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Issue No. 24, Article 6/November 3, 2006

Fall-Applied Herbicides

Over the past several seasons, the practice of applying herbicides in the fall, specifically to control winter annual weed species, has gained popularity across many areas of Illinois. No-till fields, particularly in central and southern Illinois, can sport robust vegetative growth before spring planting if early preplant or burndown herbicide applications are delayed. Interest has thus grown in attempting to control fall-emerging weeds soon after crop harvest.

We, like researchers across many midwestern states, have investigated the efficacy of fall-applied herbicides for controlling winter annual weed species. Our trials have ranged from north to south in Illinois, beginning during the fall of 1999 and continuing into this fall. We've looked at many aspects, concepts, and products and product combinations during these years and offer the following points of consideration:

1. Fall herbicide applications seem to "fit" better in central and southern Illinois compared with northern Illinois. This probably is attributable to generally milder average winter temperatures the farther south in Illinois one ventures, as well as to earlier resumption of vegetative growth in the spring. Fall-emerging weed species in the south may be able to produce more growth in the fall before entering winter dormancy as well as to resume growth earlier in the spring. Thus, at any given date in spring, weed growth in no-till fields in southern Illinois typically is ampler than in no-till fields in northern Illinois.

2. Application timing can be very important in achieving the goal for fall applications. For example, if you are interested in applying a treatment that does not have much soil-residual activity, such as 2,4-D or glyphosate, the application should be timed to occur after the majority of weeds have emerged following harvest. Instead of applying such a treatment in mid-October, waiting until early to mid-November might provide better results. If, on the other hand, your fall application will include a herbicide with soil-residual activity, application timing could occur sooner.

3. Be sure that the products you are considering applying in the fall have activity on emerged weeds. For example, if you are thinking about applying simazine on fields where corn is to be planted in 2007 and weeds have already emerged, you might want to consider tank-mixing another product with simazine to control the already emerged weeds.

4. Fall applications that include soil-residual herbicides may not always produce a clean field by the time planting occurs next spring. Delays in fieldwork caused by adverse environmental conditions may allow the fields to green-up before the crop can be planted. Additionally, on several occasions we've observed that if we successfully control the suite of winter annual weed species, summer annual weed species (such as common lambsquarters and smartweed) emerge sooner than if the winter annuals were still present.

5. It's perhaps even more tenuous to expect much control of waterhemp from fall-applied herbicides. Given the extended emergence duration of waterhemp, better control from a soil-residual herbicide often results when the application is made closer to planting compared with several weeks (or months) prior to planting.

6. With the increasing prevalence of marestail, including glyphosate-resistant populations, fall herbicide applications may prove more efficacious than spring applications. Glyphosate alone may not provide adequate control of marestail when applied in either fall or spring, but a fall-application timing provides an opportunity to utilize higher application rates of products such as 2,4-D than are feasible in spring. However, keep in mind that emergence of marestail may not be restricted to fall months. Anecdotal observations suggest that some percentage of marestail populations may emerge during spring months. We plan to begin experiments next spring to determine the emergence characteristics of marestail populations along a north-south transect in Illinois, so stay tuned for future updates.--Aaron Hager

Aaron Hager

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