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Fall Herbicide Application Considerations

As the end of harvest approaches, it’s time to start thinking about fall herbicide applications. Fall herbicide applications may save you time next spring and reduce your workload, but there are a few factors, such as weed species, to consider before deciding if it's right for your operation. 

“Herbicides applied in the fall often can provide improved control of many winter annual weed species compared with similar applications made in the spring,” says Aaron Hager, University of Illinois Extension weeds specialist. It shouldn’t be considered as a fail-safe option, though. Some weed species are better controlled with herbicides applied in the fall compared with spring – such as marestail, he says.

“Targeting emerged marestail with higher application rates of products such as 2,4-D in the fall almost always results in better control at planting compared with targeting overwintered and often larger plants with lower rates of 2,4-D in the spring,” says Hager.

What about soil-residual herbicides

“Typically, the earlier the fall application is made, the more benefit a soil-residual herbicide can provide since emergence of winter annual weeds is often not complete,” says Hager. “However, delaying the herbicide application until later in the fall (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 nonresidual herbicides.”

Consider the weather before moving forward. Cooler temperatures bring benefits. “Cold winter conditions can reduce herbicide degradation in the soil and increase herbicide persistence,” says Hager.

Fall applications are a gamble. There's no guarantee that the weather will cooperate with your plans. 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, explains Hager.

Hager recommends targeting fall-applied herbicides to fall-emerging winter annual species, biennials, and perennials. He doesn’t recommend fall applications of residual herbicides to control spring-emerging annual weeds.

The Extension weed science program at the University of Illinois does not recommend fall application of residual herbicides to control Amaranthus species for two main reasons, says Hager.  

  • Inconsistent performance. The performance consistency varies greatly on the weather and soil conditions after the application, says Hager. “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,” says Hager.
  • Increased selection for herbicide-resistant biotypes. Soil-applied herbicides are not immune from selection for herbicide-resistant biotypes, says Hager. “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,” says Hager.

“Applying these herbicides when they will be most effective against these challenging summer annual species is a critical component of an integrated management program,” says Hager.

 

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