With sub-$4 corn, fungicide decisions were a bit harder this year, says Erin Marlow. But in the last two to three weeks, disease pressure has increased with identification of southern rust, gray leaf spot and tar spot.
“In some cases, you get into fields that have a little bit of everything, and it really starts to add up,” says Marlow, a crop consultant in Maryville, Mo.
But on Wednesday, she got three calls throughout the day that were easy answers. Farmers asked her if they should make a late season herbicide application.
“Most people aren’t going to spend money on something that they don’t see the value in, in a lot of cases,” she says. “This is being fed by emotion. There’s just kind of that psychological pull for it.”
Late-season weeds popping through the soybean canopies and tall green weeds standing out in browning corn fields have some farmers questioning if a late-season application can be a weed management tool.
“It’s difficult for a person to drive by a field they farm and see that,” Marlow says. “There’s this overwhelming feeling to try and do something. The education part for us is trying to explain to them that you’re going to hurt yourself more than you’re going to help yourself in this scenario.”
Marlow says for her geography, overall fungicide application is up this year over the average. And while talking with other consultants, she says this idea of a late season herbicide application is being fielded in Kansas, Iowa, Nebraska, Illinois, Indiana and Ohio as well.
Aaron Hager, Extension specialist, weed science/IPM at University of Illinois, gives three reasons why applying a herbicide right now is a bad idea.
1. The Weeds Are Too Developed
“Farmers are trying to clean up a mess that’s the result of either not getting complete control or another weed emergence event,” he says. “While fields may have looked clean on July 1, by the third or fourth week of July, weeds spike above the canopy. Now these are old plants — by now, rarely are these plants not already flowering.”
Marlow says the most expensive herbicide program is the one that doesn’t work.
“If we have late-season weeds, what we’ve been doing isn’t working. There are better and different options, which means we have to have a management shift,” she says.
2. It’s Illegal
“Each post product has its own application timing. For glufosinate, R1 is the latest application in soybeans,” Hager says. “There’s a reason for preharvest intervals — you can’t have pesticide residue on harvest crops. I’d hate to be the farmer who, because of a later-season application, meant an entire bin of soybeans was going to be condemned.”
3. There’s Adverse Impact on Yield
“The damage these weeds have caused in yield was weeks ago. They aren’t causing much additional loss for yield potential,” Hager says. “Applying a herbicide is throwing away your money, and you run the risk of adversely impacting the soybean yield.”
Marlow adds an example to illustrate the economic cost.
“If it costs $10/acre to add the herbicide to the tank, the yield loss in soybeans can be as much as 12 bu., and let’s assume it’s half of that you’ll lose 6 bu. at $10 beans. So, this could end up costing a farmer $70/acre.”
This is Nothing New
“These late-season herbicide applications have happened for decades,” Hager says. “That’s not saying it’s alright to do. Thinking there’s ROI on throwing in a herbicide with a fungicide application — it’s a complete misnomer.”
Hager says if waterhemp is the target species, this reinforces best practices to fight herbicide resistance.
“We’re never going to spray our way out of this,” he says. “We’ve tried for 30 years, and the only thing we’ve accomplished is speeding up the evolution for resistance against all chemistries.”
Marlow says farmers need to apply an updated lens for their weed management expectations.
“If we can get 90% to 95% control, that’s doing really good, and that could be a great expectation for chemistry,” she says. “However, if you have a field that has an area where waterhemp produced 100,000 seeds last year, with 90% control, that still leaves 10,000 waterhemp plants.”
As such, she says perfect weed control may be difficult to get to or may not exist. Once weeds start growing, they become even harder to manage.
What Can Farmers Do Different?
First, Hager and Marlow both say farmers need to understand the weed species they are targeting.
“We call them driver weeds because they drive your decisions,” Marlow says.
Second, choose the right chemistry programs.
Marlow advocates farmers use overlapping residuals in both corn and soybeans.
“We know we can kill weeds before they come up,” she says. “It’s important to choose a strong chemistry program and get applications done in a timely fashion.”
