Herbicide-resistant weeds threaten farmers across the Cotton Belt and some are already facing weed populations that exact heavy yield losses along with reductions in fiber quality.

Using several herbicides with different modes of action will be the key to managing these resistant weed populations, said University of Tennessee weed specialist Larry Steckel during the annual Cotton Consultants Conference, which kicked off the recent Beltwide Cotton Conferences, held this year in New Orleans.

Steckel said herbicide resistance resulted from use of a single herbicide to control weeds — primarily in cotton, soybeans and corn — and that moving away from a one-product control strategy with burndown applications will be essential for weed control.

“Traditionally, growers used three herbicides for burndown treatments,” Steckel said. “Gramoxone, glyphosate and 2,4-D were widely used in the late 1980s and 1990s.” Roundup (glyphosate) came on in the late 1990s as the price went down. Gramoxone price remained about the same, he said. “Some farmers kept 2,4-D in the mix.”

But with Roundup Ready crops, glyphosate use increased significantly and the reliance on one mode of action paved the way for weed population shifts and herbicide resistance. Resistant marestail first showed up in the Delta and Southeast as farmers “used Roundup almost exclusively in the late 1990s.”

But resistance did not happen in a vacuum. Departments of transportation and municipalities also used Roundup and that helped select for resistant weeds. “We see a lot of resistant horseweed in cities,” Steckel said.

Weed control programs in many areas other than farming were using one herbicide and selecting for resistance and that “ramped up resistance issues.”

Steckel said the problem is compounded by the presence of several ALS-resistant weeds. “We will have to deal with that, too,” he said.

For most cotton farmers, horseweed is the primary concern. “But we’ve also seen glyphosate-resistant Italian ryegrass in Tennessee. Ryegrass has always been hard to control with Roundup.”

He said other concerns include primrose, bluegrass, Palmer amaranth, and giant ragweed.

Marestail seems to be the biggest threat. “It was identified in Tennessee in 2001, and we found a bad field in 2003. It got worse. Population in one field showed 30 to 40 horseweed plants per square foot.”

He said the weed is persistent. “It has a very long germination period, 10 months, with most emerging in the fall but with another bump in April and into May. Weeds that germinate in the fall develop a strong root system and are harder to control.”

Management is possible, he said. “In early February, fall-germinated marestail has not gotten large, and we can hit it with a burndown and see fewer control issues.” But growers need a multi-pronged approach. Steckel recommends Clarity with glyphosate. “That works best for us,” he said. “Tennessee farmers do not use 2,4-D much. It’s not as effective as Dicamba. Ignite works well if we have enough heat, about 80 degrees. It’s not as effective in cool weather.”

He said Roundup and Clarity early followed by Gramoxone and Caparol behind the planter is a good weed management program.

But not all herbicides, even the best ones, work under all conditions. “In 2006 and 2007, we used dicamba on a large (horseweed) population and struggled to get control. But we had a dry spring and dicamba is taken up through foliage and roots and during the drought we had less uptake from roots.”

He said a higher rate may have improved control.

Steckel said a new product from BASF, Sharpen, also holds promise. “It’s good on marestail and provides two weeks of residual activity. It’s labeled for cotton, corn and soybeans.”

He said controlling glyphosate-resistant Palmer pigweed may require something like Prowl to aid the burndown. “We have developed weed biotypes that escape burndown herbicides,” he said.

“Giant ragweed has shown glyphosate resistance.” It’s also changed habits. “Giant ragweed typically started to emerge in February and was done by April,” Steckel said. “We were home free after that. Now, we’re seeing an extended window of germination, a lot of emergence into June. And that late flush is very competitive and our burndown efficacy has changed. We will need to switch herbicides to manage weed escapes in season.”

He said ryegrass is another emerging problem. Ryegrass typically has been easy to control with several herbicides prior to tillering. “After it matures past two tillers, it’s more difficult.”

He also said that fields with several species of glyphosate-resistant weeds offer unique management problems. If a field has resistant Palmer pigweed and resistant marestail, growers likely will need two different herbicides for adequate control.

He said crop selection also plays a role in resistance management. “Soybeans are more competitive with horseweed than is cotton,” he said.

e-mail: rsmith@farmpress.com