By Evan Mascitti, Dr. Andy McNitt & Tom Serensits
Nowhere is the familiar hierarchy of field safety, playability, and aesthetics more crucial than in the National Football League. In the philosophical sense, a safe field is like an insurance policy for NFL teams. Pro athletes are pricey investments (as well as human beings!) and stable turf reduces the chance for player injuries, which are already common in football. A tightly knit turf with a dense network of roots and rhizomes provides divot resistance and surface stability. Aesthetics are also important, and a safe field can certainly look good. However, the research outlined in this article demonstrates a scenario where maximum safety and performance were achieved by sacrificing some degree of visual quality.
Nearly all grass fields in the NFL are re-sodded during the season, with some being resodded multiple times per season. Many professional stadiums also host large concerts, college and/or high school games. The additive effects of wear and non-football events eventually reduce turf cover despite the field manager’s best efforts. The wear is both intense and concentrated: the majority of play occurs between the hash marks, which are only 18 feet, 6 inches feet apart in the NFL compared with 40 feet in college and 53 feet, 4 inches in high school. Heavy field use combined with low temperatures and decreasing sunlight significantly reduce turf recovery. In many cases, thick-cut sod (up to 1.75-inches thick) is installed to provide a “new” playing surface. When installed correctly, thick-cut sod can be played on almost immediately.
Once the new turf is installed, the field’s initial performance is determined by the care and maintenance that it received at the sod farm. The new field must be safe and playable right out of the gate. Thus, the sod grower must institute practices in the production field that result in a turf with excellent divot resistance.
To our knowledge, no prior research had studied the best way to pre-condition thick-cut Kentucky bluegrass sod for divot resistance. We designed a 2-year study at Penn State to test various cultural practices and their relationship to performance of thick-cut sod. Our goal was to determine which treatments during the production cycle would yield the most divot-resistant sod immediately after installation.
Treatments were chosen based on input from sod growers, including James and John Betts of Tuckahoe Turf Farms in Hammonton, NJ, NFL field managers, and other researchers. The experiment was conducted at University Park, PA. A duplicate study was also conducted at Tuckahoe Turf Farms. Plots were established atop a sand-based rootzone to mimic the native soil found at Tuckahoe and other farms that produce turf for sand-based fields. A four-cultivar blend of Kentucky bluegrass was seeded in September 2012 and 2013 (30% P-105, 30% Everest, 30% Boutique, and 10% Bewitched). The turf was fertilized with 2 lb N/1000 ft2 to quickly develop ground cover. Treatments began the spring after seeding. We chose three treatments: cutting height, sand topdressing, and nitrogen rate/timing. The treatments were applied over the second growing season to simulate a typical 14-month production cycle (Sept. seeding and Nov. harvest the following year). The experiment was duplicated over the 2012-2013 and 2013-2014 production cycles.
The four cutting height treatments ranged from 1.0-1.75 inches. Sand topdressing treatments included either three split applications to total 40 tons/acre or an untreated control. Nitrogen treatments ranged from 2-6 lb N/1,000 ft2 and were further split by application timing (Table 1). Each N application was made with ammonium sulfate at 1 lb N/1,000 ft2. We applied the plant growth regulator Primo Maxx at the label rate of 0.60 fl oz product/1,000 ft2 on 28-day intervals to the entire experimental area. Research at Penn State has shown Primo can precondition Kentucky bluegrass to improve divot resistance, and this practice has been adopted by many NFL field managers and sod farms.
In late November, 14 months after seeding, we used a sod cutter to harvest the sod at 1.75-inch depth and measured divot resistance for each plot. Divots were produced by dropping a weighted pendulum fitted with the head of a golf club pitching wedge. Smaller divots indicated high divot resistance.
Of all the treatments, nitrogen rate had the greatest effect on divot resistance. When we began the study, a typical production cycle would include fall seeding, with 4 lb N/1,000 ft2 applied during the subsequent spring and another pound applied before the November harvest. We designated this treatment 4-1. The 4-1 treatment generally resulted in the worst divot resistance of the six N treatments. The most effective nitrogen regimes delivered just 3 lb N/1,000 ft2 during the entire season (2-1 and 3-0 treatments); these were nearly 40% more divot resistant than the “standard” 4-1 treatment.
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.
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.
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.