The increasing prevalence of fall herbicide applications to control winter annual weeds may indicate that growers are becoming more aware of the benefits that these applications provide. Fall-applied herbicides can be very effective at managing annual and perennial weeds, including some tough-to-control/resistant weeds like marestail. Additionally, fields where weeds are managed in the fall are often the most weed-free in the spring. Elimination of weed cover and growth in the fall allows the soil to dry out and warm up more rapidly, and ultimately allows for an earlier start of the season the following spring. Winter annual weed control can also provide additional benefits as it relates to insect, disease, and rodent populations. Insects, such as black cutworms, will lay eggs in fields with green vegetation in the spring, but will be less attracted to fields without vegetative growth. Wildlife animals, such as voles, are less likely to invade weed-free fields. Additionally, a few winter annual weeds, including purple deadnettle and henbit, are hosts of soybean cyst nematodes and potentially can increase SCN populations if allowed to persist in fields.
When choosing a fall herbicide program, the weed spectrum must be considered. For control of annual and perennial broadleaf weeds, the combination of dicamba + 2,4-D would provide adequate control.
If the spectrum of weeds contains both winter annual grasses and broadleaf weeds, a mix of a growth regulator (2,4-D or dicamba) and glyphosate should be considered. However, the lack of an adequate residual component with these programs will allow new weeds to emerge later in the season, especially if the application is made early. If looking to make a single application towards the end of fall, residual herbicides are less pertinent.
For early fall applications, the use of a residual herbicide is necessary to manage weeds that germinate later in the season. In situations where glyphosate-resistant marestail is present, the use of a residual herbicide will help provide fall and early spring control. Control of marestail at germination or at the rosette stage in the fall is much more effective than spring burndown applications after bolting. The potential downside of using a fall residual herbicide is reduced rotational flexibility. Some residual herbicides will allow you to plant corn and soybeans without restrictions the following season, while others will limit crop rotation options. Crop rotation restrictions will also vary with the rate of the herbicide used, so always read and follow the product label and plan accordingly.
Finally, it is important to remember that fall herbicide applications are not a substitute for spring residual herbicide applications. Weeds are more easily controlled if never allowed to emerge, so incorporate spring residual herbicides followed by in-crop applications to control summer annual weeds, including waterhemp. For questions about fall herbicide applications on your farm, reach out to your local Southern States Agronomy Specialist.