Submitted to: Weed Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/22/2001
Publication Date: 2/22/2001
Citation: Interpretive Summary: The use of herbicides to control weeds in crops incurs costs to the producer and potential environmental impacts. The monetary costs are minimized when herbicide applications are effective in killing the targeted weeds. Environmental impact is limited when highly effective applications reduce the number of applications required for control. In this study the effect of environmental temperatures on the effectiveness of the herbicide Staple, was analyzed. A previous study determined that the ability of Staple to kill pigweed was affected by the temperature of the plant at the time of the application. Based on that data it was determined that application of Staple at temperatures between 20 and 34C, referred to as the "Thermal Application Range" would optimize efficacy. In this study the utility of a Thermal Application Range was analyzed with several years of environmental data from five geographic regions. The results indicate that tthe amount of time that temperatures are outside the Thermal Application Range varies from 1 to 73% across locations and from 6 to 46% within a location. The study establishes that consideration of thermal effects on herbicide action could improve efficacy and reduce application costs and environmental impacts.
Technical Abstract: Previous field studies have shown that pyrithiobac efficacy evaluated over 2 growing seasons on the Texas Southern High Plains was correlated with air temperature at the time of application (R^2=0.90). Based on these studies, a temperature-based recommendation that provided >90% reduction in dry weight 14 days after postemergence pyrithiobac applications was created. This thermal-based recommendation suggested that postemergence pyrithiobac applications be made when air temperatures are 20-34C. However, the potential utility of this recommendation to producers has not been examined. The objectives of this study were to determine the probability, frequency, & duration of the temperature-based postemergence pyrithiobac application range, & investigate the utility of using computer visualization to convey the significance of postemergence pyrithiobac efficacy thermal dependence Historic air temperature data sets collected over 11 growing seasons were analyzed. The recommended thermal range occurred during 59-93% of the daylight hrs in a typical growing season. Conversely, up to 41% of pyrithiobac applications might be adversely affected by application temperature. In some years, the temperature exceeded 34C on more than half the days evaluated for more than 6 hrs. Long durations of temperatures exceeding 34C provide a narrow window for applications. However, the duration of temperatures below 20C was generally <2 hrs. Cooler temperatures predominantly occurred in the early hours of the morning, increased, & remained within range for several hours. Therefore, delaying application until the minimum temperature was reached could allow producers to obtain acceptable weed control.