Fall Herbicide for Winter Weed Control

Fields with winter annual weeds going into winter can be difficult to manage in spring. Not only can dense mats of winter annuals make planting challenging but they can also take up soil moisture and nutrients, restricting access for developing crops.

University of Missouri research found that soil moisture content at planting was as much as 13% higher in corn and 6% higher in soybeans where winter annual weeds were removed with a fall or early spring preplant herbicide application compared to locations with a dense cover of winter annual weed species. Similarly, Kansas State University research found the average nitrogen (N) uptake from winter annual weeds was approximately 16 pounds per acre. 

Farmers with persistent weed problems, such as winter annuals or perennial species, should consider a burndown, says Aaron Hager, weed science specialist at the University of Illinois. “We always have good intentions to try to get in early in the spring, when weeds are small, but Mother Nature always holds the trump cards,” he says. “Sometimes, we simply can’t get in there to get them under control until they’re fairly large or have already flowered and set seed.”

Jared Roskamp, a technical agronomist with BASF, recommends targeting no-till acres for fall burndown applications. “No-till acres are typically some of our more challenging acres for winter annuals, especially if we get the right moisture in the fall,” he says. 

Fall herbicide applications work with nature to manage winter annual weeds. When temperatures cool in November, plants channel their food reserves from their leaves to their root systems, explains Ron Geis, a market development specialist for Corteva in northern Iowa. “The herbicide goes along for the ride,” he says, “and then we get a very complete kill of those weeds.”

Application timing

Fall herbicides can be applied anytime after harvest, before soils freeze; Roskamp typically recommends between Halloween and Thanksgiving as the ideal target window. “By the time we get to Halloween,” he points out, “winter annual weeds will have emerged, and we can burn them down before we start to get heavy frosts. Once we start to get heavy frosts, weeds slow their growth and become difficult to kill.”

Geis recommends applications on days when the morning low temperature is above freezing. He adds that the best foliar herbicide activity occurs during a few days of daytime temperatures in the 50s or higher. “Pick your days to get the maximum absorption of herbicides — days that are a little warmer and sunnier,” he recommends. 

Selecting fall herbicides

When selecting burndown treatments, consider the spectrum of winter annual and perennial species present, and tank-mix glyphosate with full-labeled rates of options such as 2,4-D and dicamba. “Generally, in the spring we’re limited by up to about a pint of 2,4-D, but in the fall we don’t really have those limitations,” Hager says. “If you’re targeting some of these tougher species, don’t be afraid to increase that rate in the fall.” 

When to consider a residual

Typically, Hager says, benefits of a soil-residual herbicide increase the earlier you apply it in fall such as mid- to late-September. That’s because often, winter annual weeds haven’t yet emerged completely. “If you’re chasing the combine out of the first field harvested with a sprayer, it may not hurt to include a residual, since there may be additional weed germination and emergence,” he says. “On the other hand, if you are getting ready to go to the grocery store to finish out your Thanksgiving shopping, then the need for residual herbicide is probably much less.” 

Because of extended germination patterns, Roskamp also factors geography into the decision. In southern Missouri and southern Illinois, for example, a residual has more value in the fall burndown to hold weeds back because those areas stay warmer a little longer. “Knockdown is important for both,” he adds, “but as I move to the north [northern Missouri, southern Iowa, western Illinois], I focus more on the knockdown. As I move to the south, I focus on both knockdown and residual.” 

Tools to fight herbicide-resistant marestail are limited in spring, so fall applications offer opportunities for effective management. “The fall program helps get a better start [to] remove those weeds, and gives us that clean start to get a good agronomic edge for next year,” Roskamp says.

With a residual herbicide controlling 90% to 95% of new weed flushes, spring burndown is easy, Geis says. “You’re controlling just a few escapes instead of a literal salad bowl of weed populations and densities,” he notes.

Benefits beyond weed control

Fall burndown applications offer numerous additional benefits, Roskamp says, including a more streamlined workload, with fewer hours of spring work; better-prepared fields at plant-ing; and improved pest management.

Winter annual weed species, like henbit and chickweed, can harbor soy-bean cyst nematodes and other insect pests. “When we get these dense, green mats of winter annuals in the spring,” Roskamp explains, “insect pests, like cutworm moths, may be attracted to weedy fields more than others. That leaves the larvae to hatch and feed on the developing corn crop.”

Hager says weeds left uncontrolled during fall contribute to the weed seed bank: accumulated seeds in the ground from previous weed generations. “If we can eliminate seed production by winter annuals with a fall application, there’s a benefit of reducing the soil seed bank,” he says. “Over time we can see these populations begin to lower.” 

Other considerations

Fall burndown herbicide applications can help spread out the spring work-load. However, it can be challenging to fit in a fall burndown. “Fall is a busy time, with fertilizer applications and other fieldwork,” Roskamp points out. “We may struggle to find the right window, and then Mother Nature has to cooperate with us too.”

Geis cautions that using crop-specific residual herbicides can lock you out of a crop rotation option if markets or planting conditions dictate changing crops the following spring. 

Finally, applying residual herbicides in fall does not replace the need for a spring residual herbicide. Residual herbicides applied in fall generally provide little if any control of summer annual weeds the following year, especially in soybeans. “There’s not enough persistence,” Hager says, “especially for species like waterhemp or Palmer amaranth that have a very long emergence period.”