Application temperature influences pyrithiobac (Staple®) efficacy in a semi-arid environment

Weed control by Staple® has been inconsistent since its commercial introduction in 1996. The influence of temperature at the time of post-emergence Staple® applications was investigated as a source of the variability in field activity. Over two growing seasons, sixteen independent and random applications of Staple® were made to 2-4 inch Palmer amaranth (Amaranthus palmeri S. Wats). The herbicide was applied at a rate of 0.063 lbs. a.i./A in a carrier volume of 15 GPA with 1% crop oil concentrate using a backpack sprayer. Plant/soil scene temperature was monitored with an infrared thermometer and air temperature was monitored with a thermocouple. These temperatures were recorded at 15-minute intervals from mid-June to late September of both years. Field activity, expressed as a percentage of the non-treated dry weight, was quantified as the dry weight accumulated by treated plants for a 14 day period following herbicide application. Accumulated dry weight ranged from 0.1% to 71.5% when compared to non-treated Palmer amaranth. Field activity differences were correlated with plant/soil scene temperature at herbicide application. Applications at temperatures above 93°F resulted in poor activity.