Why Do Cover Crops Sometimes Fail? Allen Williams, Ph.D. & Gabe Brown

June 26, 2019

The term “cover crop” is everywhere today. It’s in the farming magazines and the ag news, and is the hot topic at many agricultural conferences, workshops and field days.

We farmers and ranchers are told that we need to plant cover crops to protect the soil, build soil organic matter, reduce input costs, reduce runoff and erosion, and even feed our next cash crop. So more producers are trying out cover crops in an effort to improve their bottom lines.
And sometimes they fail. Occasionally an article is written about these people. The question then becomes: Do cover crops really deliver on the promises, or are they just an effort to sell more seed? Often the farmers being quoted will say that cover crops don’t work for them.
Obviously, cash crops like corn, wheat and soybeans also fail. But do we stop planting them? No, we plow ahead (pun intended). We expect some failure in our cash crops, and readily accept it as a part of doing business.

But many farmers give up after their first unsuccessful attempt at planting a cover crop, even though often the failure was due to their own mistakes and inexperience.

The success rate can be greatly improved. Let’s examine the most common causes of failure of a cover crop, and what we can do to enhance our chances of success.

Not determining your resource concern first
Often producers do not take the time to determine what they want the cover crop to achieve, which leads to purchasing and seeding a cover crop that may not address the resource concern. Too often this leads to a poor experience.

Here’s an example:
Gabe’s phone rang one March day, and the caller complained that he had an issue with cover crops. He had an irrigated field where the winter wheat was harvested and the straw baled before being seeded to a monoculture of purple top turnips. The turnips grew well and he turned his cows onto them for the winter.

Gabe interrupted, “Let me guess. And now your fields have no residue.”
“That’s right,” he exclaimed. “And now those fields are blowing! How did you know?”
Gabe told the caller that he had seeded the wrong cover crop for his resource concern. By baling the straw he had removed most of the residue. He then planted turnips, which are nitrogen scavengers and low in carbon. The turnips took up leftover nitrogen from the winter wheat crop and then released that N as they went through their life cycle, which accelerated the breakdown of the remaining residue. Add to this the fact that the cattle ate what little aboveground biomass there was, and this caller had a recipe for failure.

To prevent problems, you must determine what you are trying to accomplish. For example, are you trying to improve nutrient cycling? Increase organic matter? Leave more soil armor? Feed livestock? The list goes on. We have to know our goals before we put seed in the ground.

Not allowing adequate time for growth
In northern environments, many try planting cover crops after a small grain harvest, which means there will not be enough time before first frost to get much growth. In that case, one needs to be using fall-seeded biennials such as cereal rye and hairy vetch, which can tolerate colder temperatures and provide early spring growth.

In the Corn Belt, many try fitting cover crops into their corn-on-corn or corn-soybean rotation. Yes, you can get a cover crop to grow and it will advance soil health a little. But why not diversify the crop rotation by adding a cash crop such as winter wheat? Once harvested, it will allow a much greater window of time for a diverse cover crop to take up solar energy and, through photosynthesis, pump that energy into the soil.

Not using the right planting method and machinery
Are you aerially seeding, broadcast seeding, or drilling the seed? The method you should use is highly dependent on moisture. If you have irrigation or live where moisture is plentiful, aerial or broadcast seeding are options for many species of cover crops.
But not all species. Some, such as peas, soybeans and mung beans, perform much better if they are drilled into the soil. Also, in drier environments it is best to drill to ensure good seed-to-soil contact.
When drilling, you must ensure the settings are correct for the seed mix. Are you using the right inoculants for the legumes? Are you planting after harvest of the cash crop, or into the standing crop?
For instance, we know farmers who are experiencing good success with planting diverse cover crop mixes into standing corn at the V4-V5 stage. They are not experiencing any significant yield drag, and the covers are ready to grow with vigor once the corn reaches dry down and the canopy is opened, or after the corn is harvested for silage.

