You might not be able to tell it from all the media reports these days, but there’s still some life left in glyphosate, the active ingredient in the herbicides used to control weeds in Roundup Ready systems.
Compared to other herbicide modes of action, the number of documented cases of resistance to glyphosate — 20 — is still low, says Alan York, professor of weed science at North Carolina State University. The incidence of ACCase- and ALS-inhibitor resistance is much higher.
“When you look at the number of cases for glyphosate resistance, you think that that’s not such a big deal,” says York. “We don’t have as many as we do of ACCase inhibitors (Poast and Fusilade — 25), and certainly we’re nowhere near where we are with triazines or with ALS inhibitors (90 and 120 cases).”
That means farmers still have a chance to avoid full-blown resistance to glyphosate in such problem weeds as Palmer amaranth, York told participants at the annual meetings of the American Society of Farm Managers and Rural Appraisers, the National Alliance of Independent Crop Consultants and the American Society of Agricultural Consultants in Atlanta.
Roundup Ready production systems have only been around for a decade, said York. The ALS inhibitors (imidazolinones, sulfonylureas, triazolopyrimidines and pyrimidinylthiobenzoates) have been out two decades and the triazines (atrazine, etc.) five decades.
“The question is where glyphosate will be 10 years from now,” he said. “And the answer to that depends on how well we can convince our farmers that it’s a real threat, and how we can convince them to begin to take some action.”
Why glyphosate resistance when the conventional wisdom for years was that the risk of getting resistance to glyphosate was much lower than with some of the other herbicide chemistries that farmers use?
“Nobody said it would never happen,” says York. “You put enough selection pressure on any herbicide, and you’re probably going to find resistance. So even though there is still, in my opinion, relatively low risk of resistance with glyphosate, we are putting so much selection pressure on it that we’re going to find it.”
In the Mid-South and Southeast, Roundup Ready crops have come to dominate with growers planting nearly 100 percent Roundup Ready cotton with the soybeans in those states not far behind.
Look at the changes that have occurred since Roundup Ready crops arrived, he says. Fifteen years ago, farmers were using conventional tillage and cultivation, DNAs, Cotoran pre-emergence with Command or Zorial and Cotoran plus MSMA and Bladex plus MSMA post-directed.
“Now that we have Roundup Ready, most farmers are doing conservation tillage — the Southeast is 50 percent no-till,” says York. “No one cultivates any more. Now we depend almost 100 percent on glyphosate.”
Although few farmers want to go back to the old ways, they should reduce their reliance on herbicides where practical, he says. They should also plant competitive crops and cover crops and, yes, use cultivation when they can. Crop rotation, with appropriate herbicide selection, can also help.
“Multiple modes of action will be a phrase farmers will hear more and more as we try to keep resistance from spreading,” says York. “We think farmers should use at least two different modes of action in corn and soybeans and three in cotton. Residual herbicides and full use rates will also be important.”
Glyphosate-resistant Palmer amaranth is giving weed scientists the most concern now. An extremely prolific plant, Palmer amaranth can become the dominant weed in a field in short order, says York.
The first admonition for farmers who even suspect they might have a problem with glyphosate resistance is to stop depending entirely on glyphosate in their Roundup Ready cotton or soybeans or corn.
“We must begin incorporating herbicides with multiple modes of action in our weed control systems,” he said.
For openers, that can include residual herbicides preplant or pre-emergence. For preplant, Direx and Valor are choices, and for pre-emergence, growers can consider Cotoran, Direx, Prowl, Reflex and Staple.
When farmers get to the postemergence stage of weed control, tank mixes of Dual Magnum, Envoke or Staple with glyphosate are possibilities. At layby, residual herbicides such as Direx, Layby Pro, Suprend or Valor can be tank-mixed with either MSMA or glyphosate.
“With glyphosate-resistant Palmer amaranth, one of the keys will be to reduce the seedbank,” said York. “If you can get three or four years of good control, you can reduce the populations down to where you can live with them.”