Performance and economics of selected weed management systems in corn, cotton, and grain sorghum and the behavior of pyrithiobac on three Texas High Plains soils

Selected levels of herbicide and non-chemical weed control measures were employed in cotton, corn, and grain sorghum on the Texas High Plains in 1992 and 1993 to examine their efficacy, crop response, and the subsequent economic effects. Weed pressure was heavy in the cotton plots, and moderate in the sorghum and corn plots. Records of all weed control expenses were maintained. Yields were obtained from each plot in order to calculate the gross crop value and return over weed control costs. Cotton production and returns over weed control costs were greatly improved by herbicide use in both years, and the highest herbicide input gave the highest consistent return of any treatment. In both 1992 and 1993, sorghum yields were increased when herbicides were added to weed control regime. However, none of the weed control measures in sorghum showed any economic advantage over using no weed management at all. This was related to unnecessary herbicide inputs in some treatments and low weed pressure. Results with corn were very similar to sorghum. Traditionally, the Texas High Plains, having an average yearly rainfall of approximately only 15 to 17 inches, is considered to have low weed pressure and low weed species diversity as compared to the eastern half of the country. Most certainly higher rainfall areas with greater weed pressure and diversity would meet with greater costs in controlling weeds, and that must be taken into account when considering the results of this study. Sorption and mobility of 14C labeled pyrithiobac were evaluated for three Texas High Plains cotton-growing soils by utilizing a batch slurry method and soil thin layer chromatography. Sorption studies showed pyrithiobac was moderately adsorbed to all three soils, regardless of clay or organic matter content. However, the herbicide was easily desorbed from the soil, indicating the herbicide to be moderately mobile. Soil thin layer studies also indicated pyrithiobac to be a mobile herbicide, regardless of soil clay or organic matter content.