The organic arsenical herbicides, DSMA and MSMA, are used in cotton (Gossypium hirsutum L.) as directed sprays primarily to control yellow nutsedge (Cyperus esculentus L.), purple nutsedge (C. rotundus L.), and common cocklebur (Xanthium strumarium L.). However, since they have a narrow spectrum of weeds controlled, other herbicides are often included in tank-mixed combinations. Since nutsedge species and common cocklebur infest approximately 1.7 and 2.4 million acres of cotton, respectively, and 6.3 million acres receive directed sprays of DSMA or MSMA alone or in a tank-mix with another herbicide, some cotton acreage is being sprayed where these weeds are not a problem. The objectives of this study were to determine if MSMA enhances control of prickly sida (Sida spinosa L.) and pitted morningglory (lipomoea lacunosa L.) when tankmixed with herbicides applied as directed sprays and to document crop response to treatments used.

Experiments were conducted in 1987 and 1988 on a Goldsboro loamy sand with a split-plot arrangement of directed-herbicide treatments with and without MSMA. Main plot treatments were broadleaf-directed herbicides and consisted of 8 rows 30 ft long with four replications. The split-plot treatment factor was the presence or absence of MSMA randomly assigned to rows I to 4 or 5 to 8. Herbicides were applied with a tractor-mounted sprayer at recommended rates in a spray volume of 20 gal/ac. Spray pressure was maintained at 20 psi by compressed air. Treflan was applied both years preplant-incorporated at 0.5 lb ai/ac. Early directed sprays were applied when cotton reached 3.5 to 4.0 in tall, and when prickly sida and pitted morningglory were 0.5 to 1.0 in and 1.0 to 2.0 in tall, respectively. Late treatments were applied when cotton was 6.0 to 7.0 in tall, and when weeds were 2.5 to 3.0 in tall, respectively. An untreated check with and without MSMA was included. Plots were cultivated with a sweep cultivator June 3, 1987 and June 30, 1988.

Crop injury and weed control were visually evaluated 1 and 3 weeks after treatment (WAT) on a scale of 0 (no injury or weed control) to 100 (complete crop loss or weed control). The two center rows of each split-plot were harvested with a spindle picker and weighed to obtain seed cotton yield. Data were subjected to analyses of variance and least significant differences were used to separate treatment means. Since no treatment by year interactions were detected, data presented were averaged across years.

Crop injury and weed control were similar with and without MSMA, therefore, treatments were averaged across split-plots. Crop injury 1 and 3 WAT ranged from 2 to 4% and was similar with Bladex, Caparol, Karmex, Cotoran, and Probe. Crop injury from Classic (6%), Cobra (12%),and Goal (13%) 1 WAT was greater than that observed with the other herbicides and this injury was also evident 3 WAT where Cobra (6%) and Goal (8%) were applied.

Good to excellent prickly sida control (84 to 97% 1 WAT; 78 to 96% 3 WAT) was achieved with the early application Probe and late applications of Probe, Bladex, Cobra, and Goal. However, Cotoran (early and late), and applications of Caparol, Classic, and Karmex to 6-inch cotton provided poor (23 to 59% 1 WAT; 20 to 60% 3 WAT) prickly sida control.

Probe (early and late), Bladex, Cobra, Goal, and Classic provided good to excellent (86 to 98% 1 WAT; 78 to 96% 3 WAT) control of pitted morningglory, but Cotoran, Caparol, and Karmex did not adequately control this weed species.

Seed cotton yields with and without MSMA averaged across broadleaf herbicides were 1570 lb/ac and 1550 lb/ac, respectively. The presence of MSMA did not enhance weed control or seed cotton yields.