Farmer Manages Resistant Weeds Popping Up in Iowa

Jim Fitkin prefers to be known for his FIT-POP Popcorn, not the resistant weeds he suspected in his fields a few years ago. The fourth-generation corn, soybean and popcorn farmer near Cedar Falls, Iowa, places a high priority on managing soil for productivity for generations to come. 

So, he was disappointed to discover what he believes were the beginnings of both herbicide-resistant giant ragweed in the early 2000s and waterhemp a few years later. 

Fitkin is not alone when it comes to dealing with herbicide-resistant weeds in Iowa. According to Iowa State University weed scientists, glyphosate resistance was first confirmed in Iowa in 2011 in three weed species: giant ragweed, marestail and waterhemp. 

ISU Extension notes giant ragweed is usually the first summer annual to emerge —and it emerges quickly. As populations of the weed increase in Iowa, so does resistance to both ALS-inhibiting herbicides (group 2) and glyphosate (group 9).

“Like a lot of farmers here, I was using glyphosate in my soybeans and was very pleased with the complete kill I saw on all of my weeds,” says Fitkin. “Then, when Roundup Ready corn came out and later Roundup was off patent, I used the same glyphosate to kill weeds in those fields.”

Fitkin says the glyphosate-only system worked well in both corn and soybeans for several years. 

“I wasn’t rotating herbicides, so there was no break in the cycle,” he says. “I slowly began to realize the immediate burndown I had seen on giant ragweed was no longer there. The weed would reappear after the first kill, but a second application of glyphosate would take care of it.”

Eventually, Fitkin got no giant ragweed control with glyphosate. Then, the same evolution began to occur with waterhemp. “I knew what had happened,” he says. “I didn’t contact anyone to confirm it. The resistance was obvious. I knew I was going to have to change course.”

Fitkin tried using a glyphosate burndown, but the resistant weeds came back. The second application worked for a few years, but the weeds ultimately won the battle. So, when a new strategy became available in 2017, he gave it a try —using a soybean variety with both glyphosate and dicamba tolerance. Adding a new mode of action has addressed the problem.

Research confirms the effectiveness of adding dicamba. Purdue University studies in 2010 with dicamba and glyphosate found dicamba with glyphosate had 30% to 65% better control over glyphosate-resistant Palmer amaranth, waterhemp, horseweed and giant ragweed versus glyphosate alone. 

And while dicamba drift has been an industry concern, Fitkin applied it according to label and documented all of his activity. He used dicamba on soybean fields only next to cornfields and did not see any damage to his crops. But, he is concerned about dicamba’s future.

“Drift will become less of an issue as more crops are bred with dicamba tolerance, like we saw with glyphosate and glyphosate-tolerant soybeans. Right now, I can give weeds two shots of dicamba, if needed,” he says. “I planted cover crops in 2019 and will no-till soybeans into them this year and use the same program, but I believe I could eventually see resistant weeds again.”

Fitkin says he knows now that weed populations shift and allow the resistant ones to survive. “Resistant weeds are always there. Farmers just need to learn to better manage them,” he says.

The content in this document was last updated 8/4/2020. As details continue to emerge and change for farmers regarding 2020 dicamba over-the-top applications in soybeans, Take Action is committed to providing unbiased articles and researcher-vetted resources to help navigate this growing season. Find the most updated dicamba information at iwilltakeaction.com/dicamba-resources.