In recent years, interest in adding cover crops has increased and potential benefits have become more widely recognized. From nitrogen (N) and nutrient management to soil health and carbon credits, there are numerous reasons why growers may consider adding cover crops to their farms. But cover crops are not for every field, or even every grower.
“What’s your goal with cover crops?” asked Kevin Fry, Pioneer Field Agronomist. “While forages and erosion control are the top methods in most places, growers need to define their goals before planting a single seed.”
Cover crop capabilities and management requirements vary by species. The most commonly used cover crops fall into one of three broad groups: grasses, legumes and brassica.
- Grasses, including winter cereals such as rye, wheat, barley and triticale, are the most widely used cover crops in corn and soybean cropping systems. In general, these grasses are best suited for scavenging soil nutrients, preventing soil erosion and suppressing weeds.
- Legumes are valued as cover crops primarily for their ability to fix N. Common legumes used as winter cover crops in corn and soybean cropping systems include hairy vetch, field pea, lentil, crimson clover, red clover and berseem clover.
- Brassica cover crops, such as radish or winter canola, have grown in popularity recently due to their ability to provide many of the same benefits as grasses but with residues that break down more rapidly in the spring. Certain brassicas are also becoming well known for their ability to produce a large taproot that is effective at minimizing soil compaction.
It is important to note that cover crop suitability varies by region. Minimum annual temperature is a good predictor of how well adapted a cover crop is to a location. U.S. Department of Agriculture plant hardiness zones can be used as a guideline to help select the best adapted cover crop for specific locations.
Growers also need to consider how and when to terminate their cover crops. Winterkilling, tilling, mowing and herbicides are the four main methods. Winterkilling is highly effective but only applicable to certain cover crops. Similarly, while tilling legumes can help increase N availability, it is less desirable for grasses that produce greater quantities of low-N biomass. Due to simplicity and efficacy, many growers prefer to terminate cover crops using herbicides.
Research studies on the effects of cover crop on grain yields vary depending on environment, cover crop species and management. Getting the greatest benefit out of cover crops requires a management level on par with corn and soybeans. Growers should start by testing a cover crop on a single field and expand.
Cover crop selection
As noted above, the most commonly used cover crops fall into one of three broad groups based on species:
Grasses, including winter cereals such as rye, wheat, barley and triticale, are the most widely used cover crops in corn and soybean cropping systems. Winter cereals are typically planted in late summer through late fall and produce a small to moderate amount of root and above-ground biomass before going dormant in the winter. Vigorous growth resumes in early spring, and large amounts of biomass are produced by mid to late spring.
In some instances, winter cereals produce more biomass than is easily managed prior to planting the next grain crop. For this reason, some growers prefer non-winter-hardy cereals like oats, which establish rapidly in the fall, but winterkill and leave behind little residue to manage in the spring.
Annual ryegrass is another option if spring residue levels are a concern. Annual ryegrass is a bunch-type forage grass that produces less above-ground biomass than winter cereals but more root biomass. Annual ryegrass is slower to establish in the fall compared to winter cereals and must be seeded by mid-September in most locations if it is to survive the winter. Because it produces a large amount of shallow root biomass, annual ryegrass is a good fit for no-till systems.
In general, grass cover crops are well suited for:
- Scavenging soil nutrients, especially N
- Preventing soil erosion
- Producing large amounts of biomass that helps to increase soil organic matter
- Suppressing weeds
Legumes are valued as cover crops primarily for their ability to fix N. Common legumes used as winter cover crops in corn and soybean cropping systems include hairy vetch, field pea, lentil, crimson clover, red clover and berseem clover. Legumes can be seeded in early summer through early fall but in many regions must be planted earlier than cereals to survive the winter. The amount of N added by legumes varies among species but is directly proportional to the amount of biomass produced. For this reason, spring management of legume cover crops can involve a trade-off between early corn planting and waiting for more biomass and N production by the legume. A future article will provide guidelines for estimating N credits from legume cover crops.
Brassica cover crops have grown in popularity recently due to their ability to provide many of the same benefits as grasses but with residues that break down more rapidly in the spring. Certain brassicas are also becoming well known for their ability to produce a large taproot that is effective at breaking soil compaction. Common brassicas used as winter cover crops in corn and soybean cropping systems include canola, mustards, forage radish and turnip. Like most legumes, brassicas must be planted earlier than cereals in order to successfully establish and provide maximum benefits. Many brassica cover crops winterkill in locations with subfreezing winter temperatures, which helps accelerate residue decomposition in the spring.
