Effect of Pendimethalin Formulation and Application Rate on Cotton Fruit Partitioning

Because of the development of glyphosate-resistant weed species, the lack of new herbicide chemistry, and the late-season emergence of annual grass species, efforts are underway to expand the use of currently available herbicides for use in cotton. Field studies were conducted in 2005 and 2006 to evaluate the effect of POST-applied pendimethalin formulation and application rate on cotton fruit partitioning. Oil- and water-based pendimethalin formulations as well as S-metolachlor were applied to cotton that had four true leaves. All pendimethalin and S-metolachlor applications included glyphosate for broad-spectrum weed control. Pendimethalin formulation and application rate had no effect on seed-cotton partitioning to horizontal fruiting zones, on second- or third-position horizontal fruiting sites, or on monopodial branches. However, increased seed-cotton partitioned to plants that had lost apical dominance was observed when the water-based pendimethalin formulation was applied at rates of 1.7 kg ai/ha and higher as well as when the oil-based pendimethalin formulation was applied at 3.3 kg ai/ha. Application of water-based pendimethalin at rates of 1.7 and 3.4 kg ai/ha and oil-based pendimethalin at rates of 0.8, 1.7, and 3.3 kg ai/ha resulted in reduced seed-cotton located at position 1 fruiting sites compared with the untreated check. POST application of S-metolachlor had no effect on fruit partitioning to horizontal fruiting positions or vertical fruiting zones. Minor differences in seed-cotton partitioning to cohorts and individual fruiting nodes were observed from application of glyphosate, pendimethalin, and S-metolachlor. However, no differences in seed-cotton yield were observed from application of glyphosate, S-metolachlor, or pendimethalin, regardless of formulation or application rate. POST pendimethalin application at rates less than 1.7 kg ai/ha is relatively safe and should provide cotton producers with an additional tool for herbicide-resistant weeds and late-season annual grasses.