By Chris Proctor

Controlling weeds for next year's cropping season probably isn't the first thing on your mind right now, but now is a good time to start thinking about controlling winter annual weeds. To successfully manage resistant weeds, in particular, you have to stay one step ahead and remember that timing is critical.

One reason weeds can be so challenging to manage is each species has unique biology that affects its emergence, growth and reproduction. This means there is no one good time to spray herbicides that will effectively control all weed species. Often, weed management plans focus on crop growth and development, and time control measures to align with critical crop development stages. However, I would argue from a weed management perspective, it is more effective to focus on weed growth and development and time control strategies to target weeds when they are small (under 4 inches). For winter annual weeds, this would mean applying herbicides in the fall as weeds emerge.

Marestail (horseweed Conyza canadensis L.) is one of eight resistant weed species in Nebraska. In a survey of Nebraska farmers on the occurrence of glyphosate-resistant weeds, marestail was reported to infest the most acres across the state. In addition to the high incidence of glyphosate resistance in marestail populations, it is a weed that becomes very difficult to control with herbicides after it bolts and enters the reproductive stage of growth. Given these facts, marestail is one of the most challenging weeds to manage in corn and especially soybean production systems, making proper timing of control imperative.

Our preliminary one-year data evaluating marestail emergence across Nebraska show that well over 90% of marestail emerge in the fall for eastern and central Nebraska. However, in the Panhandle, marestail emerged entirely in the spring. This data suggests the optimal time for controlling marestail for most of the corn and soybean acres in Nebraska is the fall.

To support this idea, we have also conducted studies that evaluated the effectiveness of several different herbicides for controlling marestail, applied either in the fall or the spring. What we found was 2,4-D, dicamba or Sharpen was significantly more effective for controlling marestail when applied in the fall compared to the spring. When comparing these three treatments applied in the fall, dicamba was consistently more effective at controlling marestail.

An additional concern when fall-applying herbicides is heavy residue can decrease the efficacy of some herbicides. For example, Sharpen is a herbicide with very low water solubility, and if applied over heavy corn residue, it will remain on the residue even after rain. Dicamba, on the other hand, has very high water solubility and will easily wash off residue, improving control of weeds caught under corn residue.

A final consideration for controlling fall-emerging marestail is the use of fall-planted cover crops. Some of our research with fall-planted cereal rye following corn has shown that even late-planted cereal rye can provide enough competition to significantly reduce marestail populations in the spring.

By designing your weed management plan with the biology of hard-to-control weeds in mind, you can greatly improve your success. For winter annual weeds, focus on fall-applied herbicides or the use of cover crops to manage weeds when they are small and most susceptible.

Proctor is a Nebraska Extension weed science educator.