Use of Spray Adjuvants to Reduce Herbicide Rates While Maintaining Weed Control.

Field experiments were conducted at the Pine Hills Golf Club in Winder, GA during the summer of 2013. The trial was located on a common bermudagrass rough maintained at 5.1 cm with a natural infestation of Carolina false dandelion [Pyrrhopappus carolinianus (Walt.) DC.]. Plots measuring 1.5 x 1.5 m were arranged in a 3 x 2 x 2 factorial within a randomized complete block design with four replications. The main factor was herbicide chemistry (Celsius, 4 Speed XT, and Blindside), the sub-factor was herbicide rate (low and high), and the sub-sub-factor was spray adjuvant (Adigor and MSO). Herbicide treatments were applied on June 12, 2013 and consisted of thiencarbazone + iodosulfuron + dicamba (Celsius) at 0.08 or 0.12 kg ai ha^-1, 2,4-D + triclopyr + dicamba + pyraflufen (4 Speed XT) at 0.84 or 1.2 kg ai ha^-1, and metsulfuron + sulfentrazone (Blindside) at 0.21 or 0.3 kg ai ha^-1. Methylated rapeseed oil (Adigor) or methylated seed oil [modified vegetable oil and surfactant blend (MSO)] at 0.5% v/v was added to each herbicide treatment. A non-treated check plot was added for comparison. Treatments were applied using a CO₂ pressurized backpack sprayer equipped with XR8004VS nozzle tips calibrated to deliver 375 L ha^-1at 221 kPa. Visual ratings of % bermudagrass phytotoxicity and % false dandelion cover were recorded 1, 2, 4, 6, and 10 weeks after treatment (WAT). Percent bermudagrass phytotoxicity was rated on a scale of 0 (no bermudagrass phytotoxicity) to 100% (complete bermudagrass desiccation). Percent false dandelion cover was converted to % control by comparing back to cover ratings recorded at the initiation of the trial. No bermudagrass phytotoxicity was observed throughout the length of the trial, regardless of treatment. However, the trial was conducted in a heavily weed infested area containing low bermudagrass cover with initially poor turfgrass quality. At 4 WAT all treatments resulted in 100% Carolina false dandelion control except Celsius at 0.08 kg ai ha^-1 + MSO (99%) and Celsius at 0.12 kg ai ha^-1 + Adigor (98%). At 10 WAT 4 Speed XT treatments maintained 100% false dandelion control regardless of herbicide rate or spray adjuvant. Regrowth of false dandelion was observed in response to the other two herbicides 10 WAT. Celsius at 0.12 kg ai ha^-1 + Adigor resulted in the lowest control (87%), while all other Celsius treatments exhibited ≥ 90% control 10 WAT. Blindside at 0.3 kg ai ha^-1 + MSO still exhibited 100% false dandelion control, while all other Blindside treatments resulted in 97% control 10 WAT. The addition of spray adjuvants may maintain herbicide efficacy for the control of Carolina false dandelion while allowing for the reduction in the amount of active ingredient necessary for control. However, this may be herbicide specific.