Conservation tillage systems utilizing winter wheat as a cover crop are gaining acceptance on the Texas Southern High Plains as a means to reduce soil erosion, protect young cotton plants, and increase profitability. Effective weed control is a major limitation to the adoption of the system. Experiments were conducted in 1990 to evaluate trifluralin and pendimethalin applied as either chemigation treatments using a small-plot chemigation simulator or sprayed on the soil and water incorporated by center pivot irrigation.

Winter wheat was planted in September, 1989 and terminated with glyphosate at 0.4 lb/A in April, four weeks before cotton planting. Trifluralin and pendimethalin were applied to a clay loam soil at rates ranging from 0.75 to 2.0 lb/A using a chemigation simulator applying 0.75 inch/A of irrigation water. Similar rates of trifluralin, pendimethalin, and granular trifluralin (TR-10) were applied with a conventional spray at 15 gpa and incorporated with 0.75 inch of irrigation water. Mechanically incorporated, conventional tillage plots were treated with trifluralin or pendimethalin at 0.75 lb/A. At planting, a portion of each plot received a preemergence overlay of prometryn at 1.2 lb/A. At planting, 0.9" of irrigation was applied for seed germination. During the growing season, approximately 6 inches of irrigation water was applied.

Ratings made 64 days after chemigation treatment showed pigweed control ranged from 63 to 100% as trifluralin rate increased from 0.75 to 2.0 lb/A Pigweed control for chemigation application of pendimethalin ranged from 60 to 83% for the rates evaluated. The addition of prometryn as a preemergence overlay increased control substantially for all treatments with the exception of the highest trifluralin rate.

When herbicides were sprayed ont he soil and water incorporated, control ranged from 70 to 85% with trifluralin and 50 to 100% for pendimethalin. For both herbicides, the 2.0 lb or highest rate was required for control equal to the conventional tillage mechanically incorporated treatment. Pigweed control with application of trifluralin granules (TR-10) ranged from 63 to 93%. As with the chemigation treatments, addition of a preemergence overlay increased control at all but highest trifluralin or pendimethalin rate.