Thermal dependence of pyrithiobac efficacy in Amaranthus palmeri
Light, Ginger G.
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Variability in weed control following pyrithiobac applications has been observed under field conditions. The influence of temperature on this variability was investigated. Results from field studies performed over two growing seasons identified plant and air temperatures at the time of herbicide treatment that correlated with differences in whole-plant efficacy (R^ = 0.90). Based on the field data, weed control with pyrithiobac was acceptable at application temperatures of 20 to 34° C. 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 seedling Amaranthus palmeri. Relative inhibitor potency (I50) values were obtained at saturating substrate conditions for temperatures from 10 to 50° 0. Regression analysis of field activity against I50 values showed the two data sets to be highly correlated (F^ = 0.88). Historic air temperature data sets collected over eleven growing seasons were evaluated to assess the probability, duration, and frequency of the temperature range where acceptable weed control occurred. The recommended thermal range occurred during 59 to 93% of the daylight hours in a typical growing season. Conversely, up to 41% of pyrithiobac applications are potentially adversely affected by application temperature. Computer images of seasonal temperature data with a color oveday corresponding to the temperatures below, within, and above the recommended application temperature range were developed to provide a visualization of seasonal efficacy. The thermal dependence of enzyme/herbicide interactions may provide another means of understanding environmental factors limiting herbicidal efficacy and predicting herbicide inhibition at the whole-plant level.