Weed management is an evolving issue as the science of managing these yield robbers changes. And the rise of tougher-to-control weeds enhances that challenge.

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One tactic that's become more popular is making a fall herbicide application designed to get you off to a good start next spring. However, is that an approach that makes sense for your farm? That's a decision you'll have to make, but we contacted key Midwest weed specialists to explore the topic in more depth.

"It's kind of a mixed situation," says Mike Owen, weed specialist, Iowa State University. "I have never been a strong proponent of fall weed control with the exception of where you have a lot of winter annuals." He notes that generally, fall applied herbicide is "never as good as a spring application of a residual herbicide."

No skipping

One issue is the idea that a fall application suggests you can skip a spring herbicide application. That's a worry given the changing nature of weeds and the fact that even the best herbicide won't be waiting around to stop a spring weed germination flush. However, the rising concern of winter annuals, along with the rising issue of marestail control, complicates the issue.

"Weeds like marestail that behave as an early spring germinator and a winter annual, a fall herbicide application along with what you're using from a spring-applied residual herbicide perspective will help," Owen adds. "It also depends on the likelihood that you'll go forward with the fall application and follow with a spring application. This year a lot of people saw that intention get lost in the shuffle of wet weather. As Bob Burns said, the best laid plans…"

Bill Johnson, Purdue weed scientist, asks whether you're using conventional or no-till as a cultural practice. If you have weeds that can be controlled with a fall tillage pass your need for fall herbicide diminishes. But those winter annuals are a concern. "They can slow spring soil warming and drying," he notes. In a no-till situation that can be problematic.

As for that pesky marestail? "We are sort of in an evolution phase here with the marestail issue," he says. "Here in the Eastern Corn Belt we're almost to the point of making a blanket recommendation for a fall-applied, limited residual herbicide."

Sticking around

Johnson adds that the residual you might get next spring does depend on weather factors. "You can get some residual control into spring once temperatures warm up for weeds to germinate, but that gets difficult to predict," he notes. For example, depending on weather conditions, a fall-applied residual could give you control as late as the first two to three weeks of April.

Owen adds that the decision to treat doesn't have to be a whole-farm issue. "Scouting is important, if you're not scouting there's no need to consider treating," he says. "If you have 5,000 acres of ground, scout and treat those that have real problems with mairestail and other winter annuals. Compartmentalize and use precision when making weed management decisions."

One piece of good news? "There is no shortage of active ingredients to consider nless you have evolved herbicide resistance issues in the marestail popluations, which again emphasizes the need for scouting," Owen adds.

The decision tree with this story walks through the decision choices to consider if you're pondering a fall weed control pass.

The changing nature of a weed

There are natural and artificial factors that can shape the life cycle of a weed. Kevin Bradley, University of Missouri weed specialist, says one weed that's changing is marestail and that's beyond its rise as a weed resistant to glyphosate. "We used to call marestail a winter annual weed," he says. "There's a shift in its biology."

Bradley explains that marestail was a popular reason for fall weed control. The plant is easier to knockdown when in its early rosette stage. However, it's germinating in spring and in a later timeframe than in the past. "There's no reason to believe that a species can't adapt to the practices we use and change its life cycle a bit," Bradley says. And it appears that marestail may be shifting to more of a summer annual mode in some parts of the country.

A fall herbicide application may make sense, but Bradley also cautions: "A grower cannot be under the assumption that by applying fall herbicide that we're rotating modes of action for our pigweed species. A fall herbicide application isn't going to last long enough to have any activity on pigweed species that emerge in the late spring and summer, and therefore cannot be considered as a part of an effective  pigweed management program.  .  Fall herbicide applications are mostly about marestail and winter annual weed species."