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Pigweed problem: What happened?

Unfortunately, we were not as concerned about resistance management as we were about short-term bottom line.

I get more questions about pigweed control in cotton than about any other weed or crop. Pigweeds have become the number one weed problem in the northern half of Arkansas.

At some point in every conversation, I always seem to hear statements such as, “Now, don't tell me to use a yellow herbicide or Staple because it doesn't work.” I also hear that although the pigweed problems are getting worse across the state, they are still easily controlled on some farms while on others it seems that nothing works.

Pigweeds are definitely easier to manage on tighter soils than on deep sands. Most pigweed seed germinate in the top 1/4 inch of the soil surface and it is hard to find a soil-applied herbicide that will remain in the top 1/4 inch of a sandy soil. Herbicides applied to sandy soils are usually moved below the germinating zone with rain and irrigation or else broken down by sunlight in only a few days. This may account for differences in control between farms or fields.

Another major setback in our pigweed control programs is ALS resistance. To us, this means Staple resistance because Staple is the only herbicide currently in the cotton market with this mode of action. A high percentage of our pigweed population is now resistant to Staple.

Some of our problems were created when Staple first came out and we quit using Zorial. Staple was cheaper and worked very well, so it quickly became the standard. Unfortunately, we were not as concerned about resistance management as we were about short-term bottom line, and now Staple is no longer a viable herbicide for pigweed control. In fact DuPont has moved pigweed from the list of weeds controlled to list of weeds suppressed only on the Staple label. Staple still works well on the non-resistant pigweeds — there are just not very many of them left.

If I can humbly get up on my soap box for a minute — and I say humbly because my soap box is not financed by profits from my cotton crop as is most farmers — this is a example of what can happen when we fail to look past the immediate situation. The university scientists, the chemical company representatives nor the farmers showed adequate concern for resistance management in this case.

Most resistance-management strategies suggested by chemical companies include adding a tankmix partner with their products. This adds extra cost without an immediate, visible increase in weed control, and that is a hard sell in any farmer's field. Very few chemical salesmen will suggest that a farmer not use his product this year because it was used last year.

It is too late to get the ALS-resistant pigweed out of Arkansas, but this problem should serve to remind us about the importance of resistance management in the future.

More expensive and slower herbicide registrations, fewer herbicide companies and a smaller overall herbicide market have slowed the number of truly new herbicides coming into the market. We are continuing to get quite a few new formulations and mixtures of old chemistry, but very few new chemical families. This steps up the importance of protecting what we have and making resistance management a part of farming and not just good rhetoric.

Although there are no confirmed cases, what would happen if pigweed became resistant to glyphosate? That is a scary thought. I hope it will never happen, but when we become too dependent upon one chemistry, strange things could happen.

Ken Smith is a weed scientist with the University of Arkansas Extension Service. E-mail:

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