Economic evaluation of long-term weed control systems and of pitted morningglory control systems with glyphosate-tolerant cotton

Scope and method of study. Field research was conducted to evaluate efficacy and economics of glyphosate and conventional herbicides in a glyphosate-tolerant cotton system and to measure weed population shifts over a 3-yr period. One experiment was conducted for 3 years under dryland conditions with herbicides applied to the same plots each year. Weed control and weed count data were collected each year and economic comparisons were made for both glyphosate and conventional herbicide treatments. The second experiment was conducted for 2 years under irrigated conditions to compare both glyphosate and conventional herbicide systems and the economics of these systems for pitted morningglory control.; Findings and conclusions. In the dryland experiment, a single postemergence glyphosate application was the most economical treatment in all 3 years of the experiment. Weed shifts were observed over the 3-yr period. In the untreated checks, populations shifted from the predominance of johnsongrass in 1998 to common lambsquarters in 1999, to Palmer amaranth in 2000. Plots treated with pendimethalin preplant incorporated followed by a single postemergence glyphosate application had a small increase in pitted morningglory and yellow nutsedge populations. In the irrigated experiment, a single postemergence application of glyphosate did not provide season-long pitted morningglory control. Three postemergence glyphosate applications when compared to two postemergence pyrithiobac applications controlled pitted morningglory equally, thus resulting in similar cotton yield and net return. These data indicate that weed species shifts can occur in as few as 3 years with repeated applications of the same herbicide or herbicides. Glyphosate and glyphosate-tolerant cotton can be an effective and economical alternative to conventional herbicides used in Oklahoma under dryland and irrigated conditions.