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Burndown & Cover Crop Termination

Apr 15, 2021

By: Jeff Blauwet, CFE Technical Agronomist

Cover crops are becoming much more common all the time. The added carbon credit bank being established and payments potentially made available with their usage may continue to expand cover crops.  No-till planting, especially soybeans, I always becoming common. These practices will change our herbicide strategies vs conventional tillage. Tillage usually knocks out a flush of early emerging weeds, especially winter annuals like Cereal Rye, Marestail and Mustards or annuals like kochia, ragweeds & lambsquarters etc. Eliminating tillage means we need to deal with these weeds with another method and in most cases that’s herbicides. 

Cereal rye is very commonly used for a base cover crop in the area with some other legumes occasionally added to the program. Rye is a winter annual, so it is planted in the fall with the goal to germinate and get a slight amount of growth. The following spring, as soon as the snow is melted and the frost comes out, rye will green up and start spring growth quickly. Some years like this year growth starts as early as March. The recommendation in corn is to terminate rye at least 14 days ahead of planting to minimize the allelopathic affect from rye slowing early corn growth. It can be a challenge to get the burndown done some years as we can be cold enough in the morning to have frost on plants and getting a good kill in this environment can be difficult. Most burndown herbicide programs are built around Roundup and a growth regulator like Xtendimax (dicamba) or Enlist (2,4-D). These systemic herbicides need to be taken into the plant’s growing points to kill them and slow growth due to cold slows herbicide uptake. This makes it important NOT to skimp on rates. There may also be a desire to apply these herbicides with liquid nitrogen (on corn acres) to save an application pass. This can work sometimes, especially if the mix is >50% water mixed with 32% N, but UAN can burn plants quickly thus slowing uptake even more. If you apply herbicide with liquid N, be aware you may need to reapply a second application if control is less than desired and weeds start to green back up in a week or so.

Herbicide burn down applications in soybeans can be slightly easier as you won’t need liquid nitrogen thus the risk of burning weeds and cover crops does not exist plus applications are typically made later in the spring in a warmer environment. Many no-tillers are also planting “green” into a very recently terminated stand of cover crop rye with success. The allelopathy problem seems to be a corn issue not seen in soybeans.  Part of the confusion with allelopathy could be attributed to early season Nitrogen tie up and that is why it is extremely important to have a well planned nitrogen program with a certain amount of early season nitrogen being available to the plant to avoid this allelopathy affect. Warmer soils with adequate nitrogen will generally minimize the allelopathy affect and corn usually outgrows this once roots become well established. Burndown applications in the absence of cover crops that only include winter annuals and early germinating weeds is also much easier now with the latest technologies of either dicamba or Enlist soybeans. You don’t have to wait the required number of days after applying dicamba or Enlist to plant as before because these traits were not previously available. Having the added flexibility to plant a few days before OR after applying your burndown can be a huge benefit with these technologies.

Consult your CFE agronomist for the best fit for your operation.

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