Prevention and Control of Palmer Amaranth in Cotton
ID
2805-1001
Palmer amaranth (Amaranthus palmeri), a member of the "pigweed" family, is one of the most troublesome weeds in many southern row crops. Seed can germinate all season and plants can grow to over 6 feet in height. Plants have either male flowers that shed pollen or female flowers that can produce up to 600,000 seed per plant. One Palmer amaranth per 30 foot of row can reduce cotton yield by 6 to 12%.
In Virginia, the weed has only been documented in four southeastern counties, but will likely enlarge its territory. Its spread can be rapid because of custom harvesting, failing to clean cotton pickers and tillage implements after exiting infested fields, and spreading of gin trash containing the seed. More troublesome is that extensive use of herbicides has led to resistance to glyphosate and ALS-inhibiting herbicides. We currently have populations of ALS-resistant Palmer amaranth in Virginia and glyphosate resistance was observed as close as Gates County, North Carolina.
Palmer amaranth is threatening Virginia's cotton cropping systems. Aggressive steps are necessary to minimize its spread and delay the development of herbicide resistance. No longer can Virginia cotton growers settle for the easiest, least expensive, or short-term (single-season) control strategies. Cotton weed management must quickly become a carefully planned and integrated control program that preserves herbicide and genetic technologies. For effective and long-lasting Palmer amaranth control, one needs to look 3 to 5 years into the future and not focus solely on herbicides and varieties. Long-term management of Palmer amaranth requires a multiple strategy approach that includes: integrating crop and herbicide rotation, diversifying in-season herbicides, closely monitoring fields, completely controlling the weed in rotational crops, cleaning harvest and tillage equipment, and removing escapes before seed production.
Of those strategies listed, integrating herbicides of different mechanism of actions will have the greatest benefit within the cotton growing season. This can be accomplished by including a soil residual herbicide with the burndown application, using soil-applied residual herbicides at planting, tank-mixing postemergence herbicides, and making a late-season layby application. The following are suggestions for controlling Palmer amaranth in Roundup Ready® and Liberty-Link® cotton (Table 1).
Rights
Virginia Cooperative Extension materials are available for public use, re-print, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Publisher
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Rick D. Rudd, Interim Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Wondi Mersie, Interim Administrator, 1890 Extension Program, Virginia State, Petersburg.
Date
May 1, 2009