What to do When Your Cover Crop Gets Away from You

Unpredictable spring weather can present a lot of challenges when terminating cover crops, and not getting in the field while a cover crop is quickly growing can be a stressful event. Eventually, the cover crop may “get away from you,” reaching a height or biomass level that you are not prepared to address. If that happens, modifying the termination and management plan for the following crop may be necessary, following a few of the points listed here.

Don’t Plant into a Partially Dead Cover Crop

When properly equipped, planting into living cover crops or ‘crispy’ dead cover crops is generally successful. However, cover crops in between those two stages often cause problems. Material that is losing color or becoming rank is hard to cut through, can wrap around planter parts and cause issues with hairpinning and seed slot closure. It is better to wait until the cover crop is completely dead or commit to planting “green” into the living cover and terminating the cover crop after planting.

Adjust Your Termination Program

If there is a short window for terminating cover crops, applying herbicides with fast activity can be beneficial. Paraquat typically results in a quicker kill than glyphosate, however it is most effective when combined with a PSII inhibitor (group 5), such as atrazine or metribuzin1. This option is typically more effective than glufosinate, but it is generally not as effective as glyphosate and a follow-up application may be necessary, particularity the cover crop is tall or thick.

Don’t Kill, then Plant

When dealing with rapidly growing cover crops, it may be tempting to terminate a few days ahead of the intended planting date, particularly if you are dealing with wet fields. While the cover crop is growing, it is actively removing soil moisture through transpiration, and once it is killed that process stops. Problems arise when weather or equipment issues close the window between termination and planting, resulting in wet fields where there is no removal of moisture from the cover crop and crop residue acting as a mulch to hold soil moisture in, resulting in further planting delays.

Figure 1. Corn planted into tall, unrolled cover crops can result in spindly plants, non-uniform emergence, and missing plants. Photo credit: Zachary Larson.

Switch to Soybeans if Possible

Corn planted into tall, thick cover crops may have challenges with emerged population and evenness of emergence (Figure 1). Therefore, switching to soybeans is preferable, as it will likely result in more stable yields when compared to corn.2 Soybeans can adapt to lower populations better than corn, and plants can emerge over a wider window while maintaining yield potential. Additionally, soybeans are less affected by nitrogen tie-up that is common with higher amounts of biomass and higher carbon-to-nitrogen ratios typical in mature grass and brassica cover crops.

Remove, but Don’t Mow Down, Cover Crops

Mowing of large covers is generally not advisable as it can often result in clumping or windrowing of the residue, creating uneven planting conditions. Removal of the cover crop through chopping or baling eliminates this issue and typically results in suitable planting conditions. However, removal leaves the soil bare, increasing evaporation and potentially reducing soil available water later in the growing season. If the material is for livestock feed, ensure that the cover crop is suitable for feeding and that there are no feeding restrictions from prior herbicide applications.

If You Commit to Planting Corn “Green”, Roll the Cover Crop Down

Tall, dense cover intercepts light from emerging plants. To overcome this, plants go through the process of etiolation, where stems and leaves elongate, resulting in “spindly” plants. Corn seems especially susceptible to this, and when combined with uneven emergence, potential yield reductions may occur. Rolling the cover crop can help ensure that emerging plants receive an even light source. Rolling should be completed in the direction in which you intend to plant, and tools such as cultipackers, or something as simple as a telephone pole drug behind a tractor can be effective. Splitting a field in half and rolling and planting in “loops” may be preferable to passes back and forth, as it can minimize planting into material facing the wrong direction.

Cover crops should be terminated when they begin to “bounce back,” typically a few days after planting. Glyphosate provides good control of most grasses, while a plant growth regulator (group 4) herbicide, such as dicamba or 2,4-D, can help control legumes and brassicas. Exercise caution with PGR herbicides; seedling injury can result if seeds are exposed via open seed slots, which can be an issue when planning green, particularly if planters are not properly equipped.

Soybeans generally do not experience the same amount of etiolation that corn does. Therefore, some taller, yet less dense cover crops may not have to be rolled. However, if cover crops are thick, rolling may be the best option.

Shift Nitrogen Applications to Earlier in the Season

The decay of heavy cover crops, particularly in small grains, can tie up nitrogen as soil microbes begin to digest the fresh residue. This nitrogen will eventually be released, but it may occur after the peak nitrogen demand of a corn crop. To help compensate for this tie-up, nitrogen application should be shifted to earlier in the growing season, with at least 50 units of nitrogen applied at planting, and more if the cover is heavy. Depending on the amount and type of cover crop biomass, yield loss may occur for a subsequent corn crop. However, yield loss appears to be less prevalent with soybeans planted into heavy living cover.

Figure 2. In this rolled down cover crop, solid closing wheels are used to pinch the seed slot back together. Photo credit: Zachary Larson.

Adjust Your Planter Accordingly

Many planters may not be equipped for planting into heavy cover crops, so modification of existing equipment is recommended if the correct parts are available. In assessing your planter, look for points where potential wrapping may occur. Disk or shark fin style row cleaners generally cut through residue better than finger style ones, and are less likely to wrap. Fertilizer openers are a possible point of wrapping. If soil phosphorus is ideal and the application of starter fertilizer is only beneficial as a boost in cool soils, removing the fertilizer units and dribbling nitrogen fertilizer next to the row can eliminate a wrapping point. Additionally, closing wheels may need to be replaced. Wheels with horizontal fingers or fins are more likely to wrap around material. If using row cleaners, then most closing wheel setups should work. However, if row cleaners will not be used, then heavy, solid press wheels may help, as cover crop roots can hold soil together in a sod-like fashion, resulting in the seed slot needing to be pinched rather than crumbled shut (Figure 2).

Figure 3. Slug damage in corn. Slug mouthparts scrape away crop tissue leaving a windowing appearance. Photo credit: Zachary Larson.

Watch for Crop Pests

Planting into heavy, overgrown cover crops may result in some unexpected crop pests. In addition to usual crop pests for your region, slugs can present a challenge in heavy cover crop residue. Planting green has been shown to reduce slug pressure compared to early-terminated cover crops, while others have observed greater slug damage in crops planted green.2 Scout fields after planting to monitor damage, especially in areas with seedlings in poorly closed seed slots. Damaged emerged plants have a “windowing” appearance as slugs scrape the green material off plants, leaving translucent tissue (Figure 3). There are no over-the-top options available for controlling slugs, only baits which dissolve with rainfall. Fortunately, rapidly growing crops generally outgrow slug damage, and some early-season damage can be tolerated.

References:

1. Oseland, E., & Bradley, K. 2020. Final results from a multi-state study on cover crop termination with herbicides. University of Missouri Integrated Pest Management. https://ipm.missouri.edu/cropPest/2020/3/coverCropTermination-KB/.

2. Reed, H. K., Karsten, H. D., Curran, W. S., Tooker, J. F. & Duiker, S. W. 2019. Planting green effects on corn and soybean production. Agronomy Journal, 111(5): 2314-2325.

Legal Statements:

ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.

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