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Considerations for Spring Burndown Programs for Weed Control

April 6, 2021 11:06 AM
Blog Post

As it has started to warm up, folks are ready to implement early-spring weed management programs, especially in fields with infestations of winter annuals like field pennycress and marestail (horseweed). Spring burndown (preplant) herbicide programs should target winter annual plants at the rosette stage, prior to stem elongation (bolting), for consistent control. Cold nights (<40°F) will reduce activity of herbicides, particularly for glyphosate. With cool weather conditions, it is recommended to wait for a few days until it warms up before applying herbicides.

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2020 Summary of Herbicide Evaluations for Marestail (Horseweed) Control in Soybean

February 11, 2021
Marestail infestation in a soybean field at the ISU Research and Demonstration Farm near Ames, IA in 2020.

Marestail is one of the most widespread and troublesome weeds in Iowa croplands. It can grow to a height of 1.5 to 6 feet, produce up to 200,000 seeds, and can reduce soybean yields up to 80% if not controlled (Figure 1). Marestail seeds are light and disperse across landscapes with winds. Seeds have little dormancy and can germinate soon after seed shed. In general, 75% of seedlings germinate in fall, remain in rosette-stage until spring, begin stem elongation in April, and start flowering in July. About 25% of seeds germinate in the spring.

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Fall Marestail (Horseweed) Management

October 2, 2017

Marestail (Conyza canadensis) is one of the most difficult weeds to manage in no-till soybean.  While classified as a winter annual, the plant has significant emergence in both late summer/early fall and in the spring.  This extended emergence period greatly complicates management since the success of postemergence products is closely tied to plant size.  Attempting to control populations at the time of planting often results in control failures as fall-emerged plants are too large for acceptable control.

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Horseweed (marestail) management in no-till

April 19, 2016 9:49 AM
Blog Post

The focus has been on planting corn, but don't forget about the weeds. In addition to getting preemergence herbicides applied to the corn acres, burndown applications on no-till soybean fields should be a top priority. Timely application of burndown herbicides simplifies controlling many winter annual weeds, particularly horseweed (marestail). Once the stems on horseweed belong to elongate the probability of successful control diminishes rapidly.  

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Managing Winter Annual Weeds in No-Till Fields

April 8, 2009
Image of horseweed in rosette stage

Bob Hartzler, Department of Agronomy


As no-till acres have increased in Iowa, so have the acres infested with winter annual weeds. The primary cost associated with these weeds is interference with crop establishment and early-season growth.


Some growers question whether it is better to control winter annuals prior to planting or just wait to deal with them at planting. In most situations, the best option will be to control winter annuals as soon as it is fit to get into the field.