Growing Degree-Days Optimize Trinexapac-ethyl Reapplications for Ultradwarf Bermudagrass Putting Greens

Abstract

Applying trinexapac-ethyl (TE) is a standard practice for growth suppression and quality improvement of ultradwarf bermudagrass [Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy] putting greens, but research is lacking on proper reapplication frequency and rate. Making properly-timed reapplications is necessary to maintain turfgrass suppression and quality benefits. Ability to predict the maximum suppression point (MSP) that follows a TE application is helpful for making a proper reapplication; however, predicting the MSP is difficult because suppression duration is affected by environmental conditions, especially temperature. Previous research shows that growing degree-days (GDD), a variable unit that accounts for temperature, effectively predicts the suppression of creeping bentgrass (Agrostis stolonifera L.) following a TE application. Research was conducted to identify the optimal variable for predicting the MSP after a TE application on a ‘MiniVerde’ ultradwarf bermudagrass putting green. Tested variable units included: calendar days, GDD (base temperatures of 0 to 12ºC), soil temperature (2.5 cm), global horizontal irradiance, and photosynthetically active radiation. Pseudo-R2 values from the resulting models suggest that GDD0 (GDD with a base temperature of 0ºC) predicts the MSP better than the other variables. The GDD0 model (pseudo-R2: 0.564, SE: 0.195) indicates that the MSP occurred at 262 GDD0. From these results, we hypothesized that reapplying TE before the MSP on a 200-GDD0 interval would maintain suppression and quality benefits throughout the season. The objective of the second experiment was to test a GDD0 reapplication schedule for an entire growing season. We included 4 GDD0 intervals (100, 200, 400, and 600) and 2 TE rates (0.022 and 0.044 kg ai ha-1). As expected, the 100- and 200-GDD0 intervals resulted in consistent suppression throughout the experiment, and suppression magnitude increased with the higher TE rate. The 400- and 600-GDD0 intervals caused fluctuation in suppression magnitude from day-to-day. Phytotoxicity occurred after initial applications and was more severe for the high rate.