URBANA, Ill. – Herbicides applied in the fall often can provide improved control of many winter annual weed species compared with similar applications made in the spring. Marestail is a prime example. More and more Illinois marestail populations are resistant to herbicides, including glyphosate and ALS-inhibiting products. University of Illinois weed scientist Aaron Hager recommends targeting emerged marestail with higher application rates of products such as 2,4-D in the fall to achieve better control come spring.
Hager is frequently asked whether a fall application needs to include one or more herbicides that provide residual control of winter annual weed species.
“Typically, the earlier the fall application is made—say, early October—the more benefit a soil-residual herbicide can provide, since emergence of winter annual weeds is often not complete. However, delaying the herbicide application until later in the fall—say, mid-November—often diminishes the necessity of a soil-residual herbicide, since most of the winter annual weeds have emerged and can be controlled with non-residual herbicides,” Hager says.
Applying a soil-residual herbicide late in the fall in hopes of having a clean field prior to planting is akin to gambling on the weather. Cold winter conditions can reduce herbicide degradation in the soil and increase herbicide persistence. This might not always be favorable since, depending on the residual herbicide, increased persistence also can cause injury to the following crop. A more moderate winter and early spring warming will increase herbicide degradation, which could result in the need for a burndown herbicide to control existing vegetation before planting.
“We recommend fall-applied herbicides to target fall-emerging winter annual species, biennials and perennials,” Hager notes. “We do not recommend fall application of residual herbicides for control of any spring-emerging annual weed species.”
Hager notes that some products have 2(ee) recommendations that suggest the product will control certain summer annual weed species following application in the fall. Certain products list “pigweed species” among these summer annuals, but Hager specifically recommends against fall application of residual herbicides to control Amaranthus species, for the following reasons:
Inconsistent performance: Performance consistency of soil-residual herbicides applied in the fall is greatly dependent on weather and soil conditions after application. “Our data suggest the greatest and most consistent control of Amaranthus species either at planting or several weeks after planting was achieved when residual herbicides were applied in the spring, not in the fall,” Hager says.
Increased selection for herbicide-resistant biotypes: Soil-applied herbicides are not immune from selection for herbicide-resistant biotypes. Following a fall application, the concentration of herbicide remaining in the spring when Amaranthus species begin to germinate will be much lower compared with the same product rate applied closer to planting.
Populations of several summer annual broadleaf weed species in Illinois demonstrate resistance to herbicides from more than one site-of-action herbicide class. Their effective management requires an integrated approach that often includes soil-residual herbicides.
“Applying these herbicides when they will be most effective against these challenging summer annual species is a critical component of an integrated management program,” Hager says.
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