Cover Story
Growing Degree-Day Based Plant Growth Regulator Application on Ultradwarf Bermudagrass Putting Greens By Kate Parkes and Eric Reasor, Ph.D.
Introduction
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. 10 • Mississippi Turfgrass • Winter 2019
GDD Research
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 GDD10C. 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 CO2 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/ 1000ft2 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
Figure 1
Visual turfgrass quality of ‘MiniVerde’ ultradwarf hybrid bermudagrass putting green in Starkville, MS from 1 June to 29 October treated with 4 fl oz per acre per week, 2 fl oz per acre twice weekly, or 4 fl oz per acre every 220 GDD10C. A non-treated check was included for comparison.
