Cover Crops: What They Are and How to Kill Them

By: Wilks Wood, Ph.D, Field Agronomist and Andrew Hunt, Research Agronomist

If you haven’t added the use of cover crops to your farming strategy, you’ve no doubt come across this age-old method at some point. Perhaps you learned about it in agronomy school. 

Alternatively, maybe you stumbled upon some research when looking for new ways to improve your soil health. 

If your farming strategy doesn’t include planting cover crops, you may want to reconsider. According to a nationwide survey, farmers showed acreage planted in cover crops has nearly doubled over the past five years.

Cover crops can help you boost soil health, reduce erosion, and enrich the soil with populations of “good” microbes.

If you’re contemplating planting cover crops this year, or simply want to know what they are, keep reading. 

What is a cover crop? 

A cover crop isn’t planted to be harvested. It’s grown to protect the soil and keep living roots in the soil. It’s planted when cash crops aren’t being grown, which is during the winter for the majority of cash crops, or when the land is fallow. 

For instance, let’s say you have about 500 acres of corn and soybean. Maybe you use hairy vetch as your cover crop. One study proves that the cover crops’ contribution to fixing nitrogen (N) in the soil is significant, potentially supplying about half the necessary N for corn.

The cover crop also gives your soil protection, suppressing weeds and other pests, improving soil compaction, increasing soil organic matter, and improving water infiltration and moisture conservation.  


While it may be an investment upfront, the cover crop enhances how you grow (reducing inputs) and what you grow (increasing yield). 

When do I need to kill my cover crops? 

Cover crops have the potential to become weeds, which slows soil drying and insulates the soil against warming (keeping the surface of the soil too cool). In addition, some cover crops can get too large and create too much biomass, which leads to problems with planting.

In some locations, frost or other factors may kill your crops naturally. Otherwise, you’ll need to kill your cover crops to plant your cash crops.

 

If you want to get the full benefit from cover crops, timing is important, and it depends on which cover crop was planted. Regardless, if you have a winter cover crop in the field, we recommend checking it now to stay on top of spring management. 


If you kill your cover crop too early, you may not meet your objectives. If you kill your cover crop too late, it can lead to potential yield losses for your cash crop, due to factors like delayed planting, planting complications, and weed issues. 

For most regions with summer rainfall, a cover crop can be killed before the cash crop emerges. However, some agronomists and farmers would argue that cover crops need to be killed a week or three before planting, which allows for some decomposition and drying out.

Also, if spring periods are drier, earlier termination may be necessary to conserve soil moisture. 

There’s really not a one-size-fits-all approach to when you should kill your cover crops. Timing is often based on when you need/want to plant your cash crop. Other times, you may base your decision on specific heights/growth stages of the cover crop. 

Here are 6 reasons you may want to kill your cover crop early:

  1. It allows time to replenish soil water (if you have a drier spring or are in a drier location)
  2. It improves soil warming
  3. Decomposition breakdown reduces phytotoxic effects of residues on cash crops
  4. It reduces the survival of diseases that were on the cover crop
  5. Greater decomposition occurs before planting, which decreases interference with planters
  6. It increases nitrogen mineralization (release) from lower C:N ratio cover crops

Here are 3 reasons you may want to wait longer before killing your cover crop:

  1. There’s more residue available for soil and water conservation
  2. There’s increased weed control
  3. You have greater N contribution from legumes

How to kill your cover crop

To kill your cover crops, you can use herbicide, tillage, mowing (which doesn’t work for everyone), and rolling/crimping. 

Regardless of how you kill your cover crops, it’s important to have a plan in place to minimize issues in the spring. Want more resources on prepping for the 2020 planting season?

 

Check out our article on the 5 steps you need to take now to prep your equipment for a successful season. Read now.

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