The presence of dinitroaniline (dna)-resistant Palmer amaranth (Amaranthus palmeri) was confirmed in South Carolina cotton in 1989, and this species has become established as the most troublesome weed in South Carolina cotton. In a 1993 survey, 77% of South Carolina s cotton growers reported Palmer amaranth among their top three most troublesome weeds; 92% reported this species among their top five most troublesome weeds. Growers and weed scientists have been unable to achieve adequate control of dna-resistant Palmer amaranth with commercially available herbicides. Extra herbicide applications, extra cultivations, handweeding, and rope-wick applications specifically for control of Palmer amaranth were performed on 31%, 30%, 27%, and 35% of our cotton acreage in 1993. Grower estimates of the cost of these extra inputs exceeded $2.5 million. Significant additional losses occur due to reductions in yield and quality and increased cost of harvesting.

Rotation is often recommended to prevent or slow the appearance of herbicide-resistant weeds, and to aid in control of these weed biotypes. However, less than one-half of South Carolina s cotton growers rotate their cotton acreage. Also, the average years of continuous cotton reported is 13. Therefore, rotation appears to be a control tactic with limited potential.

Staple, an experimental herbicide from DuPont has provided good to excellent preemergence and early postemergence control of dna-resistant and -susceptible Palmer amaranth. Palmer amaranth seedlings have achieved heights of 18 to 24 inches within 3 wk after planting and respective cotton and Palmer amaranth heights of 3 inches and 8 to 10 inches often occur simultaneously. Therefore, timing of early postemergence applications of Staple must be based on weed height, not cotton height. Sequential (preemergence followed by postemergence) applications of Staple appear to be necessary in some cases.