Herbicide after harvest
If you're looking for an additional opportunity to manage problematic weeds, applying herbicides after harvest could be a good idea. Before applying any herbicide, however, be sure to scout fields to determine what weeds are present and if it's worth any treatment at all. Also, be sure to read product labels. Many herbicides can be applied in the fall, but not all. And some labels specify more narrow application timing restrictions.
"If you are considering applying one or more herbicides without much soil-residual activity (for example, 2,4-D or glyphosate), time the application for after most winter annual species have emerged," says Aaron Hager, weed science specialist with the University of Illinois Extension. In some cases, applying such a treatment in mid-to-late-October might provide better results than in early October. However, if your fall application will include a herbicide with soil-residual activity, then the application can be made sooner, Hager says.
Recently, horseweed/marestail populations have increased in minimum and no-tillage cropping systems across much of the southern Corn Belt. "Horseweed completes its life cycle in one year, but, different from many annual species, it may exist as a winter or summer annual" Hager explains. "Populations of winter annual horseweed typically emerge in fall, within a few days or weeks after seed is dispersed from the parent plant. In northern areas of Illinois, most horseweed demonstrates a winter annual life cycle, whereas areas south of (approximately) Interstate 70 see a substantially higher proportion of spring emergence."
Fall herbicide applications may prove more efficacious than spring applications, due to the continuing populations of horseweed, including glyphosate-resistant types. Glyphosate alone may not provide adequate control when applied in either fall or spring, Hager says, but fall application provides an opportunity to use higher rates of products such as 2,4-D than are feasible in spring.
Hager continues, "We do not recommend fall herbicide applications as an avenue to provide residual control of summer annual weed species. Control of such species, such as waterhemp, is often improved when applications of soil-residual herbicides are made closer to planting, compared with several weeks (or months) before." If a soil-residual herbicide will be part of a fall herbicide application, Extension specialists suggest selecting an application rate that will provide control of winter annuals throughout the remainder of 2012, and recommend not increasing the application rate in hopes of obtaining control of summer annual species next spring.