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Issue No. 23, Article 4/October 9, 2009

Fall-Applied Herbicides

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

University of Illinois weed scientists, like researchers across many other midwestern states, have investigated the efficacy of fall-applied herbicides for control of winter annual weed species. Our trials have ranged from northern to southern Illinois, beginning during the fall of 1999 and continuing into this fall. Having examined many aspects, concepts, and products or product combinations during these years, we offer the following points to consider:

  • Fall herbicide applications seem to "fit" better in areas of central and southern Illinois compared with northern Illinois (approximately north of Interstate 80). This probably can be attributed to generally milder average winter temperatures the farther south one ventures (contributing to better winter survival of fall-emerged weeds) as well as to earlier resumption of weed growth in the spring. Fall-emerging weed species in the south may be able both to produce more growth in the fall before entering winter dormancy and 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 more ample than in no-till fields in northern Illinois.
  • Application timing can be very important in achieving goals for fall applications. For example, if you are interested in a treatment without much soil-residual activity, such as 2,4-D or glyphosate, the application following harvest should occur after most weeds have emerged. 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, the application could occur sooner.
  • Be sure to know if the products you are considering applying in the fall have activity on emerged weeds. For example, if you are thinking about using simazine on fields where corn will be planted in 2010 and weeds have already emerged, you might want to consider tank-mixing another product with simazine to control the emerged weeds.
  • Fall applications that include soil-residual herbicides may not always produce a clean field by planting time next spring. Delays in fieldwork caused by adverse environmental conditions may allow fields to green-up before the crop can be planted. On several occasions we've also observed that if we successfully control the suite of winter annual weed species, summer annual species (such as common lambsquarters and smartweed) emerge sooner than if the winter annuals were still present.
  • It's perhaps even more tenuous to expect much control of summer annual weed species such as waterhemp from fall-applied herbicides. Given waterhemp's extended emergence duration, better control from a soil-residual herbicide often results when the application is made closer to planting compared with several weeks (or months) before.
  • With the increasing prevalence of horseweed (marestail), including glyphosate-resistant populations, fall herbicide applications may prove more effective than spring ones. Glyphosate alone may not provide adequate control when applied in either fall or spring, but fall timing provides an opportunity to use higher application rates of products such as 2,4-D than are feasible in spring. However, keep in mind that emergence of horseweed may not be restricted to fall months. Repeated observations suggest the farther south you venture, the higher the proportion of horseweed that emerges during spring.--Aaron Hager

Aaron Hager

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