Just because it's possible to wait until later in the season to apply Roundup (glyphosate) to Roundup Ready Flex cotton varieties doesn't mean it's a particularly good idea.

In fact, yields may suffer significantly, up to a 30 percent loss, if weed pressure is allowed to build and compete with cotton.

“Late application of Roundup on Roundup Ready Flex cotton usually results in reduced effectiveness,” said Shane Osborne, weed control specialist at the Oklahoma State University Southwest Research and Extension Center in Altus.

Osborne discussed Roundup Ready flex cotton programs during a recent field day at the center.

Competition with weeds for resources, including water and nitrogen, may hamper the crop, Osborne said, if application is delayed.

He pointed to findings in an herbicide comparison program in which three applications of Roundup were made to two significantly different weed sizes.

Treatment one included three applications of Roundup Weathermax at 22 ounces per acre on morningglory ranging from two to three inches high.

The second treatment included three applications of Roundup Weathermax at 43 ounces per acre on seven to eight-inch tall morningglory.

Control was virtually the same for each treatment, but treatment two, higher rates at a later weed growth stage, resulted in a 250 pound per acre yield loss, almost 30 percent.

“That's significant,” Osborne said.

He said the Roundup Ready Flex system provides growers significant weed control advantages but he offers some cautions.

“The flexibility encourages delayed application, with potential for reduced yields,” he said.

“Also, we have to consider potential for developing weed resistance. That's already a concern in Oklahoma. We have to manage this system carefully.”

Advantages include a familiar over-the-top broadcast application that's “simple and familiar.”

He said the system controls a broad spectrum of weeds and increases control options in mid-season, when cotton is at the six to 12-leaf stage. “It's an economical mid and late-season grass control. Also, our ability to tank mix with other products helps eliminate extra trips across the field. That could be a big advantage with rising energy costs. Tank mixes allow us to kill two (or more) birds with one stone.”

Osborne said tank mix compatibility also works in product stewardship. “Integrating other chemistry into the Roundup Ready Flex system reduces potential for developing weed resistance,” he said.

He said residual herbicides also play a role in the program and may be extremely important when rainy spells keep farmers out of their fields.

“We had a lot of rainy spells this year,” Osborne said. “We could see quite a difference in spots where residual herbicides were applied.”

Osborne said growers can take care of larger morningglory plants when they use tank mixes instead of Roundup alone.

It works with other products as well, including:

Orthene for thrips control.

Vydate or Trimax for fleahopper control.

Pix or Mepiquat Chloride for regulating cotton plant growth.

Tracer or Karate for worm control.

“We have observed no visual injury or stunting so far,” Osborne said. “We'll know more when we harvest plots this fall and compare yields.”

e-mail: rsmith@primediabusiness.com