The Great Debate: Cover Crops

So, what’s the big deal with cover crops? It is widely known that the National Resource Conservation Service (NRCS) and other government agencies are in favor of cover crops and their contribution to the 4 R’s (Right Source, Right Rate, Right Time, Right Place) of nutrient management. With all of the environmental and water quality issues coming to fruition in the Midwest, there is a push to increase the number of cover crop acres. A recent example is the NRCS soil health initiative, which has improved incentives to grow cover crops and increase acreage.

As a result of the increased demand, cover crops are increasingly a hot topic at coffee shops throughout the Midwest. This is because there is no perfect solution when it comes to planting a cover crop. Additionally, what works for your neighbor, may not work for you. This article is intended to give you a better understanding of what cover crops are and are not by addressing the following:

• What is considered a cover crop?
• What do cover crops do?
• What are the benefits of planting cover crops?
• Where do cover crops fit in the traditional corn/soybean crop rotation?

What is considered a cover crop?
A cover crop is any plant that is broadcast, drilled or planted with the purpose of covering the soil and acting as a protective barrier against water runoff and soil erosion. Cover crops are often planted as a monoculture system (single species) or polyculture (mixture of species). Common species that are widely used across the US include, but are not limited to: annual ryegrass, barley, canola, hairy vetch, oats, radish (tillage), rye, sunn hemp, triticale, winter pea and winter wheat. Usually, if one is planting a cover crop mixture with four to five species, two or three species will likely dominate (annual ryegrass, triticale). Consequently, when planting mixtures, two to three species in a blend is optimum.

What do cover crops do?
Research has shown that cover crops may improve soil conditions by improving soil structure, increasing water infiltration, building organic matter, promoting soil microbial activity and increasing nutrient cycling. Besides acting as a protective barrier against soil erosion and runoff, cover crops can often take up excess nutrients that are not used by the primary crop. This allows them to scavenge whatever nutrients are left, and through mineralization, release nutrients back into the soil for uptake by the next year’s crop.

What are the benefits of planting cover crops?
Since a cover crop can be a multitude of different plants or a combination of plant species, there are many benefits when it comes to promoting better nutrient stewardship.

Legume plant species such as sunn hemp, clover and hairy vetch are often used as cover crops for their nitrogen fixation capabilities. Legumes such as these are able to fixate atmospheric nitrogen in the root nodules and then release nitrogen through a process called mineralization, making it available to the next crop. In order for mineralization to occur, environmental and soil conditions have to be optimum, meaning not too hot or cold and not too wet or dry. Determining the amount of nitrogen released from mineralization of cover crops can be tricky because it involves many different factors and processes. It is often better to err on the side of caution when trying to predict nitrogen mineralization than assuming a cover crop will release “x” pounds of nitrogen per acre. It truly depends on the cover crop used and on the environmental conditions over time.

Grass species such as annual ryegrass, oats, triticale and others are often used for their nutrient uptake capabilities, as well as their forage production capabilities. Grass cover crops may reduce nutrient loss by taking up nutrients, like nitrate nitrogen (NO3--N), that are prone to leaching into groundwater sources. Brassicas such as radish have become a popular cover crop because of their ability to biologically reduce compaction in severely compacted soils.

Where do cover crops fit in the traditional corn/soybean rotation?
In the traditional corn/soybean rotation, cover crops can be aerial seeded, broadcasted or drilled (after harvest) in September or October at a rate of approximately 60 lbs/ac, but this varies greatly by crop. You can use either a solid seeded planting of one species or a mixture without any problems. If using a legume species, drilling is the preferred seeding method as they need good seed to soil contact for ideal establishment and are more expensive than grasses. Also, remember that if you are using a legume, an inoculant is needed to promote nitrogen fixation. It is also possible to spread cover crops when broadcasting dry fertilizer in the fall. However, keep in mind that broadcasting seed in heavy residue conditions could affect germination and stand establishment of the cover crop.

It is also important to remember that these cover crops can become weeds if not disked, grazed, or chemically terminated before the plant reaches the reproductive phase and seed formation occurs. If this happens, it could be very problematic come spring or summer.

If you already have or are thinking about incorporating cover crops, start by keeping things simple and start off slow. They need water and nutrients, so be mindful if you are in a water limiting environment, or if your soils are nutrient depleted. Using cover crops as another tool in your management system can help you reach the maximum potential that your soils and land have to offer.

- Kevin Meeks, Helena AGRIntelligence Agronomist