Glyphosate-resistant Italian ryegrass and glyphosate-resistant goosegrass became notably more prevalent in Tennessee in 2014. 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, were much more frequent this past growing season.

Toss in a few soybean fields that looked like a good grass pasture after multiple glyphosate applications, and one can easily draw the conclusion that these new glyphosate-resistant weeds are much 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. They can 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.

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. However, we are in the beginning stages of putting a tremendous amount of selection pressure with the clethodim products on these glyphosate-resistant grass species. How much longer will it be before clethodim no longer works on those grass species?

I know we are all waiting on the new herbicide trait technologies to help us manage these weeds. I have enjoyed reading the recent Delta Farm Press articles on the new herbicide trait technologies by my friend and colleague, Bob Scott. I agree with both his hopes and concerns with the new traits.

One additional concern I have is that there is no answer for ryegrass or goosegrass with the Enlist or Roundup Xtend technologies. 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. 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.

So what are some answers to help us manage the glyphosate-resistant grass species regardless of the herbicide trait technology in the crop? First, in soybean, 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. 

Finally, some tactics that do not include a herbicide, like 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 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.