By Carroll Smith
We don’t have a lot of options in cotton and soybeans for managing glyphosate-resistant weeds, but we do have several good pre-emerge and post-emerge herbicide options in corn,” says Bill Williams, LSU AgCenter state weed specialist.
Williams adds that he doesn’t know of any weeds in corn in Louisiana that can’t be controlled in the absence of glyphosate. However, one weed control strategy that Extension is recommending to the state’s corn farmers who do plan to use glyphosate is to include other modes of action (MOA).
“We already do that with atrazine, but we also need to include a good grass herbicide like Dual,” he says. “Package mixes, such as Halex GT that contain multiple modes of action, are becoming more common, offer excellent weed control and fit well into resistance management. Still, it is important to reduce selection pressure on glyphosate.
“So, another suggestion for farmers growing Pioneer Herculex hybrids is to apply the non-selective herbicide Ignite instead of glyphosate,” Williams adds. “But, it will also be to a good idea to apply atrazine and an effective grass herbicide.”
Another option is to use a pre-emerge program, followed by a post-emerge program in which combinations of herbicides such as Steadfast, Option and Callisto can be used on any of the hybrids.
“However, if a pre-emerge herbicide goes out at planting, it may play out by the time the weeds come up in early April,” Williams says. “That is why the glyphosate and Liberty Link programs work well for us. But, Steadfast or Option mixed with atrazine and/or one of the bleachers offers excellent post-emerge weed control.”
Use Different Modes Of Action
In Mississippi, weed scientist Jason Bond says, “Corn has somewhat been our answer to glyphosate resistance because of the MOA we have access to in corn, relative particularly to cotton. One treatment for, say, Palmer amaranth in corn is Lexar herbicide pre-emerge, but it’s relatively expensive.
“However, you can use Bicep II Magnum, which is less expensive than Lexar and a good treatment for pigweed because it contains Dual, which works well pre-emergence on pigweed. Atrazine will, too.”
Another option Bond lists is Callisto – a relatively economic treatment for pigweed.
“Under the right conditions, these treatments should control resistant pigweeds,” he says. “Because these alternative treatments control the species when applied alone, if you do apply glyphosate over the top of Roundup Ready corn, no more selection pressure is placed on the pigweeds as long as you have achieved good pigweed control from these other treatments.
“Any of the herbicides tankmixed with glyphosate as part of a resistance management program has to provide excellent control of the target species when applied alone,” Bond concludes.
Arkansas Extension weed scientist Ken Smith agrees wholeheartedly.
“Be sure to use the herbicide rate that is effective in controlling the targeted weed,” he says. “Don’t drop to a lower rate in the tankmix and depend on bumping the efficacy of Roundup a bit. This is not the right approach to achieving a good resistance management program.”
As far as corn, Smith has done a lot of work with late-season morningglories.
“Much of the corn that we now plant in Arkansas contains the Liberty Link gene and/or the Roundup Ready gene,” he says. “Late season, we’ve found that Ignite is more effective on morningglories than glyphosate.
“If we use a mixture such as Lexar or atrazine + Dual or Bicep up front, then come back after the corn reaches 12 inches with Ignite + one of the bleacher herbicides, we haven’t used any glyphosate in the program, and the field is very clean.”
Smith notes that if a farmer plants a Roundup Ready hybrid, he can come back with a tankmix of, say, glyphosate + Callisto in 20-inch corn and get good results with that combination as well.