Horseweed control in cotton continues to be a major problem, says Shane Osborne, Oklahoma State University extension specialist. Osborne says an Extension Service study from last spring shows that weeds defy some of the best technology.

“Regardless of the latest and greatest technological advances in the cotton industry, weed problems still plague cotton farmers,” Osborne says. “Weeds in cotton can rob the soil of both moisture and essential nutrients. Although Roundup Ready Flex cotton varieties (which allow broadcast applications of approved formulations of glyphosate for most of the growing season, according to the label) have simplified most producers’ weed control programs, new problems continue to emerge.

“Horseweed (often referred to as mare’s tail) has quickly become one of the biggest weed problems across much of the cotton belt,” he says. “For the past several years, glyphosate-resistant populations of this weed have grabbed headlines at one time or another in agricultural publications. Even the plants that do not have true resistance to glyphosate (currently the case for most of Oklahoma) can still be very difficult to control.

“Cotton Incorporated, through the OSU State Support Committee, funded a project for OSU’s Extension Cotton Program this past spring. Two trial locations, one in Tillman County at the Roger Fischer farm and one at the Doc and Danny Davis farm in Washita County, were no-till, dryland cotton production systems with heavy populations of horseweed.

‘”Data from other regions suggested that pre-season control was essential to get a clean start and most programs included either 2,4-D or Banvel. In these tests, four treatments were applied approximately 45 days prior to planting.”

At the Tillman County location, Banvel plus glyphosate plus Valor provided excellent control of horseweed. At the Washita County location, since a cover crop was still growing at application time, glyphosate was dropped out of the mix. However, Banvel plus Valor was again very effective in horseweed control, Osborne says.

“The key to controlling this weed is in application timing. Typically, effective control can be achieved when horseweed is the rosette stage or just beginning to bolt upward. Most successful applications are made on small horseweed less than six inches tall. With drought stress, control improves as plant size decreases.”

Osborne says growers who want detailed information on chemical application rates, along with all the treatments evaluated should call the OSU Southwest Research and Extension Center south of Altus at 1-580-482-2120 or email shane.osborne@okstate.edu.