In our final installment on prepping harvested and fallow ground for next year, we’re discussing fall herbicide applications. Growers who experienced heavy weed pressure this year should consider a fall herbicide pass before a hard winter freeze sets in. Fall herbicide applications can help control weeds that emerge in fall in addition to winter annuals that can grow during the winter months.
Key Weeds Controlled
Weeds such as Palmer amaranth and waterhemp are highly prolific and aggressive, and some weeds like marestail can overwinter in the soil as massive seedbanks. Weed control is a must year-round, and growers can benefit from fall herbicide applications to help control the seedbanks before they become problematic for next year. This is especially important in fallow fields where weeds didn’t have competition from crops. Weeds thrive when there aren’t other plants competing for nutrients, water and sun. The more they’re allowed to thrive, the more they’re going to reproduce.
Fall herbicide passes are crucial in helping tackle winter annuals such as marestail, field pennycress, tansy mustard, henbit, downy brome, prickly lettuce and shepherd’s purse. Winter annuals typically emerge later summer or early fall and can survive through winter. They’re often hard to spot in the fall when there’s crop residue left behind from harvest, so growers should always scout their fields post-harvest to look for small rosettes close to the ground.
Marestail — one of the more troublesome winter annual weeds for growers — germinates in the fall and can produce up to 200,000 seeds per plant. University of Nebraska Extension experts note that “fall emerging marestail seedlings tend to experience higher mortality while established plants produce more seeds and are more competitive to crops when compared to spring emerged plants.” Tackling them in the fall with a burndown herbicide can help keep them at bay come spring. Growers should also consider a herbicide with residual activity so that the chemistries can stay active in the soil for a time after application. It’s also important to remember that marestail throughout the Corn Belt has shown some resistance to glyphosate.
Another burdensome weed that growers can help control with a fall herbicide pass is kochia, a summer annual weed. Growers in the upper Midwest have recently struggled to control kochia in row crops. South Dakota State University Extension experts note that a fall-applied residual herbicide can be more effective than a pre-herbicide applied in the spring because wet and cool spring weather in the region may not allow a pre-herbicide to fully activate before planting. A residual herbicide applied in fall, on the other hand, has more time to activate in the soil and can help block emerging summer annual weeds.
Applications
Ohio State University agronomy experts recommend growers use two methods of action to achieve full effectiveness of your herbicide pass. Specifically, they note, “Our philosophy has generally been to start with 2,4-D, and then add another herbicide that results in more comprehensive control.” They suggest herbicides like glyphosate, metribuzin and simazine as viable secondary options.
Other experts recommend using a herbicide with a residual to control winter annual weeds. The University of Missouri Extension suggests growers “choose a program with enough residual activity to control winter annual weeds that may come up later this fall as well as any that may emerge next spring.” They also note that fall burndown passes lessen your workload come spring because they should replace the need for a spring burndown.
Timing and Application Recommendations
A fall herbicide application should be part of a larger weed control strategy. It should be considered in tandem with a pre-emerge application for spring, post-emerge applications and foliar applications during the growing season. Fall applications are typically for burndown only.
Ohio State University agronomic experts recommend anytime up to Thanksgiving is a good time for a fall herbicide application, and even into December so long as a hard freeze hasn’t occurred. They note, “Once hard freezes start to occur, there is usually a substantial change in the condition of certain weeds, such as dandelion and thistle, that renders them less sensitive to herbicides.” Rainfall can also impact the efficacy of a herbicide, so pay close attention to the weather forecast. And, as always, growers need to read and follow label directions before making any applications.
Fall herbicide applications may not completely eliminate the seedbanks, but they can control some of the population so it’s more manageable next year. Follow your fall application(s) with a pre-emerge application in the spring for maximum effectiveness. And, if you have any questions, please don’t hesitate to contact your local Stine agronomist.