorous, but excess top growth comes at the expense of valuable belowground biomass. The leaf tissue is also succulent and more easily torn from the soil. MOWING HEIGHT EFFECTS We were surprised to find no significant differences in divot resistance across cutting heights, although cutting height did affect shoot density and surface shear strength (data not shown). Prior research at Penn State has shown better divot resistance with lower mowing heights. Although the turf produces fewer total roots when clipped closely, most of those roots are concentrated in the top inch of soil and help stabilize the turf. This strategy is not recommended for facilities that receive less intense but more frequent use, such as multi-use high school fields. There the cutting height should be higher to counteract the frequent, abrasive wear. The lack of a mowing height effect in our recent project could have been related to the cultivar blend or the use of Primo masking the effect. TOPDRESSING EFFECTS NFL field managers prefer sod with little thatch because the thatch-soil interface represents a shear plane along which divots may form. Unfortunately, the aggressive nature of Kentucky bluegrass cultivars used for NFL sod production also makes them heavy thatchers. We included topdressing as a treatment because light, frequent sand applications can mitigate thatch buildup without the need for mechanical cultivation. Aerification or verticutting to remove thatch would temporarily reduce surface stability and prevent the sod from being harvestable. We hypothesized that divot resistance would be improved by light topdressing, just enough to dilute the thatch as it formed. However, the data in this project did not support our hypothesis. Divot resistance was not significantly affected by the topdressing treatment in year 1, and in year 2 topdressing actually reduced divot resistance by a slight margin (6%). This part of the project raised more questions than answers, and the interactions among thatch, topdressing, and divot resistance warrant further study. CONCLUSIONS For NFL field managers and the sod growers they trust, surface stability always trumps visual appeal. While the two are not mutually exclusive, our research demonstrated that sacrificing some color and density by lowering N inputs could improve divot resistance by 30% or more. Applying less nitrogen means the grass will not be as dark green or as dense. But beneath the surface, roots and rhizomes will flourish and help to improve divot resistance. Divot resistance cannot be predicted solely by shoot density, root mass, soil moisture or any other single characteristic. It is a combination of many turf properties, and there remains much to learn about the effects of sod pre-conditioning on divot resistance. Evan Mascitti is assistant sports turf manager with the Triple A Gwinnett Braves; he completed this project while earning his MS in Agronomy at Penn State. Dr. Andy McNitt is program coordinator-turfgrass science major, professor of soil science, and director, Center for Sports Surface Research at Penn State; and Tom Serensits is manager of the Center for Sports Surface Research. 1.00 1.20 1.40 1.60 1.80 2.00 2.20 2.40 2-0 2-1 3-0 3-1 4-0 4-1 Below-ground biomass (g/plug) N program (no. of applications in spring-fall) Root/rhizome mass vs. N treatment-2013 10.0 15.0 20.0 25.0 30.0 35.0 2-0 2-1 3-0 3-1 4-0 4-1 Divot length (cm) N program (no. of applications in spring-fall) Divot length vs. N treatment-2013

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