Horseweed is one of those weeds that can slip up on you when you're not looking, according to Mid-South weed scientists. The plants can emerge in fall or early winter but not become evident in the field until planting time.

Andy Kendig, weed scientist with the University of Missouri's Delta Center, has followed fall herbicides, first as a Midwest program for corn and soybeans and most recently in the Mid-South for control of glyphosate-resistant horseweed in cotton.

Kendig recalls the early 1990s when former northern Missouri weed specialist Mike DeFelice expressed concern that fall herbicides “would still require a full burndown in the spring, leave the soil bare through the winter and mostly just cost the grower extra money.”

“We tried several different fall herbicides on cotton, but found the same thing — bare soil on Feb. 1, but thick stands of horseweed at cotton planting,” said Kendig. “Envoke has been an exception. Over the last two winter seasons we've seen Envoke provide excellent horseweed control from November or December well into spring.”

Speaking at a field day at the Delta Center in early spring, Kendig showed plots treated with Envoke alone and with other contact herbicides and with competing residual herbicides. As they were last year, the plots will be planted no-till and rated for the presence of horseweed after cotton emerges.

Last November, EPA granted an amended label allowing farmers to make fall/early winter applications of Envoke in cotton. The label permits Envoke to be applied at the rate of 0.1 ounce per acre at least 90 days before cotton planting.

Kendig said the history of glyphosate resistance in horseweed in southeast Missouri can almost be traced by looking off to the west from his research plots at the Delta Center's Lee Farm.

“The first documented case of glyphosate-resistant marestail was found in that field in 2002,” he said. “Oddly, the resistant marestail stayed out of my research field through 2005. In 2006, we would kill half the marestail with glyphosate and then this year had an even higher survival rate.”

Farmers have tried a number of herbicide strategies ranging from adding dicamba (Clarity or Oracle) to their glyphosate burndown in the spring to applying other residual herbicides in the fall.

Waiting to control large marestail in the spring may not be a good option at this point, according to Larry Steckel, Extension weed scientist with the University of Tennessee Experiment Station in Jackson.

“What we saw this past spring was the horseweed established back in the previous August/September was very large, had taproots that went 8 to 12 inches and survived some burndown applications I thought would have killed them in April,” he said.

University of Tennessee researchers have applied Envoke at rates of 0.5 to 2.0 ounces per acre. The ultra low rate provided good control until planting, but the higher rates resulted in some stunting of the cotton.

“I would recommend combining the 0.5- to 1.0-ounce rate of Envoke with dicamba or a high rate of 2,4-D for application shortly after cotton harvest in November,” says Steckel. “You'll be set until spring. You can hit it with a little bit of Gramoxone or glyphosate just prior to planting and be good to go.”

Farmers in the lower Delta have also been making fall herbicide applications to try to “thin the horseweed populations in the fall so they're manageable in the spring,” says Dan Poston, Extension weed scientist with the Delta Research and Extension Center, Stoneville, Miss.

“We've got a lot of products that seem to hold horseweed into March, but very few hold until planting. We saw some very good results from Envoke in terms of being able to eliminate those horseweed populations in the fall all the way through planting.”

“Most farmers like to start clean, so they may want to put out Envoke with Touchdown Total or Gramoxone,” says Keith Driggs, technical support representative for Syngenta. “With big marestail that has emerged, they may want to add dicamba.”

Fall applications are an added expense, but “some growers are having to spray burndown herbicides twice in the spring. We think fall applications can provide more consistent weed control and not force growers to depend on spring applications when they're already very busy.”