After a relatively slow fall field season, harvest is now winding down in much of the state. As folks continue to wrap up there have been some reports of late-season fall herbicide applications. Previous C.O.R.N. articles have covered the benefits of fall herbicide applications: Fall-applied Herbicide Considerations. The results of the 2023 preharvest weeds survey of late-season escapes in soybean found that marestail was among the top five most commonly encountered species. Fall applications are an essential part of managing marestail and other overwintering weeds.
Here are some general reminders about late fall herbicide applications:
- Scout fields after harvest to evaluate weed emergence and growth to see if the populations present justify an herbicide application. The need for fall applications will vary from field to field.
- Treatments made at this time should target emerged weeds. OSU research has found little benefit from adding metribuzin or other residual herbicides in the fall, especially this late (the exception being chlorimuron which can persist into spring).
- Following frost events, perennial species like dandelion and thistles are less sensitive to herbicides. Winter annual weeds will still be sensitive.
- Foliar herbicides are generally most effective when daytime temperatures are in the 50s or higher, and nighttime temperatures are above 40.
- In general, herbicides are effective across a range of environmental conditions. Herbicides applied into December are still effective, though applications made in cooler temperatures will likely result in slower plant death.
Table 1 in the Weed Control Guide provides ratings for various overwintering weed species in response to fall-applied herbicides. For questions related to the identification and control of weeds, reach out to Alyssa Essman at Essman.email@example.com.