Tackling the Jungle of Winter Weeds
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Winter weeds, in particular winter annuals and some simple perennials, have been the topic of discussion over the last few years. These weeds have increased in prevalence and growth across most of the Midwestern landscape. Increases in these weeds may be attributed to increased acreage in no-till crop production and, possibly, the use of more herbicides that have little or no residual activity. In addition, the past few winters have been relatively mild, extending the growing season for winter vegetation. This winter vegetation can be a major headache to growers in the spring, since winter annuals can form dense vegetative mats that can physically interfere with planting and tillage, potentially harbor destructive insects, and decrease moisture evaporation from the soil surface. Decreases in soil moisture evaporation equates to wetter soils in the spring that can cause delays in planting and tillage operations.
Problematic Winter Weeds
Several winter annual and simple perennial weed species commonly infest corn and soybean fields throughout the Midwest. Some of these species tend to be more problematic in identification and control than others. Currently, winter annuals such as common chickweed, purple deadnettle, henbit, butterweed (cressleaf groundsel), horseweed (marestail), and several mustard species and the perennial dandelion top the list of weeds that cause concern. Identification of these weed species is the first step in controlling them.
A few good early-season weed guides are available to help aid in identifying these weeds. The first is the North Central Regional Publication No. NCR 614, Early Spring Weeds of No-Till Crop Production, published by the University of Missouri. This guide contains colored photographs of 45 different weed species, a taxonomic key, and a guide to herbicides to help aid in control. It can be purchased through the Information Technology and Communication Services office at the University of Illinois. The second guide is a new publication, A Pocket Identification Guide of Early-Season Weed Species, that will first be available at the 2003 Corn and Soybean Classics. It contains colored photographs of 19 different winter weeds that are common to Illinois.
Spring Control of Winter Weeds
Until recently, control of winter annual weeds has generally taken place in the spring with the use of tillage or burndown herbicide applications. In many cases, burndown herbicide applications have been very effective in controlling winter annual weeds. However, there have been instances where winter annual weed control in the spring has not been adequate. These difficulties have arisen anywhere from insufficient spray coverage to timeliness of the burndown application due to uncooperative spring weather. One of the more challenging problems with controlling existing vegetation in the spring is making applications in cool weather when weeds are not actively growing. To address this problem and determine how much of an effect air temperature has on herbicide activity, an experiment was initiated in the spring of 2002.
Three herbicide treatments—glyphosate (Roundup UltraMax), paraquat (Gramoxone Max), and paraquat (Gramoxone Max) plus metribuzin (Sencor)—were applied at six different application timings based on daytime high air temperatures ranging from 47 to 87°F. Weeds evaluated included common chickweed and henbit. Overall, the effect of temperature was not significant for common chickweed control with glyphosate and paraquat plus metribuzin. However, temperature had a significant impact on common chickweed control with paraquat; control increased as temperature increased. Conversely, temperature greatly affected henbit control. Henbit control was less than 80 percent with all herbicides until applications were made when temperatures were above 76°F.
Overall, increases in temperature at the different timings significantly enhanced weed control and reduced weed biomass. These results show the treatment least affected by temperature was paraquat plus metribuzin. This may be attributed to the soil residual activity of metribuzin. Some weed control provided by this treatment at lower temperatures may have come from this residual activity. There also appear to be differences in how application temperature affects control between the two species. Temperature had little effect on common chickweed control with glyphosate; however, application temperature significantly affected glyphosate activity on henbit.
Fall Control of Winter Weeds
Since winter annual and perennial weed control has not always been consistent in the spring, the practice of applying herbicides in the fall to control winter annual weeds has gained widespread popularity over the last few years. This practice started in the fall of 1999 in Illinois, with only a few products labeled for fall applications. Since that time, a number of products have been added to the fall-applied arsenal. With this increase in products, there also has been an increase in interest from many producers—mostly growers who have had a difficult time controlling winter annual and perennial weeds in no-till fields in the spring.
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 annuals that 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, are 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 (such as 2,4-D) can be used in the fall, allowing for greater control of perennial weeds like dandelion.
There are three basic approaches to fall herbicide applications: 1) apply a herbicide with soil residual activity before most of the winter annual weed species germinate; 2) apply a non-residual 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 approaches 1 and 2. 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. While 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 last three years, we have conducted several experiments looking at fall herbicide applications for winter annual weed control, and these are summarized below.
Winter Annual Weed Control
During the fall of 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 pts + 0.5 pt) was also applied alone to see how this treatment would work without a residual herbicide.
As expected at the outset, results with these fall applications were quite variable at soybean planting. The 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, butterweed (cressleaf groundsel), 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 (such as common lambsquarters and ragweed spp.) flourished in these plots due to lack of residual activity and lack of winter annuals (such as chickweed) to suppress summer annual weed growth.
Fall Applications Based On Soil and Air Temperature
In the fall of 2001, two new experiments were initiated 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 to 60°F. The second experiment examined winter annual weed control from fall herbicide applications based on air temperatures ranging from 25 to 65°F. Based on one year's data, herbicide applications with soil temperatures of less than 55°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, there was no difference in common chickweed control based on air temperature.
Even though there were very little differences in weed control based on application timing this year, these experiments were repeated in the fall of 2002.
Additional Fall-applied Research
In addition to the experiments described above, we also have conducted a number of research trials examining fall herbicide treatments that manufacturers are currently promoting. This experiment was conducted at Brownstown and applications were made in mid-November of 2001.
Conclusions
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, there may be little to no benefit to 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.