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Corn+Soybean Digest

For Diverse Weeds, Diversity Control

For Jim Rygh Jr., single-pass postemergence herbicide applications don't cut it anymore.

Like many Midwestern soybean growers, Rygh, of Lake Mills, IA, has returned to a sequential application program with multiple modes of action. His goal is to control tough weeds such as waterhemp and ragweed.

It's a move that extension weed scientists like Jeff Gunsolus, University of Minnesota, and Bob Hartzler, Iowa State University, applaud.

"Total postemergence programs can treat weeds like pigweed, lambsquarters and foxtail, but when you get into common ragweed and cocklebur, you need the sequence," Gunsolus says. "Different weeds germinate at different times, and some are not ready when a single pass is made. Sequentials work because different herbicides have different strengths over different weeds."

Research at the University of Minnesota, Waseca, showed that sequential weed control tends to be a money-maker for soybean growers, even with the extra application costs.

"With any total-post program, there is a risk of weather causing problems," Hartzler says. "That risk is lessened with a sequential program. You don't have all your eggs in one basket, so there is a greater margin of safety."

Increasing his margin of safety was one reason Rygh fall-applied Prowl impregnated with fertilizer to 200 acres of corn stubble to be rotated to soybeans. Then he applied Pursuit early post the following spring.

"I didn't have early grass competition for my beans, and this program gave better control on pigweeds and just about everything else," says Rygh. "I tried the sequential program because I wasn't satisfied with control of tall waterhemp and some of the grasses in a total-post program. I fear if I don't control those early weeds, I will lose yield."

Single-pass weed control is a great goal, says Hartzler, but because of the diversity of weed species, it is difficult to find a one-pass postemergence program that will sustain itself. Even tankmixes are not immune from breaking control as weed species shift. He also points out that products that normally provide excellent control by themselves can lose activity in a tankmix.

"There are situations where one pass will work, and if it does, go for it," says Hartzler. "But keep in mind that if you continue to use one herbicide indefinitely, weeds will adapt and species will shift rather quickly."

Even newer single-pass programs, especially the non-selective herbicides with tolerant crops, will succumb to weed selection at some point, warns Hartzler.

"We need to manage these technologies and not expect them to last for 10 years," he says. "I think that, even with the non-selective herbicides, there are weeds that can tolerate them. That doesn't mean the products aren't great tools, but we need to keep expectations realistic."

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