NATIONAL AG WEEK: Weeds took advantage of Iowa's wildly mild winter

February and early March 2024 will go down in the history books as unusually warm and dry. Many winter annual, biennial and perennial weeds have resumed growth earlier than usual. Managers will need to be especially vigilant to treat these “early” weeds in a timely manner this spring.

The primary concern this spring is that temperatures have been warm enough for these species to resume growth but too cool overall to achieve effective, consistent control with herbicides. Herbicide applications for winter annual and biennial control should be targeted prior to bolting, or stem elongation, and flowering. Control of winter annual and biennial weeds with herbicides becomes less consistent as they bolt and flower.

The optimum timing of herbicide application will vary by weed species and field conditions, so site-specific scouting and knowledge of the infestation is important to make appropriate decisions. For example, winter annuals like henbit, common chickweed, and field pennycress generally flower earlier than the winter annual horseweed and biennials like musk thistle and wild parsnip.

Effective burndown treatments should follow herbicide label suggestions for carrier type and volume, nozzle type, and weather considerations. Treatments made on sunny days with warm daytime and nighttime (>40F) temperatures will generally be more successful than applications made during cooler conditions.

When selecting herbicide burndown treatments, choose herbicides labeled for control of the target species and consider the likelihood of resistant biotypes in the field, particularly when treating horseweed. Herbicide group (HG)1 9 (glyphosate) and HG 2 (ALS) resistant horseweed populations are widespread across Iowa. Including 0.5 lb. a.e. 2,4-D LVE, 0.25-0.5 lb. a.e. dicamba (HG 4) or 1 oz. Sharpen (HG 14) or another saflufenacil product labeled for burndown control to glyphosate will increase the consistency of horseweed control.

Check herbicide labels for planting restrictions if treating winter annuals in row crop fields and for haying or grazing restrictions when treating weeds in forages. Most 2,4-D labels have a 7-14 day planting restriction for corn or soybean following application. Ester formulations of 2,4-D have a shorter interval to crop planting after application than amine formulations. In addition, esters often perform better under the cool conditions commonly encountered with spring applications. Dicamba can also be used preplant in soybean but has longer replant restrictions than 2,4-D (14 days at 0.25 lb a.e. and 28 days for 0.50 lb. a.e.). When dicamba or 2,4-D resistant soybean varieties are planted, a dicamba or 2,4-D product that is registered for use on those varieties can be used preplant without any planting delay.

The use of an effective early burndown herbicide treatment for winter annual, and early spring weeds is an important first step to achieving a clean field for crop planting. Due to the earlier green-up of these weeds, the timing of burndown herbicide applications will need to move earlier than in a typical year. Timely biennial weed control in pastures and hay fields is important to improve forage yields and reduce concern for feed quality. In row crop fields, the inclusion of a herbicide with residual activity should keep fields weed-free and will allow for a delay in the next herbicide application until after crop planting. Keep in mind that moving residual herbicide applications earlier will mean the next herbicide application will likely need an earlier application as well. While this burndown application may not be necessary in many fields, no-till fields with known winter annual weed issues are good targets for an early burndown herbicide application this spring.

The group number refers to the site of action of a herbicide. The Herbicide Group number is displayed on the first page of the herbicide label and is important information for developing resilient weed management programs.