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 one example of a weed species that is often better controlled with herbicides applied in the fall compared with the spring. An increasing frequency of marestail populations in Illinois are resistant to glyphosate, and within the past year we have confirmed that resistance to ALS-inhibiting herbicides also is present in Illinois populations. 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.
One question typically posed is whether or not 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. 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 (similar to last winter) 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 target fall-emerging winter annual species, biennials, and perennials. We do not recommend fall application of residual herbicides for control of any spring-emerging annual weed species. We are aware that some products have 2(ee) recommendations that suggest the product will control certain summer annual weed species following application in the fall. Particularly concerning to us is that “pigweed species” are listed on at least one product label. The extension weed science program at the University of Illinois does not recommend fall-application of residual herbicides to control Amaranthus species the next spring for the following reasons:
1) Inconsistent performance: as previously described, the 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.
2) Increased selection for herbicide-resistant biotypes: soil-applied herbicides are not immune from selection for herbicide-resistant biotypes (please see the April 16, 2013 article titled: “Herbicide Resistance: Are Soil-Applied Herbicides Immune?”). 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 of the most challenging 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.