Growing Degree-Day Based Plant Growth Regulator Application on Ultradwarf Bermudagrass Putting Green

By Kate Parkes and Eric Reasor, Ph.D.

Plant growth regulator (PGR) programs are routinely implemented on ultradwarf bermudagrass putting greens to decrease mowing requirements and increase putting green playability. Current PGR programs are based on regular calendar application frequencies (i.e., weekly or biweekly) that are relatively unchanged by climate and environmental factors. PGR applications at the aforementioned frequencies have the potential for an overaccumulation of the PGR within the turfgrass plant that can lead to decreased traffic tolerance, increased disease severity, and increased weed presence.

To combat the problem of PGR overaccumulation, the concept of using a growing degree-day (GDD) model to predict PGR reapplications has been researched (Kreuser and Soldat, 2011; Reasor et al., 2018). The GDD model determines when to reapply by using heat accumulation to estimate the amount of PGR remaining in the turfgrass. Temperature has been shown to be a driver for the rate at which PGRs are metabolized in turfgrasses. As a result, if weather is ever-fluctuating on a daily and certainly weekly basis, shouldn’t it be taken into consideration when reapplying a PGR?

What are the benefits of reapplying PGRs using a GDD model and how does this method compare to weekly or biweekly reapplication frequencies? Research at Mississippi State University (MSU), University of Tennessee, and North Carolina State University was conducted in 2018 to answer some of these questions.

Three PGR programs were implemented on ultradwarf bermudagrass putting greens from May to November 2018. The PGR programs consisted of trinexapac-ethyl (Primo MAXX, Syngenta) applied at 4 fl oz per acre every week, 2 fl oz per acre twice weekly, and 4 fl oz per acre every 220 GDD 10C . The weekly applications were made every Monday and twice weekly applications every Monday and Thursday. A non-treated check with no PGR was also included. Trinexapac-ethyl was applied using a CO 2 pressurized backpack sprayer with 11004VS nozzles (TeeJet) calibrated to deliver 40 gallons per acre. The experiment was conducted on ‘MiniVerde’ in Starkville, MS, ‘TifEagle’ in Knoxville, TN, and ‘Champion’ in Durham, NC. Plots were mowed five times per week at 0.125" and maintained with 0.1 lb N/ 1000ft 2 of urea (46-0-0) applied every 14 days.

Onsite weather stations at each location measured daily air temperatures that were used to calculate cumulative GDD. First, the daily mean air temperature was calculated from the maximum and minimum daily air temperature. Next, the base temperature (10°C; 50°F) was subtracted from the daily mean air temperature, and the difference was the daily GDD value. Consecutive daily GDD values were then added to calculate cumulative GDD. The base temperature of 10°C was selected as that is the temperature where photosynthesis becomes minimal for C 4 plants. In contrast, the GDD reapplication interval for creeping bentgrass utilizes a base temperature of 0°C (32°F) (Kreuser and Soldat, 2011). The 220 GDD 10C was utilized in this research based on previous research conducted in the summer of 2017. Previous research indicated that peak growth regulation occurred at 166 to 177 GDD 10C (Reasor et al., 2018) and Kreuser and Soldat (2011) suggested PGR reapplication at 1.3 times the GDD at peak suppression to maximize plant health and putting green playability (i.e., reapplication on ultradwarf every 216 to 230 GDD 10C ). The reapplication target was 220 GDD 10C ; however, based on weather, scheduling, and other factors, applications were generally made between 210 and 230 GDD 10C . The reapplication timing of Kreuser and Soldat (2011) was based on creeping bentgrass putting greens with an observed “rebound effect” or growth above the normal rate. No rebound growth was observed on ultradwarf bermudagrass cultivars (Reasor et al., 2018); therefore, the reapplication interval could likely be expanded beyond 230 GDD 10C . Visual turfgrass quality was assessed weekly from 1 June to 1 November on a scale of 1 to 9. Golf ball roll distance using a USGA Stimpmeter was measured three times per day, twice a week. Morning ball roll distance was measured from 7 to 9 AM, mid-day 10 AM to 1 PM, and afternoon from 2 to 5 PM.

Throughout the summer, mean turfgrass quality of plots receiving trinexapacethyl on a weekly basis and on GDD interval was significantly greater than the non-treated check and twice weekly treatments (Fig. 1). Weekly and GDD treatments were not statistically different; however, there was a trend of the GDD interval having greater turfgrass quality.

Golf ball roll distance of the four treatments are presented in Fig. 2. As expected, the non-treated had significantly less ball roll distance throughout the growing season than the three trinexapac-ethyl treatments. Among the three trinexapac-ethyl treatments, the GDD interval treatment had significantly greater ball roll distances early in the summer, but then was significantly lower in the latter part of the season. The ball roll distance might have been statistically lower; however, the difference between it and the weekly and twice weekly applications was only six inches. According to Karcher et al. (2001) the average golfer cannot detect ball roll distance differences of six inches or less.

Different golf ball roll distances were measured in the morning, midday, and afternoon (Fig. 3). The morning distances were consistently faster than those in the mid-day and afternoon. However, mid-day and afternoon were similar throughout the growing season. These ball roll distances among times-of-day did not change with the different trinexapac-ethyl treatments. Regardless of weekly, twice weekly, or GDD reapplications, mid-day and afternoon ball roll distances were similar.

Implementing a GDD model for trinexapac-ethyl reapplications could be beneficial to turf managers. GDD based reapplications provided increased plant health and visual turfgrass quality, while still maintaining desired golf ball roll distances. The GDD based reapplications also led to fewer trinexapac-ethyl applications. Trinexapac-ethyl applications were made 44 times for the twice weekly treatment, 22 times for the weekly treatment, and 11 times for the GDD treatment. Since weather is an uncontrolled variable, and an over accumulation of PGR in turfgrass is an issue, the implementation of GDD based trinexapac-ethyl applications has the potential to increase overall quality and performance of ultradwarf bermudagrass putting greens.

Karcher, D. T. Nikolai, and R. Calhoun. 2001. Golfers’ perceptions of greens speeds vary. Golf Course Mgt. 69(3):57-60.

Kreuser, W.C., and D.J. Soldat. 2011. A growing degree day model to schedule trinexapac-ethyl applications on Agrostis stolonifera golf putting greens. Crop Sci. 51:2228- 2236. doi:10.2135/cropsci2011. 01.0034

Reasor, E.H., J.T. Brosnan, J.P. Kerns, W.J. Hutchens, D.R. Taylor, J.D. McCurdy, D.J. Soldat, and W.C. Kreuser. 2018. Growing degree day models for plant growth regulator applications on ultradwarf bermudagrass putting greens. Crop Sci. 58: 1801-1807. doi:10.2135/cropsci 2018.01.0077