Staple, the trade name for a new cotton herbicide containing pyrithiobac, proved effective in overthe-top broadleaf weed control when combined with arsenical herbicides in AAES research at three sites statewide. Though the arsenical herbicides caused some yield loss and delayed maturity at some sites in some years, this was not significantly affected by the addition of Staple in a tank mix.

The organic arsenical herbicides MSMA and DSMA have been used for many years for controlling weeds in cotton. MSMA was registered for use by U.S. cotton producers in 1964 as a postemergence-directed treatment on cotton ranging from 7.6 cm tall to first bloom (one inch equals 2.6 cm). MSMA is labeled for postemergence salvage treatments in some states, such as Georgia, for postemergence control of broadleaf weeds, grasses, and nutsedge in cotton while DSMA is labeled for the same use in Alabama, due to lower crop injury. Staple, which was marketed for the first time in 1996, controls several broadleaf weeds without adversely affecting cotton growth and yield when applied postemergence. Favorable results in many studies suggest that total postemergence programs for cotton producers may be feasible in certain situations. The weed species that may limit the usefulness of Staple in the Southeast is sicklepod, due to lack of postemergence control. Since MSMA has been shown to increase the activity of some herbicides on many weeds, it may have potential for increasing sicklepod control when tank-mixed with Staple. However, tank-mixing herbicides from different chemical families can increase the potential for crop Injury.

To evaluate the effect of Staple and MSMA or DSMA combinations on cotton growth and development, experiments were conducted in Prattville and Headland in 1994 and 1995.

Treatments included Staple, MSMA, and DSMA applied alone, and Staple tank-mixed with MSMA or DSMA and an untreated check for comparison. Herbicides were applied postemergence over-the-top of the crop canopy prior to the pinhead square stage of cotton development with a conventional-type sprayer. The varieties planted in Headland were ‘Suregrow 1001’ and ‘Delta and Pineland (DP) 90’ in 1994 and 1995, respectively. The varieties planted in Prattville were ‘DP 90’ and ‘DP 51’ in 1994 and 1995, repsectively.

Cotton response was evaluated two weeks after treatment using visual ratings where 0 = no effect and 100 = plant death. Maturity effects were determined prior to harvest by recording the number of total bolls from six meters of row (one meter equals 39 inches) and the number of open and closed bolls from two meters of row in each plot. The plots were chemically defoliated and the center two rows machine-harvested once after all mature bolls were open.

Cotton in 1994 at Prattville was injured by applications of MSMA and DSMA applied alone and tank-mixed with Staple. Although there was no difference in injury between MSMA and DSMA applied alone, DSMA tank-mixed with Staple gave less injury than MSMA tank-mixed with Staple. In 1995, MSMA applied alone or tank-mixed with Staple resulted in higher injury than all other treatments. Adding Staple to MSMA or DSMA did not increase injury over the two products applied alone, regradless of location and year.

Cotton in 1994 at Headland was injured by all treatments utilizing MSMA and DSMA (Table 1). Although DSMA is generally considered less injurious than MSMA when applied at similar rates, no differences were recorded. The addition of Staple to MSMA or DSMA did not increase cotton injury over each chemical applied alone. At the same location in 1995, no injury was recorded regardless of the treatment.

The total number of bolls was reduced by MSMA plus Staple compared to the untreated control in 1994 at Prattville. Adding Staple to MSMA and DSMA did not affect the number of total bolls over the same treatments applied alone. There were no differences in total bolls in 1995. In 1994, percent open bolls were reduced by MSMA and DSMA applied alone and MSMA tank-mixed with Staple when compared to the untreated control. No effect was measured in 1995. Staple tank-mixed with MSMA or DSMA did not delay maturity greater than the two arsenical herbicides applied alone. Seed cotton yield was was not affected by Staple or DSMA applied alone ot tank-mixed. MSMA applied alone and tank-mixed with Staple decreased cotton yield compared to the untreated control and Staple applied alone.

No treatment affected boll development wither year in Headland (Table 2). The total number of bolls averaged 221 per six meters of row for each treatment. Cotton maturity was not affected by any treatment, with an average of 71% open. Seed cotton yield at Headland was not affected by any treatment with an average of 2,280 kilograms per hectare (multiply by .893 to determine pounds per acre).

Monks is an Assistant Professor, Patterson is a Professor, and Delaney is an Extension Resource Conservation Associate of Agronomy and Soils. Moore is Superintendent of Prattville Experiment Field and Wells is Superintendent of Wiregrass Substation.