Use of a Growing Degree Model to Schedule Trinexapac-Ethyl Applications to Creeping Bentgrass Golf Putting Greens.

The plant growth regulator trinexapac-ethyl (TE) is widely used in the turfgrass industry to reduce turfgrass clipping production. Research has shown that TE efficacy declines during summer. Loss of efficacy is directly correlated with air temperature and TE metabolism by plants. The purpose of this study was to evaluate a growing degree day model (GDD) to estimate TE metabolism and indicate when TE should be re-applied to maintain constant growth regulation. This study was conducted on a creeping bentgrass (Agrostis stolonifera Hud.) golf putting green with an 80:20 sand:peat root zone at the O.J. Noer Turfgrass Research Facility in Madison, WI during the summer of 2008. The treatments consisted of four replications of TE re-application intervals of 100, 200, 400, 800 GDD, every four weeks and an un-treated control. The GDD model was based on a summation of the mean daily air temperature (°C) with a base of 0°C.  After the accumulated thermal units had been surpassed TE was re-applied. TE was applied at the rate of 0.05 kg A.I. ha^-1.  Turfgrass clippings were collected daily with sand debris removed. Visual turfgrass quality ratings and chlorophyll index were recorded weekly. The 100 and 200 GDD re-application intervals provided a consistent 20-30% mean clipping production reduction with respect to the control plots throughout the summer.  The other re-application intervals resulted in significantly increased turfgrass clipping production above the untreated control approximately 250 to 300 GDD after application of TE. Clipping production for the 400, 800, and four-week re-application interval treatments were 100, 98, and 104% of the control. The 100 and 200 GDD treatments consistently had the greatest visual turfgrass quality and chlorophyll index measurements throughout the study period. Re-applying TE every 200 GDD units provided consistent growth regulation during a growing season in creeping bentgrass putting greens.