One way to control glyphosate-resistant pigweed in cotton fields is to make sure you overlap residual herbicide applications, says University of Arkansas Extension weed scientist Ken Smith, speaking at the Judd Hill Field Day south of Trumann, Ark.
Smith's research showed that “an application of Cotoran at planting, followed at one-leaf cotton by an application of Dual, followed by our post-direct applications and a layby did a good job. But if we put out Cotoran and wait until the fourth leaf stage to put down Dual, it was just like night and day. At four leaves, the pigweeds had already broken and we lost control. Nothing else we did the rest of the year would allow us to catch up.”
Smith noted that careful planning is required to protect against the development or infestation of glyphosate-resistant pigweed. He recommends that in fields where pigweed is a problem, adopt as many of the following options as possible to help keep fields clean:
- Use a long residual herbicide such as Valor or Reflex in the preplant burndown application.
- A yellow herbicide such as Treflan or Prowl applied preplant incorporated will prevent many pigweeds from becoming established in the early season.
- Applying Cotoran, Caporal or Direx pre-emergence will also delay the emergence of pigweed.
- Metolachlor products such as Dual Magnum applied with glyphosate will control pigweed until the cotton is large enough to make post-directed applications.
- Post-directed and layby applications that include a herbicide with residual properties will keep pigweeds under control until the cotton 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 late germinating pigweeds under control.
- Escapes after layby should be removed by chopping crews.
- Crop rotation to LibertyLink cotton, conventional corn or grain sorghum is also a good resistance management strategy.
Smith says it's hard to tell if a few pigweeds in a cotton field are “misses” or resistant. “My question is what are we going to do about those plants? Are we going to leave them or take them out? If your fields are clean and there is only a scattering of one of two pigweeds out there, how much does it cost to chop those? I don't know, but I do know it could be costly once you allow them to spread.”
When asked if Mid-South growers can learn from the experiences of the Southeast growers and weed scientists, who have a few more years of experience with glyphosate-resistant pigweed, Smith said, “The problem is that our biotypes are not the same as Georgia's. Georgia has only one biotype. We have two distinct biotypes and one of them is the type that when it produces seeds, all of its offspring are going to be extremely tolerant, up to 8X.
“We have another biotype that is much more common that is creeping up on us. But the offspring won't always be resistant, only about 10 percent to 15 percent will be resistant. We're seeing more and more of this one every year.”
Pigweed is one of Arkansas' “Notorious Five” — weeds discovered and confirmed in Arkansas to be resistant to glyphosate herbicide, according to Smith. The full list:
Giant ragweed — “We have about 10 fields that contain confirmed glyphosate-resistant ragweed,” Smith said. “It's been what we call a ditch weed, and it's never really been a problem in our cropland. But it's creeping into west Tennessee, and in the Midwest, it's a serious problem. So far, we haven't had a major problem with it in Arkansas.
Common ragweed — “We've confirmed it in one location. It's still in that one field and has not spread from that field. We're keeping an eye on and hopefully it will not spread.”
Horseweed, or marestail — “The seed blows in the wind and it really makes very little difference what you do on your farm. Fortunately, we've been able to do a fairly successful job of managing it when we get on it early in late February or early March.”
Johnsongrass — “We don't see very much of it anymore. Roundup has pretty much taken it out of the picture. But it is still around, and we do have some that are confirmed resistant to glyphosate. The good thing is that they are in only a couple of locations. It has about a 2X resistance.”
Palmer amaranth, or Palmer pigweed — “It's been confirmed in 15 of our cropping counties. Male plants do not produce many seeds. But the female plant can produce up to 250,000 plus seeds per plant. 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.”
Smith said glyphosate “is still the greatest herbicide ever made, no doubt in my mind. We're not going away from glyphosate technology. It's still a valuable tool. But it doesn't take too many pigweeds to make you look at your hole card. If we have clean fields, let's make an effort to keep them clean. So never allow soil residual herbicides to break in cotton fields. They have to overlap.”