Iowa: Spring Burndown — Take Possible Resistance Into Account

Marestail prior to bolting in the spring. Photo: Iowa State University

Marestail prior to bolting in the spring. Photo: Iowa State University

With the short timeframe for fieldwork this spring prior to planting, early weed management may fall to the bottom of the priority list for many. For those who have persistent issues with winter annuals (field pennycress, horseweed/marestail) in no-till, an early burndown treatment may be worth the extra effort this spring.



Winter annuals resume growth soon after the arrival of warm temperatures, so as soon as fields are fit the weeds will be susceptible to spray.

Applications made prior to planting increase the consistency of control of these weeds, particularly horseweed. Many of these weeds are either flowering or beginning to bolt, where the stems elongate, at the time of planting. Achieving full control of weeds at these stages becomes difficult.

Effective burndown treatments should follow herbicide label suggestions for carrier type, carrier volume, nozzle type, and environmental considerations. Treatments made on sunny days with warm daytime (>55F) and nighttime (>40F) temperatures will generally be more successful than those in cooler conditions.

When selecting burndown treatments, consider the likelihood of resistant horseweed biotypes in the field. HG 9 (glyphosate) and HG 2 (ALS) resistant populations are widespread across the state. Including 1 pt 2,4-D LVE or 1 oz Sharpen to glyphosate will increase the consistency of horseweed control, even in fields without glyphosate resistance.



Check pesticide labels for planting restrictions; most 2,4-D labels have a 7-14 day planting restriction for corn or soybean following 2,4-D application. Ester formulations of 2,4-D allow for a shorter interval to crop planting than amine formulations. In addition, esters often perform better under the cool conditions commonly encountered with spring applications.

The use of an effective early burndown against winter annual and early spring weeds is an important first step to achieving a clean field for crop planting. By including a product with residual activity, fields should remain weed-free and will allow for a delay in the next herbicide application until after crop planting.

While this application is not necessary in many fields, those no-till fields with known winter annual weed issues may be good targets for an early burndown this spring.

Source URL: https://crops.extension.iastate.edu/cropnews/2019/03/spring-burndown-treatments-winter-annual-weeds