Targeting Postemergence Dallisgrass Control Using Cooling Degree Days.

Field experiments were conducted at Pine Hills Golf Course in Winder, GA during 2016 and 2017. Research was conducted on a common bermudagrass [Cynodon dactylon (L.) Pers.] (3.8 cm) rough with a mature dallisgrass infestation. Plots (1.5 x 1.5 m) contained approximately 75% dallisgrass cover. Treatments were arranged in a 4 x 3 factorial (4 herbicides and 3 rates) within a randomized complete block design with 4 replications. Herbicide treatments were initiated on September 23, 2016 (5 to 125 CDD_22C) and consisted of Drive XLR8 (quinclorac) at 1.5, 2.3, and 3.0 kg ai ha^-1; MSMA (monosodium methanearsonate) at 6.1, 9.1, and 12.1 kg ai ha^-1; Roundup PRO (glyphosate) at 5, 7.6, and 10.1 kg ai ha^-1; and Tribute Total (thiencarbazone + foramsulfuron + halosulfuron) at 0.14, 0.18, and 0.22 kg ai ha^-1. Methylated seed oil at 0.5% v/v was added to all Drive XLR8 and Tribute Total treatments. An untreated check was included for comparison. Herbicides were applied with a CO₂ powered backpack sprayer calibrated to deliver 843 L ha^-1 at 221 kPa. Visual ratings of % dallisgrass cover were recorded at 0, 4, and 21 weeks after initial treatment (WAIT) and converted to percent control by comparing back to % cover at trial initiation (0 WAIT). Initial dallisgrass control (4 WAIT) was greatest in response to Roundup PRO (100%), regardless of rate. Control with MSMA was 74 to 86%, regardless of rate, followed by (fb) Drive XLR8 (45 to 63%) and Tribute Total (25 to 47%) 4 WAIT. Long-term control (21 WAIT) was greatest in response to Roundup PRO (100%). All other treatments resulted in dallisgrass control of 85 to 95%, regardless of herbicide or rate. Targeted herbicide applications based on cooling degree days may have increased dallisgrass control; however, regrowth will be monitored through the summer.