Palmer pigweed getting revenge

Of all the things we are looking at in research plots this summer, the most pressing to me is control of glyphosate-resistant Palmer amaranth or Palmer pigweed in soybeans.

I have been contacted about 10 or 12 fields so far this summer where either one or two applications of 22 ounces of PowerMax or the equivalent has failed to control pigweed in soybeans. The problem is so widespread that we are not going to look at all the fields or try to sample them all anymore.

Syngenta personnel have told me they have treated over 40 fields for pigweed failure with Touchdown. Unfortunately, the only option left for growers is to try to burn back the survivors with 1.5 pints per acre of Flexstar. Flexstar is not a salvage product on pigweed and will not control a pigweed over about 6 inches tall. This is a tough situation.

Adding 6 ounces of Flexstar to glyphosate may help against morningglories, but it will only make a pigweed mad. It takes a full 1.25 to 1.5 pints of Flexstar on 3-inch to 5-inch pigweed for there to be a hope of control.

One reason Flexstar is our only choice is that most of Palmer pigweed we have is already resistant to the ALS chemistry such as Classic, FirstRate and Scepter.

We have over 20 studies evaluating various programs, including Ignite on LibertyLink soybeans at our Newport, Ark., location — basically a carpet of Palmer amaranth. My conclusion: it is hard to kill pigweeds without glyphosate.

The best plots in our work this year take a program approach. Yes, it is going to cost a little more. Valor (or Valor-containing pre-mix) in a burn-down a few days before planting or Prefix (Dual + Reflex) applied pre- followed by two applications of Ignite were the only treatments I would call “table top” clean.

It scares me to rely on a pre for pigweed control. Some of our plots missed the irrigation just a bit and did not get activated.

A straight Ignite program in Liberty beans works, but you have to time the first shot of Ignite on 2-inch pigweeds or you get behind real fast. I think Bayer will be fully promoting the use of residuals with LibertyLink beans from day one.

In conventional soybeans, I like Valor applied a few days before planting in a burn-down with glyphosate, then going back very early with Flexstar.

Why Valor preplant and not pre? I think applying Valor preplant gives you more time for a chance of rain to get the Valor activated and less chance of crop injury than a true pre.

Why not Prefix pre? If you do that, you do not have any Flexstar left to go back with — you would be over the labeled limit.

I am excited about LibertyLink soybeans, and Valor and Prefix are some of the best Palmer amaranth materials I have ever evaluated. But in my plots everything has to go just right for great control. I hope that the companies will continue to screen new chemistry on this weed.

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