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Getting a Start on Weed Control in the Fall

October 4, 2002
Over the past couple of years, the practice of applying herbicides in the fall to control winter annual weeds has gained widespread popularity. This practice started in fall 1999 in Illinois, with only a few products labeled for fall applications. Since 1999 a number of products have been added to the fall-applied arsenal (Table 4). With this increase in products has come an increase in interest from many producers. This interest arises mostly from growers who have had a difficult time controlling winter annual and perennial weeds in no-till fields in the spring. Winter annual weeds, such as purple deadnettle, henbit, chickweed, horseweed (marestail), and a number of mustard species, can form a dense weed mat that can be difficult to control with spring burndown herbicides. These difficulties can arise from insufficient spray coverage, fluctuating spring temperatures, and timeliness of the application due to uncooperative spring weather.

A number of potential benefits may be realized from controlling winter annual and simple perennial weeds in the fall. Controlling these weeds in the fall prevents dense mats of winter annual weeds that can physically interfere with planting and tillage, reduces vegetation where insects may harbor, and potentially allows for earlier planting due to increases in soil drying and warming. In addition, controlling these weeds in the fall prevents them from producing seed, thereby decreasing the soil seed bank and helping reduce future problems with these species. Fall control of simple perennials, such as dandelions and white cockle, is much more effective than controlling these weeds in the spring. In the fall, food reserves in these perennials are being moved to the roots; when a systemic herbicide is applied, that herbicide moves with the food reserves to the roots and can cause complete control of the roots. Additionally, higher rates of some translocated herbicides (i.e., 2,4-D) can be used in the fall, allowing for greater control of perennial weeds such as dandelion.

Three basic approaches to fall herbicide applications follow: (1) apply a herbicide with soil-residual activity before most of the winter annual weed species germinate; (2) apply a nonresidual herbicide, such as glyphosate, 2,4-D, or Gramoxone, to emerged winter annual, biennial, and perennial weeds while they are still relatively small or in the rosette stage; and (3) use a combination of the first two approaches. All of these approaches strive to reduce the amount of total vegetation that needs to be dealt with in the spring prior to planting, possibly even eliminating the need for a burndown herbicide application. Although these approaches sound good in theory, the actual end results may or may not be as good as expected in large part due to uncertain weather conditions. During the past 3 years, we have conducted several experiments looking at fall herbicide applications for winter annual weed control, which are summarized below.

During fall 1999 and 2000, we conducted an experiment at four locations to examine the efficacy of fall-applied soybean herbicides. The locations we selected were Dekalb, Urbana, Brownstown, and Altamont, which represented a good north-to-south gradient as well as some diversity in weed species. At these four locations, fall herbicide applications were made in mid-November. The herbicides that we included were Canopy (3.0 and 7.0 oz/A), Canopy XL (2.5 and 6.8 oz/A), and Sencor (4.0 and 10.0 oz/A), all with and without glyphosate + 2,4-D. Glyphosate (Roundup Ultra) + 2,4-D (1.5 pt + 0.5 pt) was also applied alone to see how this treatment would work without a residual herbicide. As we expected at the outset, results with these fall applications were quite variable at soybean planting.

What we found was that these fall herbicide applications seemed to be more suited to the southern regions of the state where winter annual weed growth was much more prevalent. Also in many cases, the higher rates of these herbicides outperformed the lower rates; however, this outcome could be overcome with the addition of glyphosate and 2,4-D to these treatments. In comparing just the residual herbicide treatments, Canopy at both rates and the high rate of Canopy XL were the most consistent at controlling common chickweed, annual bluegrass, purple deadnettle, cressleaf groundsel (butterweed), and shepherd's-purse. The addition of glyphosate and 2,4-D improved winter annual weed control for a number of these treatments. Glyphosate + 2,4-D without a residual herbicide provided good control of common chickweed and shepherd's-purse; however, at soybean planting, summer annual weeds (i.e., common lambsquarters and ragweed spp.) flourished in these plots from lack of residual activity and lack of winter annuals (i.e., chickweed) to suppress summer annual weed growth.

In fall 2001 we initiated a new set of experiments that focused on the timing of fall herbicide applications. One experiment was set up to examine residual control of summer annual weeds from fall herbicide applications based on soil temperatures ranging from 30 deg F to 60 deg F. The second experiment examined winter annual weed control from fall herbicide applications based on air temperatures ranging from 25 deg F to 65 deg F. Based on one year's data, herbicide applications with soil temperatures less than 55 deg F had little to no effect on summer annual weed control. Conversely, air temperature did have an effect on initial common chickweed control with glyphosate + 2,4-D and Canopy XL + Express. However, by 30 days before planting, no difference in common chickweed control based on air temperature existed. Even though there were very little differences in weed control based on application timing this year, we will be repeating these experiments in fall 2002.

In addition to these experiments, we also have conducted a number of research trials examining fall herbicide treatments that manufacturers are currently promoting. Table 5 compares some of the more popular fall treatments for weed control in corn, and Table 6 compares fall treatments in soybean. Both of these experiments were conducted at Brownstown, and applications were made in mid-November 2001. The weed-control ratings listed in these tables are at corn or soybean planting.

Fall herbicide treatments can be extremely effective tools in managing winter annual, biennial, and simple perennial weeds. So how do you know if fall herbicide applications are suitable for your farming operation? These applications are most effective on fields where these weeds have been a problem in the past. If spring herbicide treatments have been effectively controlling these species and they do not appear to be increasing, little to no benefit may be gained from fall herbicide applications in these fields. In addition, even though winter annual weeds may be controlled by fall applications, under certain conditions a spring burndown treatment may still be needed.--Christy Sprague, Aaron Hager, and Ryan Hasty

Author: Aaron Hager Christy Sprague Ryan Hasty

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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