Applying herbicides after harvest provides an additional opportunity to manage some problematic weed species, including winter annuals, biennials and perennials. 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 this fall. Many herbicides used before or after crop planting and emergence can be applied in the fall, but certainly not every herbicide is labeled for fall application. Also keep in mind that the labels of some herbicides approved for fall application specify 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. Instead of applying such a treatment in early October, mid-to-late-October application might provide better results. If, on the other hand, your fall application will include a herbicide with soil-residual activity, then the application can be made sooner.
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, different from many annual species, it may exist as a winter or summer annual. 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. Both winter and summer annual life cycles are found across central Illinois.
With the increasing prevalence of horseweed, including glyphosate-resistant populations, fall herbicide applications may prove more efficacious than spring ones. Glyphosate alone may not provide adequate control when applied in either fall or spring, but fall application provides an opportunity to use higher rates of products such as 2,4-D than are feasible in spring.
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, we suggest selecting an application rate that will provide control of winter annuals throughout the remainder of 2012, and we recommend not increasing the application rate in hopes of obtaining control of summer annual species next spring.