Post-harvest herbicide application provides an additional opportunity to manage problematic weed species including winter annuals, biennials, and perennials, said University of Illinois associate professor of weed science Aaron Hager.
“Be sure to scout fields before making any application to determine what weeds are present and if their densities are high enough to warrant treatment,” he advised. “Many herbicides used prior to or after crop planting/emergence can be used in the fall, but not all herbicides are labeled for fall application. Also, some herbicides approved for fall application have application timing restrictions.”
Herbicides that do not have much soil-residual activity (for example, 2,4-D or glyphosate) should be applied after the majority of winter annual species have emerged. A mid- to late-October application might be more effective than one in early October. However, if the fall application will include a herbicide with soil-residual activity, it can be applied earlier.
Horseweed/marestail (Conyza canadensis) populations are increasing in minimum and no-tillage cropping systems across much of the southern two-thirds of Illinois. Horseweed completes its life cycle in one year, but unlike many other annual species, it may exist as a winter or summer annual.
Winter annual horseweed populations typically emerge in the fall, within a few days or weeks after seed is dispersed from the parent plant. In northern Illinois, most horseweed has a winter annual life cycle. A substantially higher proportion of spring emergence occurs in areas (approximately) south of Interstate 70. Both winter and summer annual life cycles can be found across central Illinois.
With the increasing prevalence of horseweed, including glyphosate-resistant populations, fall herbicide applications may work better than spring applications. Glyphosate alone may not provide adequate control no matter when applied, but a higher application rate of products (such as 2,4-D) can be used in fall than in spring.
“We do not recommend fall herbicide applications as a method to provide residual control of summer annual weed species,” Hager said.
Applying soil-residual herbicides fairly close to, instead of several weeks (or months) prior to, planting can improve control of summer annual species, such as waterhemp. If a soil-residual herbicide will be part of a fall herbicide application, use an application rate that will provide control of winter annuals throughout the rest of 2012. “We recommend against increasing the application rate with the idea of controlling summer annual species next spring,” Hager said.