Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: February 5, 2000
Publication Date: N/A
Technical Abstract: Pyrithiobac is a preemergence and/or postemergence cotton herbicide (Gossypium hirsutum)used to control annual broadleaf weeds, including Palmer amaranth (Amaranthus palmeri). Inconsistent weed control has been observed under field conditions. This inconsistency may be influenced by abiotic factors such as rate, weed size, spray coverage, or environmental factors. The influence of temperature on this variability was investigated Results from field studies performed over two growing seasons identified a temperature range from 20 to 34 degrees C at the time of herbicide application that correlated with acceptable weed control. Thermal limitations may affect biological processes such as uptake, translocation, metabolism, and enzyme inhibition. To investigate a potential source of thermal limitations on pyrithiobac efficacy, the thermal dependence of in vitro inhibition of acetolactate synthase (ALS), the site-of-action for pyrithiobac, was examined. A crude leaf extract of ALS was obtained from Palmer amaranth. Relative inhibitor potency (I_50) values were obtained at saturating substrate conditions for temperatures from 10 to 50 degrees C. Regression analysis of field activity against I_50 values showed the two data sets to be highly correlated (R^2=0.88). Finally, historic air temperature data sets collected over several growing were evaluated to assess the probability, duration, and frequency of the temperature range where acceptable weed control occurred. The recommended thermal range occurred during 60 to 80% of the daylight hours. Therefore, 20 to 40% of pyrithiobac applications are potentially adversely affected by application temperature. Computer images of seasonal patterns were developed to provide a visualization of seasonal efficacy.