Don’t become so fixated on pigweed that you overlook other weed and grass problems

It’s easy for farmers to become so preoccupied with glyphosate-resistant Palmer amaranth that they lose focus on other weed species, such as Italian ryegrass and goosegrass.

Until recently, most farmers have continued to add glyphosate to their tank mixes to help with grasses and other non-glyphosate-resistant species. But that approach may be flawed, according to Larry Steckel, Extension weed scientist with the University of Tennessee.

“Glyphosate-resistant Italian ryegrass and glyphosate-resistant goosegrass became notably more prevalent in Tennessee in 2014,” says Dr. Steckel, who is located at the West Tennessee Research and Education Center in Jackson.

“I received many more questions on how close to corn planting to apply a clethodim product to take out ryegrass without harming corn, and what clethodim rate to clean up large grass in soybean and cotton. (Clethodim is sold as Select and under other trade names.)

Some soybean fields looked more like a good grass pasture after multiple glyphosate applications in 2014, he noted, leading him and other specialists to conclude glyphosate-resistant grasses are becoming more common.

“I have a lot of concern about the glyphosate-resistant grass species. Our weed management programs have been centered around controlling Palmer amaranth and justifiably so. The crucial component of that program has been a pre-applied herbicide. Often these pre-applied herbicides are a Valor- or Authority-based premix.”

Those provide good pigweed residual control “but typically do not contain a herbicide that is very effective on grass, the logic being we can stop the pigweeds and use glyphosate to clean up the grass in-crop,” he said.

“In retrospect, we should have been recommending tank-mixes of clethodim with glyphosate for those post-emergence applications to help keep glyphosate effective on goosegrass and ryegrass.

“This is water under the bridge at this point. So far clethodim has been effective, and it is relatively cheap.”

He’s becoming more concerned about the selection pressure being put on grasses with the heavy use of clethodim and wonders how long it might be before clethodim no longer works on those grass species.

Steckel also worries about the fact that neither the Enlist Duo or Roundup Xtend technologies, which are expected to be available in 2015 or 2016 (depending on the crop) will give growers much help with glyphosate-resistant grasses.

“Enlist will provide cotton and soybean tolerance to glyphosate, 2,4-D and glufosinate, while Xtend will provide tolerance to glyphosate, dicamba and in cotton, glufosinate,” Steckel notes.

“Those technologies will greatly help us manage pigweed. However, the only herbicide in those technologies that will have any activity on those two glyphosate grass species is glufosinate. Unfortunately, glufosinate is a poor ryegrass herbicide and is even less effective on goosegrass.”

In soybeans, he says, pre-applied herbicide applications need to have a Dual or Zidua component somewhere in the mix. “With commodity prices on a downward spiral, I know adding more herbicide expense is not appealing, but we really have no choice. On top of the grass control, those herbicides will help provide Palmer control.”

Another weed management tactic is to rotate out of continuous soybean or cotton fields into corn. “In corn, we can expose goosegrass to Callisto, Laudis or Armezon/Impact tank-mixed with atrazine, which is also an effective option.”

Strategies that do not include a herbicide, such as utilizing cover crops, can also be helpful.

“In our research where we are utilizing cereal rye or wheat as a cover, we have fewer weeds overall to control in the crop. I do not want to shout that the sky is falling, but I do fear that we are so concerned with pigweed management, we will let the glyphosate-resistant grass species run over us.”

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