Postemergence over-the-top Cotoran and Probe treatments were evaluated for weed control and their effects on cotton yield and lint quality near Portageville, Mo. during 1989 and 1990. Studies were initiated as randomized complete block designs in a split-plot arrangement of treatments. The main plots were weedy and weed-free (handweeded the entire season) and sub-plots were herbicide treatments. Herbicide treatments were Probe and Cotoran applied to cotyledon, 1 to 2 and 5 to 6 leaf cotton and post-directed to 3 to 4 inch cotton. Cotoran and Probe were applied at 0.5 and 0.25 lb ai/A at each over-the-top application, respectively. These treatments were compared to 1.0 and 0.5 lb ai/A Cotoran and Probe post-directed, respectively. Weed species evaluated were ivyleaf and entireleaf morningglories, velvetleaf and common cocklebur. All treatments received a base treatment of Treflan (0.75 lb ai/A) plus Zorial (1 lb ai/A) preplant incorporated followed by Cotoran (1.25 lb ai/A) preemergence.

Cotoran provided better morningglory control at 8 weeks after planting (WAP) when applied to 1 to 2 or 5 to 6 leaf cotton (95%) than the cotyledon or post-directed (88%) treatments. Morningglory control ranged from 69 to 83% and there were no differences between treatments at 12 WAP. Probe resulted in better morningglory control when applied to 1 to 2 leaf cotton (88%) or post-directed (84%) than at the cotyledon (73%) or 5 to 6 leaf cotton stage (67%) at 12 WAP.

Velvetleaf control was 63 and 76% when Cotoran was applied post-directed or at the cotyledon stage of cotton, respectively, at 12 WAP. The other POST Probe treatments provided less than 60% velvetleaf control. Probe resulted in better velvetleaf control when applied post-directed (89%) or at the 1 to 2 leaf (75%) compared to the cotyledon (46%) or 5 to 6 leaf cotton stage (33%).

Cotton was more tolerant to over-the top cotoran when applied during the cotyledon than at the true leaf stages of cotton. Cotton foliar injury ranged from 6 to 30% over the two year period. Probe caused foliar injury from 14 to 31% when applied over-the tip of cotton. Probe was also more injurious to cotton during the true leaf stage of growth (30%) as opposed to treatments during the cotyledon stage (14%) in 1989. In 1990 cotton foliar injury from Probe was similar at all stages of growth evaluated (26 to 31%)

Cotroan applied post-directed or during the cotyledon stage resulted in lint cotton yields similar to the weed-free control when weeds were allowed to go uncontrolled after herbicide treatments in 1990. Cotoran applied at the 1 to 2 or 5 t 6 leaf cotton stage resulted in reduced lint cotton yields compared to the weed-free control when Probe was applied post-directed in 1989 and 1990 or applied to cotyledon or 1 to 2 leaf cotton in 1990, then allowing weeds to recover or reinfest after herbicide treatments. In 1989, all cotoran treatments resulted in less lint cotton yields when weeds were uncontrolled after the herbicide treatments.

Cotoran reduced lint cotton yields in 1989 without the presence of weeds when applied to 1 to 2 leaf cotton in 1989. In 1990, none of the herbicide treatments reduced cotton yields in the absence of weeds. herbicide treatments did not lower cotton quality.