Louisiana farmers — knock on wood — have not encountered any confirmed cases of glyphosate-resistant pigweed in their cotton or soybean crops to date.
But that doesn't mean growers aren't having to deal with glyphosate-resistant plants when they rotate from cotton to corn or soybeans to cotton or soybeans to rice, according to LSU AgCenter researchers.
“I mentioned we have no confirmed resistance to glyphosate, and I meant that for traditional weeds,” said Donnie Miller, weed scientist and research coordinator at the LSU AgCenter's Northeast Research Station at St. Joseph, La.
“We do have confirmed glyphosate resistance out there with volunteer Roundup Ready cotton in soybeans, volunteer Roundup Ready soybeans in cotton fields and rice fields and volunteer Roundup Ready corn in the three above-mentioned crops. And it seems to be becoming more of a problem.”
Miller discussed research on the impact of those plants during a stop at the Northeast Station's recent Crop Production and Pest Management Field Day. Donna Lee, a former graduate student who is now an Extension agent in East Carroll Parish, conducted the research for her master's thesis.
Lee found that volunteer Roundup Ready soybeans can have a much more significant effect on yields in Roundup Ready cotton than Roundup Ready cotton in soybeans.
“We were looking at cotton as a weed in a bean crop. It doesn't appear to be super-competitive,” said Miller. “At one-half to one cotton plant per foot of row, we would expect to see a yield loss between 5 and 15 percent with those densities.
“At an even higher density of 1.5 Roundup Ready cotton plants per foot of soybean row, which she let compete for one to five weeks, she saw only a 4.5 percent to 5.5 percent yield loss. When you get on up to eight weeks, she saw a 10.5 percent yield loss.”
Volunteer Roundup Ready soybeans pose a much greater threat to cotton yields. One-half plant per foot of cotton row resulted in a 32 percent yield reduction in cotton if allowed to compete season-long. For one soybean plant per foot of cotton row, the yield loss was 50.4 percent.
“With 1.6 plants of Roundup Ready soybean weed in cotton for a period of one and two weeks after emergence, you can expect to see a yield loss of 7.2 percent and 10.9 percent,” said Lee in a paper entitled “Volunteer Roundup Ready Weeds.”
“Allowing Roundup Ready soybeans to compete with cotton for a period of four weeks after emergence resulted in a yield loss of 18.2 percent. Competition for eight weeks and season-long reduced cotton yield 32.7 percent and 61.9 percent.”
“A lot of the volunteer Roundup Ready soybeans emerge in early spring before you plant the crop,” said Miller. “That's the best time to kill them. If you wait and they grow to 6 inches or 8 inches or 12 inches high and start flowering, they can be really tough to control.”
The same will be true for volunteer Roundup Ready corn in fields rotated to cotton. Corn growers have been shifting to Roundup Ready-tolerant varieties to reduce the risk from burndown applications of glyphosate, which can lead to more volunteer corn problems when fields are rotated back to cotton.
Miller said Gramoxone Max or Ignite can be an effective treatment for volunteer Roundup Ready soybean prior to planting cotton. “In the crop, Envoke or Staple do a good job on volunteer soybeans.”
For Roundup Ready corn in cotton, researchers are recommending applications of Poast Plus, Fusilade DX or Assure II.
LSU weed scientists are also looking at preplant applications of residual herbicides such as Valor and Synchrony in Roundup Ready soybeans. They're applying those 15 and 30 days prior to planting.
“We're following those up with glyphosate applications at one, two, three and four weeks after emergence just to see how long that preplant can be expected to be effective on into the crop,” says Miller. “We also have follow-up treatments at a week followed by three weeks and a week, three weeks and five weeks to see if we can cut down on the number of applications we have to make following the preplant.”
Louisiana growers have been fortunate with glyphosate-resistant weeds, according to Miller. He said weed scientists have investigated several suspected cases but have not been able to confirm any true resistance.
In one case in 2005, researchers collected plants and seed of pigweed that were not controlled with an application of a material containing glyphosate. They were able to show the pigweeds were easily controlled if they made the application when the weeds were small.
“Once you got past that 1-inch stage or to 1.5 to 2 inches, you had to go up to 2 pounds of glyphosate to get up to 85 to 90 percent control,” said Miller. “That piqued our curiosity, so we sent the plants to a botanist on the LSU campus, and he identified it as a different species, sandhill amaranth.
“So I think we're running into situations where glyphosate technology is controlling a lot of the traditional weeds — smooth pigweed, redroot pigweed — and getting rid of the easier to control stuff. But there is also some shifting to less susceptible species, such as the sandhill amaranth.”
This year, AgCenter scientists have been investigating more complaints of pigweed and johnsongrass that were not controlled by conventional glyphosate application rates. Miller said the failures may be due to resistance or to sprayer problems or other mishaps.
“It's definitely something we want to stay on top of,” he said. “That's where we need you guys, the producers and consultants, to let us know about any suspected cases you have.”