Cotton farmers in the Mississippi Delta are often confronted with less than satisfactory performance from currently available preemergence herbicides, particularly with control of prickly sida (Sida spinosa L.) and morningglory. Consequently, these weeds are often as tall as cotton plants when herbicide label directions indicate that directed postemergence herbicides can first be applied.

Studies were conducted in 1985 and 1986 to determine the effect of over-the-top (OT) sprays of fluometuron, prometryn, methazole, dinoseb, and MSKA applied at rates lower than those recommended as directed sprays on 'DES 422' cotton and weeds. Applications were made to (1) 3- to 4-leaf morningglory plants and cotton plantswith two true leaves, 3 to 6 inches tall (stage A), (2) to cotyledon stage morningglory, cotton plants with one true leaf half expanded, 1-1/2 inches tall and repeated after 2 weeks (stage B), and (3) to cotyledon stage morningglory and repeated two times at weekly intervals (stage C). Fluometuron at 1 .5 lb ai/A was applied preemergence alone (control) and sequentially before stage A. No preemergence herbicide was used before stages B and C. Predominant weed species were entireleaf morningglory (Ipomoea hederacea var integriuscula Gray), prickly sida, and broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash]. Some smooth pigweed (Amaranthus hybridus L.), pitted morningglory (I. lacunosa L.), horse purslane (Trianthema portulacastrum L.), and nodding spurge (Euporbia nutans Lag.) plants were also present.

Weed control was determined by estimated visual control (0 = no control, 100 = complete kill) and plant counts made on each row immediately before each OT application. Cotton injury was determined by stand counts, plant height measurements, and seed cotton yields.

Cotton stand was not different from the untreated OT control with OT herbicides in 1985 or in 1986. At 7 days after final treatment, cotton height in the herbicide treatments was not different from the untreated OT control on rows treated preemergence or rows treated preemergence followed by OT application made at stage A. On rows treated OT at stage B, cotton plant height was reduced with fluometuron at 0.75 lb ai/A and methazole at 0.5 lb ai/A in 1985. No height reductions occurred in 1986. When cotton plants were treated OT at stage C, cotton plant height was reduced by fluometuron at 0.5 lb ai/A in 1986, and 0.75 lb ai/A in both years; by prometryn at 0.125 lb ai/A in 1986, and 0.25 lb ai/A in both years; by methazole at 0.25 lb ai/A in 1986, and 0.5 lb ai/A in both years; and by dinoseb at 0.5 lb ai/A in 1986.

Seed cotton yield in both years was not reduced below the untreated control when any of the herbicides were applied OT at any stage. The untreated OT control yield at stages B and C was only 49 and 33% of that at stage A in 1985 and 23 and 22% in 1986 due to weed competition (primarily prickly sida and broadleaf signalgrass). When compared with fluometuron preemergence, the average Seed cotton yield for all OT treatments was reduced 12 and 0% for stage A, 48 and 43% for stage B, and 43 and 34% for stage c treatments in 1985 and 1986, respectively. Part of this yield reduction for stages B and C was due to weed competition. However, individual OT treatments providing very effective weed control resulted in yield decreases, compared with plots receiving only preemergence fluometuron, ranging from 53 to 64% and 21 to 47% for stage B and 41% and 9 to 18% for stage C in 1985 and 1986, respectively.

Based on initial plant counts made 14 days after application, fluometuron applied preemergence gave 72% prickly sida control at 17 days and 69% control at 24 days in 1985. In 1986, prickly sida control was 86% at 21 days and 97% at 30 days after application. For annual morningglory, control values at the respective time intervals were only 28% and 27% in 1985 but were 87% and 91% in 1986; for broadleaf signalgrass, control was only 13% and 12% in 1985 but was 61% and 65% in 1986 for the respective time intervals. Control in 1985 was low because of less than normal rainfall in May followed by abundant June rainfall causing greater emergence of weed seedlings after the herbicide was applied.