Glyphosate-Resistant Weeds Are Huge Challenge

Iowa Soybean Association leaders recently went to Tennessee and the Boot Heel of Missouri to talk to farmers experiencing the challenge of trying to manage weeds that are resistant to glyphosate herbicide.

Published on: Aug 9, 2011

In early July, members of the Iowa Soybean Association executive board traveled to Tennessee and to the Boot Heel of Missouri to talk to farmers there who are experiencing serious problems trying to manage glyphosate-resistant weeds. The two main weeds those Tennessee and Missouri farmers are challenged with are glyphosate-resistant horseweed (marestail) and Palmer pigweed. Weeds that won't die, even when sprayed with higher rates of glyphosate, have become an increasingly bigger problem in the southern United States during the past several years.

The ISA farmers who went on the trip are president Randy VanKooten, president-elect Dean Coleman, secretary Tom Oswald, treasurer Mark Jackson and at-large member Ron Heck. During the same month, farmers in Iowa were reporting incidents of weeds suspected to have evolved resistance to glyphosate.

The ISA leaders who made the trip South met first with Johnny Dodson of Tennessee, who said the problem started about 7 to 8 years ago with marestail. At first, he observed a missed plant (a weed that escaped or survived the herbicide treatment) here and there in the field. Shortly after that, glyphosate resistance was discovered in Palmer pigweed (a pigweed species very similar to waterhemp).

Southern farmers now make multiple passes, spraying multiple products

Dodson, a long-time leader of the American Soybean Association, reported that farmers in his area now make multiple passes, spraying with multiple products. While his fields are very clean, Dodson said his cost of chemicals now runs close to $60 an acre.

From there, the ISA leaders crossed the Mississippi River and went to the research facility run by the University of Missouri. Weed science professor Jason Weirick showed them dicamba-resistant soybeans and explained how this new technology fits into a herbicide resistant weed management program. While this technology looks very promising, the researchers said there is no silver bullet and farmers will need all modes of action to fight the problem of glyphosate resistance in weeds.

His motto for new weed control strategy is "start clean, stay clean"

The group looked at plots where Palmer pigweed had choked out both soybeans and corn. Weirick reported that, by the time a weed is 4 inches tall, it is too late to kill it with a chemical. He also said a 4-inch tall weed will have several generations of weeds underneath it that will be shaded from a chemical application. Palmer Amaranth will grow 2 to 6 inches a day. Thus, Weirick's motto is "Start clean, stay clean."

Speaking from experience, the Southern researcher told the ISA group one plant will turn into 10,000 plants, spread by the combine throughout the field, and he pressed the point that the days of post-only applications are over. They talked about weed escapes and how one weed is one too many. In the past, farmers have been told 95% control was good enough, but even one resistant weed cannot be allowed to go to seed.

Southern farmers have had to change the way they apply herbicides

Southern farmers have changed the way they apply their weed control chemicals. They now apply 15 to 20 gallons per acre with a flat fan tip for maximum coverage. They have also developed hooded sprayers that run between the rows, applying gramoxone.

For fields that have got away from them, they've hired crews to chop the weeds out at a cost of $20 to $60 an acre. Because even the weeds that were chopped out would still produce a seed head as soon as the cut healed over, some farmers have disked fields down and replanted.

Iowans came back with conclusions on need for new weed strategy

The ISA officers came back to Iowa from their visit down South with some sobering conclusions.

"The take-away message was we need to be proactive and use a residual herbicide product to protect glyphosate-resistant crop technology," says Heck. "If you see any weeds after spraying, you need to determine why it wasn't killed by your chemical application, and what is your plan is to deal with it if it is a glyphosate-resistant genotype. We need to avoid applying glyphosate as a one-pass product applied when the weeds are too large to be effectively controlled."

Iowa State University Extension weed scientist Mike Owen accompanied the ISA farmer leaders from Iowa. Since the early 1990s, Owen has been warning that farmers must be careful stewards of the glyphosate-resistant crop technology.  ISA has funded projects through which Owen will continue surveying Iowa for the evolution of weeds resistant to glyphosate, as well as other herbicides, and working to develop management tactics for herbicide-resistant weed populations.

Iowa farmers need to make changes, diversify their weed managment

"What we saw in Tennessee was similar to what we're seeing across Iowa," Owen says. "Weeds are not being effectively controlled by the management practices farmers are using. Specifically, weeds are not being effectively managed by glyphosate alone. The weed is different, in that Palmer pigweed may be somewhat different from the waterhemp species we have here in Iowa, but the similarities are greater than the differences."

Owen adds, "There's one thing that needs to happen. That is, farmers to need to diversify their weed management program. The way to do that is to use either an early preplant herbicide, which I'd recommend, or a pre-emergence herbicide. The key is that farmers must choose carefully to be sure they have the herbicide that acts on the weeds they're dealing with and not rely on one product."

All who were on the trip agreed that being proactive by practicing good management is key. That way, "a great tool can remain effective for years to come," says Van Kooten.

ISA will continue to work with ISU to promote glyphosate stewardship. Watch for more details about the ISA officers' recent trip in an article in the October issue of the Iowa Soybean Review. In addition, a fact sheet,"Glyphosate Stewardship: Fix it Before it Breaks!," which was first distributed in 2009, is available online (www.iasoybeans.com/productionresearch/publications/glyphosate.html).