Planting a monoculture
One of the biggest mistakes we make is in thinking that our cover crops should be monocultures just like our cash crops. Monocultures following monocultures creates more opportunity for failure.
Research conducted at the Menoken Demonstration Farm in North Dakota has shown that planting a more diverse mix results in plant biomass production that is anywhere from 2.0 to 3.5 times the production of the individual crops in the mix when they are planted as monocultures.
In southwestern Minnesota, Grant and Dawn Breitkreutz (see Graze, April 2018) initially tried planting monoculture covers into their cash crops. They experienced failures in two out of three years, and thought about quitting cover crops altogether.
Then they started working with the Minnesota Sustainable Farming Association and the Pasture Project, and decided to give cover crops another go. This time they planted more diverse mixes of at least eight seeds. That made a significant difference, and they now routinely graze their covers through the winter months.
Failure to account for herbicide residues
Prior herbicide use can have profound impacts on the success of subsequent covers, and it is very important to pay attention to the herbicides used for your cash crops and their potential for carryover. You need to consider how long a particular herbicide persists in the soil and how sensitive the cover crop is to potential residue.
Typically, herbicides with a shorter half-life have less potential for negative impact. Products with a half-life of 30 days or less often pose no issue for subsequent cover crops. However, plant species sensitivity must be considered. The small-seeded legumes, grasses and mustards (clovers, ryegrass, canola) can be quite sensitive to some herbicides.
Other factors influencing cover crop sensitivity to prior herbicide use include soil biological activity, rainfall, soil texture, and even soil pH. The half-life of many herbicides can be significantly reduced IF the soil has very good to excellent soil biological activity — something that many farm soils are lacking today.
A number of good resource materials are available to help you with this issue. They include the Green Cover Seed website resources page (www.greencoverseed.com/resources), the Penn State University Agronomy Guide (https://extension.psu.edu/herbicides-persistence-and-rotation-to-cover-crops), and Iowa State University’s guide to grazing withdrawal periods for specific herbicides (http://www.extension.iastate.edu/union/sites/www.extension.iastate.edu/files/union/CoverCROP3082.pdf).
Failing to consider the carbon to nitrogen ratio (C:N)
The carbon to nitrogen ratio, or C:N, is very important in determining the specific species to include in your cover crop mixes relative to the prior crop and the next planned crop in the rotation. Understanding the C:N ratios of various plants and their residues is key to managing both soil cover and residue decomposition. Jon Stika’s book, A Soil Owner’s Manual, notes that there are times when you want plant residue to remain on the soil, and times when you want it to disappear.
Soil microbes themselves have a C:N ratio of about 8:1, but require a C:N diet of 24:1 to meet their needs. If mature alfalfa hay is fed to your cattle, what they trample and leave behind will be quickly consumed by the soil microbes, with little to no C or N remaining. That is because mature alfalfa hay has a 25:1 C:N ratio.
In comparison, wheat straw has a C:N of 80:1. This forces soil microbes to seek out other forms of N to go with the excess C. As the microbes consume other forms of N in the soil, they tie that N up in their bodies so it is not immediately available for plant uptake. Conversely, hairy vetch has a C:N of 11:1, and soil microbes will consume the vetch and leave the excess N in the soil for plant uptake.
The C:N ratio affects soil cover and nutrient cycling, which influences subsequent cash crop performance. Planting specific cover crop mixes can help you manage N and crop residue cover.
For example, planting a low C:N ratio cover crop, such as a legume- and/or brassica-heavy mix following a high C:N cash crop (corn and wheat), helps the plant residue to decompose, thus making nutrients available to the next cash crop. Likewise, planting a higher C:N ratio cover crop, such as a small grain/grass-heavy mix (grazing corn, sorghum, sorghum-sudan, millets and sunflower) helps to provide protective soil cover after a low C:N cash crop such as soybeans or peas.
Good rules of thumb are:
• Mixes with a C:N ratio greater than 24:1 will make N less available to plants.
• Mixes with a C:N ratio less than 24:1 will make N more available for plants.
• Mixes with a C:N ratio greater than 24:1 will decompose more slowly.
• Mixes with a C:N ratio less than 24:1 will decompose more rapidly.
Buying “canned” mixes
“Canned” mixes are often put together by companies that do not really understand cover crops, but simply want to cash in on the trend. Our discussion about proper C:N ratio is enough to tell us that you do not need to plant such mixes, or even plant the same mix every time.
You need to work with highly competent cover crop seed companies with extensive experience in your area/region. They can make the correct recommendations for specific cultivars that work best for your farm, soils and climate. They can also relay valuable information from the other farmers they are working with in your area so that you can avoid the common mistakes.
Don’t just talk with seed company reps. Talk with other farmers in your region who are successfully using cover crops and find out what they are doing and why. In other words, do your homework. It is your business, and you have an inherent responsibility to tend to your business and not let others make your decisions for you.
Finally, as your soil health improves, and as you figure out that you can actually plant crops besides the requisite corn and beans, you will need to alter your cover crop mixes to further enhance and complement your goals and objectives.
Failure to understand proper termination
When you use cover crops, you will have to terminate that crop before planting the next cash crop. Method and timing of termination are crucial to the success of the subsequent crop. There are several options depending on the specific mix you plant. These include:
• Herbicide burn down. Although this is common, herbicides are generally detrimental to soil biology. This is especially true of glyphosate, which is an antibiotic.
• Winter kill. This is highly effective in many northern areas, especially if a cash crop is going to be seeded early the following spring.
• Livestock grazing/animal impact. The cover crops used and timing of the grazing are critical. For the cover crop to be terminated, it has to be at the correct stage of its life cycle. Usually this means it has finished pollinating. High stock densities are required to trample the cover crop.
• Utilizing a cover crop roller. The chevron-type rolls of cover crop rollers crimp and kill the stems if they are in the correct stage. Plants must be at least pollinating to be terminated by this process. Some species, such as rye, peas, hairy vetch and other annual legumes, are more conducive to this procedure.
Failure to utilize the cover crop as a revenue generator
Your cover crops can be a really good source of revenue generation AND a good source of fertilizer inputs for the next cash crop. Livestock grazing of the cover crop can net you as much or more per acre compared to the typical cash crop. By combining the cover crop with livestock grazing, you not only have the opportunity to enhance net revenue per acre, but you can also dramatically advance soil health, including nutrient cycling. The cover crops will hold nutrients on your land and be cycled via biology, providing nutrients to subsequent crops.
Only considering yield in determining cover crop success
Too many producers consider their cover crops a success only if the yield of the subsequent crop increases. Cover crops provide a wide range of services, including feeding soil biology, keeping nutrients on your land, increasing soil organic matter, improving soil aggregation and thus water infiltration, decreasing wind and water erosion … the list goes on.
We recommend you buy and read A Soil Owner’s Manual, available at www.understandingag.com. It is a simple, straightforward read that allows you to more fully understand how soil ecosystems function, and what you can do to move that forward.
In summary, cover crops themselves are not the source of failure. It is what we do that determines success or failure. Next month we will present a number of case studies that show how success with cover crops leads to success with cash crops.

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