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Resistant Palmer amaranth ‘coast-to-coast’ in Tennessee

East Tennessee sees burgeoning resistant Palmer amaranth problem. 'Proactive' solutions discussed.

Well, it is official: We now have glyphosate-resistant (GR) Palmer amaranth coast-to-coast in the state of Tennessee.

I spent the last couple days of August in the East Tennessee counties of Franklin, McMinn and Bradley looking at soybean fields with GR Palmer amaranth. Franklin County has some soybean fields with GR Palmer amaranth at train wreck levels.

This is saying something as Palmer amaranth, let alone the GR version, could not be found there until two years ago. It is apparent now that the ability to tolerate glyphosate has helped Palmer become established into new areas.

That is not to say that East Tennessee soybean and cotton fields are as infested as the fields in West Tennessee. They are not. I am estimating the infestation level is below 20 percent. The folks in East Tennessee still have a chance to be proactive and at least delay the spread of this pest. But will they? If they do they will be among the few that are the exception to the rule!

There was a lot of debate in the academic ranks about eight years ago as to whether it would be more economic in the long run for farmers to be proactive or reactive in glyphosate-resistant weeds. There were actually several journal articles written on this subject. The general consensus was, as I remember, that it would be a little more economical in the long run to be proactive. However, I do not think anyone at the time was thinking about Palmer amaranth.

As we can easily see now with GR Palmer amaranth, “proactive” would have, hands down, been the way to go. It is not even close. One can easily see now that using, for instance, a pre-applied herbicide in soybeans at $10 an acre that could have at least delayed resistance. That would be a heck of a lot cheaper than the $45 an acre worth of herbicides we throw at GR Palmer now – and, still, in some cases, we lose the field to Palmer.

Of course being proactive is more complex than just using a PRE, particularly when your area is starting to see GR Palmer. It takes vigilance and willingness to walk across a field and pull out an escape. It also takes herbicide rotation. I talked to some folks in East Tennessee who said the only herbicide they spray on their corn is glyphosate. It does no good to rotate crops if the herbicide used in the crop is the same.

It is interesting to look back on the history of GR Palmer development in Tennessee. In 2008, I got the first true panic calls on GR Palmer taking large parts or whole soybean and cotton fields. Those came from the West Tennessee 901 and 731 area codes. In many of those cases folks were shocked that glyphosate was no longer controlling their pigweed even though they had warnings and even witnessed some Palmer escapes the year before.

In 2011, the phone calls were still primarily from West Tennessee. However, a notable change was a significant number of calls that came from East Tennessee 931 and 423 area codes.

Though the area codes were different the questions were the same. “Glyphosate did not control my pigweed and now it is 20 inches tall. What can I spray on it?” Unfortunately, my answer was the same whether the call came from near Memphis or Chattanooga: “Sharpen the hoe!”  

Looking back on this progression it really strikes me that the weed resistant issue is not so much about the weeds or even the herbicides but human nature. Most folks are not willing to change until there is a crisis.

Unfortunately, there are still a lot of growers across the state who do not realize we are in a weed control crisis. We are quite literally running out of herbicides. The change that is required to meet this problem is using all the tools available.

Crop and herbicide rotation are part of it. Corn is a good rotation crop to help manage Palmer. However, be careful as it can also hide a severe Palmer infestation.

Another good crop rotation is wheat/double-crop soybeans. A good wheat stand will delay Palmer emergence into June and a shot of Gramoxone after wheat has been cut is a good start in weed management.

We also need to get more LibertyLink soybeans into the mix, particularly in the many fields that have been continuous soybeans for the last several years. We are putting way too much selection pressure on the PPO herbicides (FlexStar, Blazer, Cobra, etc) in these fields.

Finally, we have to start utilizing ways that are not poured out of a jug. Some proactive growers in Tennessee are starting to look at row width and cover crops to help manage GR Palmer amaranth. I have walked some of these fields and we have ongoing research with cover crops.

In short, it does look like narrow soybean rows and cover crops can help manage pigweed. Tools such as these and more are going to have to be utilized for our cotton and soybean growers to stay in business.     

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