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Burndown of Horseweed (Marestail) in the Spring (05/07/20)

One of the many results of abundant rainfall last fall is the increased presence of horseweed in many no-till fields across the state.

One of the many results of abundant rainfall last fall is the increased presence of horseweed in many no-till fields across the state. The best time to control horseweed is in the fall when plants are small and more susceptible to herbicides. As we progress farther into May, horseweed will begin/continue to bolt and become more challenging to control the longer we wait. It is safe to assume that most of our horseweed is glyphosate-resistant, and we also have several populations that are resistant to ALS-inhibiting (Group 2) herbicides. Here are some of our best options for controlling horseweed prior to planting crops in the spring:

Dicamba – The largest issue with applying dicamba in the spring is that it limits the number of crops we can still plant in a timely fashion. Most dicamba labels allow immediate planting of corn. Small grains require at least 22 day plantback when applying 0.25 pounds of dicamba, regardless of formulation. We can use Engenia, Fexapan, Tavium, or Xtendimax ahead of Xtend soybean with no plantback restrictions. All other crops have at least a 4 month plantback restriction which means dicamba is not a practical choice.

Elevore – Elevore has been on the market for a few years now. It can be applied 14 days prior to planting canola, corn, soybean, small grains, or sunflowers. All other crops require at least a 9-month plantback restriction. Elevore will provide similar control of horseweed to higher rates of dicamba, but it also has a more limited weed control spectrum. Due to the limited spectrum, it is generally recommended to also include glyphosate and/or 2,4-D with Elevore. The inclusion of 2,4-D will increase the plantback restriction to some crops.

Paraquat – Paraquat (Gramoxone, others) can be applied prior to planting most of our crops. It is currently not registered for use in canola, chickpea, lentils, flax, or oats. New in 2020, applicators must have completed an online certification to use paraquat. Keep in mind that paraquat works best with higher carrier volumes. Paraquat also benefits from being tank-mixed with atrazine or metribuzin, but this will further limit the crops we can immediately plant.

Sharpen – Sharpen can be applied at 1 ounce per acre with no plantback restriction for chickpea, lentils, corn, peas, small grains, and soybeans. If soils are coarse with less than 2% OM, then a 30-day waiting period must be observed for soybeans. Higher rates of Sharpen can be used, but will increase the plantback restrictions for some crops. See page 6 in the 2020 ND Weed Control guide for more detail on rates.

 

Joe Ikley

Extension Weed Specialist

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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