I’ve fielded numerous calls over the past couple of years about fields containing pigweeds suspected of being resistant to glyphosate. Each call would create an uneasy feeling in the pit of my stomach. Each time I traveled to a suspected field, sprayed and watched the weeds die, it was a relief to know we had just dodged another bullet.
The confirmation of resistance came in an e-mail from my co-worker (University of Arkansas Extension weed specialist) Bob Scott. He had attached pictures of dead and very alive pigweeds with a note indicating all had been sprayed with glyphosate and that we needed to talk — immediately.
Most everyone has heard the story by now that the surviving plants were grown in a greenhouse from seeds collected from plants that survived farmer sprays last year. It is hard to imagine anything positive about glyphosate-resistant pigweed, but I am relieved that it was found in a soybean field where some control options are available rather than in a cotton field where few management options exist.
However, this discovery definitely heightens the importance of practicing good resistance management techniques in cotton. In a recent meeting with cotton consultants, I asked what they would do if they found pigweed that did not die after postemergence applications of glyphosate to cotton.
There were a lot of heads shaking, but no one had an answer. The fact that much of the pigweed in Arkansas is already resistant to the ALS inhibitor herbicides such as Staple and Envoke leaves no postemergence options for pigweed.
Because of the high populations and aggressive growth of pigweeds, 95 percent control is absolutely unacceptable.
The only fail-proof resistance management plan is a clean field with no escapes. Although 100 percent control is never achievable, preventing resistance is much better than trying to deal with it once it occurs.
Every weed that escapes herbicide treatment is not resistant. However, if you had several pigweed escapes last year, there is reason for concern. If they are not resistant, it indicates that there is something about your spray program that is not working, and it may be time to re-examine your spray techniques.
Escaped female pigweeds receiving pollen from a resistant male some distance away is a grave danger. Dead weeds do not produce resistant offspring.
The best guard against resistance is using more that one herbicide mode of action effective for the targeted weed. Since there are no herbicides that effectively kill pigweeds over-the-top postemergence in cotton, we are forced to utilize soil residual herbicides that control the seedling before or at emergence.
A yellow herbicide such as Treflan or Prowl applied ppi will prevent many pigweeds from becoming established early in the season. Applying per-emergent Cotoran, Caparol, or Direx will also delay the emergence of pigweed.
Metolachlor products such as Dual Magnum applied with glyphosate before the fifth-leaf growth stage of cotton will control pigweed until the cotton is large enough to make post-directed herbicide applications.
Post-directed and layby applications that include a herbicide with residual properties will keep pigweeds under control until the cotton canopy creates sufficient shade to control pigweed seedlings.
Herbicides such as Caparol and Direx in post-directed programs and Valor in layby programs have been extremely effective in keeping pigweeds under control.
Crop rotation to Liberty Link cotton, conventional corn or grain sorghum is also a good resistance management strategy.
Each pigweed plant is capable of producing a half million seeds that drop back onto the soil. One plant that is not controlled with glyphosate is sufficient to cause problems for many years to come. Escapes must be controlled prior to seed production.
All options may not be feasible on all fields, but the more of options that are adopted, the less chance there is of pigweed escapes.
Ken Smith is a weed scientist with the University of Arkansas Cooperative Extension Service.