Tackling Tough Weeds

Farmers are experiencing a real and growing problem that impacts their yields. Weed populations resistant to glyphosate continue to evolve, threatening an era of easy weed control.  

Members of the Iowa Soybean Association (ISA) executive board were invited by Monsanto in July to visit Missouri and Tennessee, where they interacted with farmers already facing a severe challenge with glyphosate resistant weeds. 

Randy VanKooten of Lynnville, Dean Coleman of Humboldt, Mark Jackson of Rose Hill, Tom Oswald of Cleghorn and Ron Heck of Perry, along with Iowa State University (ISU) Weed Specialist Mike Owen, met first with farmer Johnny Dodson in Tennessee and heard of his experiences with marestail and Palmer pigweed which have evolved resistance to glyphosate. 

Dodson said the problem started seven or eight years ago. First, he observed a marestail plant here and there. Shortly after that, glyphosate resistance was discovered in Palmer pigweed, a species similar to waterhemp. About the third year, the problem exploded.

Dodson said area farmers now make multiple passes with multiple products. Though his fields are clean, Dodson’s cost of chemicals runs close to $60 an acre.

At a University of Missouri research facility, Dr. Jason Weirick showed the Iowans dicamba-resistant soybeans and explained how they fit into the glyphosate resistant weed management program.

“There are other herbicide resistant tools coming for soybean crops, but we cannot assume those tools are the total solution,” Oswald says. “They are just one piece.”

The Iowans visited Missouri fields where Palmer pigweed grows 12 feet tall and had choked out both soybeans and corn. For fields that got away from them, Missouri farmers have hired crews to chop the weeds out. Where chopped weeds healed and still produced a seedhead, farmers have resorted to disking fields down.

The take-away message for the ISA leaders was reinforcement of the urgent need to be proactive, using good management practices to protect glyphosate-resistant crop technology. 

As Heck notes, many young farmers have never known life before glyphosate. “Before the 1990s, when we gained this technology, we spent a lot of time and money on weed control. We had to manage weeds when they were small and know which spray to use for which weeds. If some got away, we had to go back and kill them with as many as five different chemicals for five different problems. Roundup^® made us all look good.”

ISU’s Owen, who has been warning farmers since the early 1990s that they must be careful stewards of glyphosate-resistant crop technology, is already seeing glyphosate resistance problems develop in Iowa. “Palmer pigweed is somewhat different from waterhemp, but the similarities are greater than the differences. Weeds are not being effectively controlled by glyphosate alone.

“Farmers need to diversify their weed management program by using a pre-emergence herbicide, either an early preplant, which I would recommend, or a pre-emergence,” Owen says. “Farmers must carefully choose the herbicide that acts on the weeds they’re dealing with and not rely on one product.

“We must be clever about the herbicides we use and the way we use them,” Owen continues. “The more diverse the program, the safer it is for avoiding herbicide resistance. Essentially, farmers need different herbicides that control the same weed and are not used as sequential applications, but within the same treatment. They need to use tank mixes of herbicides for controlling waterhemp. Otherwise, we’re still selecting for herbicide resistant weeds.”

During harvest, farmers should note any weeds that escaped for whatever reason, whether because of resistance or not. Oswald says, “We need to be asking ourselves, ‘What weeds are these?’ and ‘Why are they there?’ Maps and notes of each field’s weed issues should be helpful for weed control planning and input purchases over the winter.”

And, the key will be flexibility. “We’ll benefit by having a Plan B and a Plan C, in case weather or other circumstances require an alternate action from the one we’ve planned,” Oswald says. 

He also suggests farmers may need to think about capacity issues. “Weed resistance will put more pressure on farmers to cover acres in a timely fashion. Older chemicals require more water than glyphosate. Farmers will need to consider whether their current herbicide application strategy has the capacity to be timely with products that require more water and what changes are needed to reduce risk of ineffective applications. Where tillage is used, row cultivation may re-emerge to help clean up fields.”

VanKooten concludes, “What is at stake is losing the inexpensive control we have with glyphosate. We must be good stewards or we may not have anything to fall back on.” 

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 population. 

A fact sheet, “Glyphosate Stewardship: Fix it Before it Breaks,” first distributed in 2009, is available online at www.iasoybeans.com/productionresearch.