Developing a Growing Degree Day Model for Trinexapac-Ethyl Applications on Miniverde Bermudagrass Putting Greens.

Trinexapac-ethyl (TE) (Primo MAXX, Syngenta, Greensboro, NC) is a commonly applied plant growth regulator on well-maintained turfgrass. Properly-timed TE applications can provide consistent growth regulation without a decrease in turfgrass quality; however, improper reapplications can lead to decreased quality and increased growth following the regulation period. Properly-timed reapplications are difficult because TE is metabolized faster as temperatures increase, which renders a calendar-based schedule ineffective. Reapplication models based on growing degree day (GDD) accumulation is theorized as a solution.

The objective of this study was to develop a GDD model for TE reapplications on ultradwarf bermudagrass (Cynodon dactylon X C. transvaalensis Pers. L.) putting greens. Two field trials were conducted concurrently on a MiniVerde putting green at the Sports Surface Field Laboratory in Auburn, AL during the summer of 2016 and repeated in 2017. One trial was designed to evaluate clipping yield following a single application of TE at 0.044 kg a.i. ha^-1. A single application was made on the first and fifteenth of May, June, July, and August. Results indicate maximum clipping suppression occurred from 14 days after treatment (DAT) in May to 10 DAT in July, but GDDs to this point was relatively constant from 305 GDD in May to 314 GDD in July (accumulation ranged from 15 GDD per day in May to 30 GDD in July).

The other trial was intended to validate a GDD interval for reapplying TE. Treatments included two TE rates (0.022 and 0.044 kg a.i. ha^-1) and four reapplication intervals (100, 200, 400, and 600 GDD). As expected, the 100 and 200 GDD intervals at both rates provided consistent suppression; however, the 100 GDD at 0.044 kg a.i. ha^-1 adversely affected quality. Both trials indicate applying TE at 0.022 kg a.i. ha^-1 every 200 GDD will provide consistent suppression and acceptable turfgrass quality.