Driving around West Tennessee the last couple of weeks, it is apparent that overall weed control is better than it was at this time last year.

Last year in August it was not hard to find soybean and cotton fields that were covered in glyphosate-resistant Palmer amaranth. Areas of some of these fields were not harvested, and overall yields in a number of these fields were reduced by 50 percent or more.

Sure we still have a lot of fields that have some glyphosate-resistant Palmer in them this year, but not many are at yield-reducing levels. I know these relatively clean fields were not achieved easily. They were much more costly in time, equipment, labor and above all money, than back when glyphosate would control Palmer amaranth at any height.

Though these cotton and soybean fields look better than they did last year, I do not think the weed control system used is sustainable much past three to five years. A number of our cotton fields received two and three applications of Ignite to try and knock down large Palmer. Quite a few of our soybean fields received multiple applications of PPO herbicides with fomesafen-based products (Flexstar, Reflex, Flexstar GT and Prefix) being the go-to-choice for many.

Exposing large glyphosate-resistant Palmer amaranth to multiple applications of either Ignite or a PPO herbicide over just a couple years is an express train way to develop resistance to those herbicides.

I have made these comments to farmers at field days this summer. The response too often from farmers is that coming traits like Dow’s DHT trait (provides 2,4-D and Ignite tolerance) and Monsanto’s dicamba-tolerant trait could bail us out if Ignite and/or PPO herbicides no longer work on Palmer.

The problem with that thinking was pointed out to me this spring with some preliminary research we are conducting with dicamba and 2,4-D on glyphosate-resisant Palmer amaranth. Dicamba and 2,4-D provided 95 percent plus control of Palmer that was less than 6 inches tall. Palmer taller than 6 inches, even with higher rates of dicamba or 2,4-D, would grow sideways for a week and then grow back toward the sun.

This research showed that 2,4-D and dicamba could be much-needed tools to help us manage Palmer. However, they are not going to control large Palmer alone. We will still need Ignite and PPO herbicides to be viable options for Palmer control, even if we are planting soybeans and cotton that can be sprayed with 2,4-D or dicamba.

So what can one do to delay Palmer developing resistance to Ignite and PPO herbicides?

In soybeans we have more options than in cotton and we need to use them. There are several good premixes that contain two herbicides that will control Palmer. Boundary (Sencor and Dual Magnum), Prefix (Flexstar and Dual Magnum) and Authority MTZ (Sencor and Authority) would be good pre-emergence choices or in the case with Prefix, a good post choice as well.

From a postemergence standpoint we have the PPO herbicides and Ignite available to us. Think about rotating these two herbicides from year to year in the same field.

Or in the case of LibertyLink soybeans, one could use one of the premixes mentioned above and follow with Ignite in-crop.

Another strategy to think about is tank-mixing a PPO in with Ignite. In theory, this should lower the odds of resistance being developed for either herbicide.

In cotton, our options are few. All we can do is rely on the preplant and pre-emergence herbicides and Dual Magnum in-crop to incorporate another mode of action.

Some are very reluctant to go back to hooded applications. These are a must if we are to delay herbicide resistance from developing.

Of course, rotating to corn, grain sorghum or wheat double-cropped with soybeans would be another good way to incorporate some different herbicides.

I do not like being the bearer of bad news. However, if you think the glyphosate/ALS-resistant Palmer is bad now, wait until it becomes resistant to Ignite and/or PPO herbicides as well. If we can keep Ignite and the PPO herbicides in play until these new herbicide traits arrive, then we may have enough herbicide diversity at that point to be able to manage Palmer amaranth long-term.

e-mail: lsteckel@utk.edu