Pyrithiobac control of Palmer amaranth on the Texas Southern High Plains was correlated previously with temperature at the time of application. In the present study, the thermal dependence of pyrithiobac efficacy was used to define a thermal application range (TAR) for postemergence pyrithiobac applications. Several years of temperature data from four cotton-growing regions of the United States were analyzed with respect to the TAR to determine the extent to which temperature limitations could affect pyrithiobac applications. Temperatures outside the TAR occurred in all years and regions analyzed. Analyses of four geographic regions utilizing 4 to 11 yr of data for each region indicated the following percentages of hours inside the TAR: Lubbock, TX, 54 to 94%; Maricopa, AZ, 27 to 33%; Raleigh-Durham, NC, 70 to 97%; and Jackson, MS, 81 to 99%. A detailed analysis of the frequency and duration of the TAR in Lubbock, TX, showed that, periodically, temperatures outside the TAR may limit the efficacy of postemergence pyrithiobac applications for several consecutive days. Finally, the TAR was shown to be useful as a postapplication diagnostic tool for evaluating herbicide applications that resulted in poor efficacy. These results suggest that long-term evaluation of historic temperatures with respect to the TAR for a given herbicide may provide insight into the potential limitations of herbicide efficacy and underscore the potential utility of developing TARs based on field and laboratory analyses of herbicide thermal dependence.
Nomenclature: Pyrithiobac; cotton, Gossypium hirsutum L.; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA.