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ATA Turf Times – Beth Guertal, Professor, Auburn University and Dave Han, Extension Specialist and Associate Professor, Alabama Extension and Auburn University

New bermudagrasses (both hybrid and improved vegetative selections of common bermudagrass) are being introduced into the turfgrass market, intended for use in athletic fields, home lawns, golf course fairways, and commercial landscaping. The grasses vary in their shade tolerance, resistance to various diseases, color, texture, and ability to tolerate traffic. For bermudgrasses used on athletic fields, traffic tolerance is one of the most important traits. So, in 2017, researchers at Auburn University installed a study to look at long-term traffic tolerance that evaluated six bermudagrass cultivars available commercially at the time under three levels of artificial traffic. This study evaluated the cultivars’ traffic tolerance when used as a typical high school athletic field.

In April of 2017 sod was laid from six different bermudagrass cultivars commonly used for southeastern US athletic fields. Cultivars were: Tifway, TifGrand, TifTuf, Latitude 36, NorthBridge (hybrid) and Celebration (vegetative common). The grasses were all obtained as sod cut from various commercial sod farms, and all sod was laid on the same day. Sod was laid in plots measuring 10 x 20 feet in size, with each main block of ‘cultivar’ replicated four times in a randomized complete block design.

Traffic was applied as strip plots across each cultivar (each traffic strip was 3 feet wide and 10 feet long), imposed using an artificial traffic simulator. Three levels of traffic were applied to the bermudagrasses: none, low (3 American style football games per week) or high (5 American style football games per week) foot traffic. The artificial traffic simulator is an aerification unit with the tines removed and replaced with strips of tire in which bolts are embedded. The traffic simulator is run up and down each plot, with the tires/bolts simulating foot wear and traffic. Traffic was applied during the typical US high school football season, from late August — November of each year (2017, 2018 and 2019). Figure 1 Plots were not overseeded in the winter, and in all other aspects (irrigation, fertilization, pest control) were managed as a higher-quality high school football field. Mowing height was 2 inches, done 3 times a week using a rotary mower, with clippings returned. Plots were not topdressed nor aerified from year to year.

Collected data included monthly color and quality using a relative 1 – 9 scale, with a score of ‘1’ for completely brown or dead turf, and a score of ‘9’ for lush, dark green turf. Additional data included shoot density, dry weight of roots, rhizomes and stolon, clipping yields, and spring density of Poa annua.

The results (to date) have shown that the cultivars do respond differently to different levels of traffic, but this mainly occurred in the fall, when the grasses were being actively trafficked. In the spring, the grasses had no traffic, and all of them tended to recover from the fall traffic, and differences in shoot density were only due to the cultivar itself. Table 1 shows shoot density of the various cultivars in the spring in various years, and only the highest level of traffic ever decreased shoot density. There were differences in cultivar, with TifGrand often having highest shoot density, and Celebration the lowest. In the fall, when traffic was being applied, Tifway and TifGrand were most affected by increasing levels of traffic, with shoot density decreasing from 5.6 to 3.4 shoots per square centimeter as traffic increased. In comparison, trafficked plots of NorthBridge and Celebration tended to not be as affected by traffic. Although their overall shoot density was often less, it did not reduce further when traffic was applied.

Table 1. Shoot density (shoots per square centimeter) of bermudagrass managed as an athletic field as affected by traffic level or cultivar. The interaction of cultivar and level of traffic was never significant for this data. Level of traffic none (NT), Low (LT) (3 American style football games per week) or High (HT) (5 American style football games per week).  Within each sampling date means followed by the same letter are not significantly from each other at an alpha of 0.05.

The weight of stolons and rhizomes can be important, because that is a key plant part (above- and underground stems) that stores carbohydrates which are then used for turfgrass recovery, growth and spring greenup. Traffic never affected the dry weight of stolons and rhizomes, and the interaction between cultivar and traffic was also never significant (Table 2). Thus, all the differences in the dry weight of stolons/rhizomes was due to the cultivar. The bermudagrass cultivar with consistently the greatest dry weight of stolons and rhizomes was NorthBridge, while Tifway always had the lowest weight.

Table 2. Dry weight of stolons, rhizomes and roots of bermudagrass managed as an athletic field as affected by traffic level or cultivar. The interaction of cultivar and level of traffic was never significant. Level of traffic none (NT), Low (LT) (3 American style football games per week) or High (HT) (5 American style football games per week). Within each sampling date means followed by the same letter are not significantly from each other at an alpha of 0.05.

Regardless of the season, bermudagrass color and quality were more influenced by cultivar than level of traffic. Typically, Celebration and TifGrand were darker in color. When no traffic was applied, bermudagrass color typically had this ranking (from most to least green): TifGrand > Celebration > Tifway = TifTuf > Latitude 36 = NorthBridge. When plots were trafficked, it affected the color of Celebration, TifGrand and Tifway the most, with color decreasing as traffic increased. In comparison, as traffic increased, the color of Latitude 36, NorthBridge and TifTuf were largely unaffected by traffic. Cultivars with best quality (determined by lushness and uniformity of the turf) were often TifTuf, TifGrand and NorthBridge. This was regardless of the level of traffic.

The plots were sodded in the spring of 2017, and a winter annual preemergent herbicide was applied in the fall of that year. The following two years (2018 and 2019) no fall preemergent was applied, allowing evaluation of Poa annua infestation as affected by traffic and cultivar. In April of 2019 and 2020, any plot that was trafficked had greater populations of Poa annua (average of 25% cover in those plots) than in any plot that did not receive traffic (average of 10%). In 2019, plots containing Tifway or TifGrand had the largest Poa annua population, with NorthBridge joining that group in 2020. All other cultivars had similar populations of Poa annua, with a general population increase of 10 to 16% from 2019 to 2020.

It should be emphasized that none of the cultivars performed poorly over the four years of the study, and all had specific seasons when they performed better than others. Some cultivars had consistently darker color, while others had higher shoot density under extreme traffic. A few cultivars recovered quickly from the fall wear period, while others took a portion of the spring to recover from that wear (but all eventually did recover). This study will continue for two more years, allowing us to evaluate the long-term effects of traffic on bermudagrass cultivar performance.