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."