Dry conditions this fall have led to timely harvest progress in much of the state. As folks start to wrap up, the window for follow up field activities like fall herbicide applications may be longer than in years past. Recent C.O.R.N. articles have covered the benefits of fall herbicide applications: Our Annual Article to Nag about Fall Herbicides and Cressleaf Groundsel, AVOID A NIGHTMARE NEXT SPRING!!!!!!!!, Another Article about Fall Herbicides?!. In the 2022 driving survey of late-season weed escapes in soybean, marestail was the second most common species encountered. Fall applications are an essential part of managing marestail and other overwintering species.
The dry pattern this fall may have reduced winter annual weed emergence, and we don’t appear to be headed into an overly wet pattern. It’s possible that weed populations are low and may not merit a fall application, although there are always more weeds out there than we think. While it’s possible any rain through the next month would stimulate additional emergence, colder weather in that time could limit emergence. Evaluation of overall weed emergence and growth at this time could help determine if an application is necessary. In OSU research there has not been a benefit of adding metribuzin or other residual herbicides this late in the fall (exception being chlorimuron which persists into the spring), so treatments at this time should address emerged weeds. Dry weather can also influence efficacy of systemic products like glyphosate and 2,4-D, which work best when applied to weeds that are actively growing. However, the main concern here should be the presence or lack of weeds. OSU research has shown that fall treatments tend to work over a variety of conditions. Although activity is slower, herbicides applied into December are still effective, so the decision about whether to apply this fall can be delayed some.
Where considerable weed populations are present, fall applications are the best way to ensure a clean start next spring. Foliar herbicides are generally most effective when daytime temperatures are in the 50s or higher, and nighttime temps above 40. Applications made in cooler temperatures can result in slower plant death or poor control overall (see comment at end of preceding paragraph). The addition of labeled adjuvants can ensure better herbicide performance in these conditions. The need for a fall application may vary across the state, and from field to field within an operation. Repeated scouting to assess weed emergence and growth following precipitation or frost events can be helpful in making this decision.