A cover crop is a plant species that is grown between cash crops, primarily to provide cropping system services rather than to produce a harvestable product. Benefits provided by cover crops include soil health improvement, soil conservation, nutrient release and capture, and weed suppression. However, like any management practice, cover crops also have challenges and limitations.
Many plant species can be used as winter cover crop in Kentucky. These include small grains, such as cereal rye or winter wheat, annual ryegrass, brassicas (radishes), and legumes (clovers).
Cover crops have many benefits not only for the soil but for the environment also. One of the greatest benefits that we can see is soil-erosion prevention. Winter cover crops provide an actively growing surface barrier holding soil in place and preventing soil erosion during winter and early spring months, when most cash crops are not growing. Grass cover crops such as cereal rye, winter wheat, and annual ryegrass produce large, fibrous root systems which help hold the soil together.
Another benefit of using cover crops is uptake and storage of environmentally harmful nutrients. Excess soil nutrients such as nitrogen and phosphorus can be lost from the soil and contaminate water resources. Most cover crops have a deeper rooting system than winter annual weeds, allowing the cover crops to uptake residual fertilizer and nutrients released from soil organic matter. Cover crops can store those nutrients and possibly recycle them to the following cash crop. Cover crops such as cereal rye can reduce soil nitrate levels in drainage water by approximately 50 percent.
The improved soil water infiltration and retention we can get from using cover crops can help provide a buffer for increasing weather variability that causes both saturated and drought soil conditions. As they grow, cover crops use water, which reduces soil saturation following high rainfall. Cover crops roots explore the soil during periods of the year when the ground is relatively soft, establishing root channels that can be used by cash crop roots during drier summer months.
In addition, research in Kentucky has shown annual ryegrass, has the potential to ameliorate fragipan soils through the release of chemical compounds from the roots. Following termination, cover crop residue helps to increase infiltration by preventing surface sealing and slowing runoff. In addition, cover crop residue can reduce the rate of soil water evaporation, improving water retention under drought conditions.
The increase of soil organic matter after termination of the cover crop is sometimes one of those unnoticed benefits. Cover crop shoots and roots decompose, release nutrients and stimulate microbial activity and soil aggregate formation. During this process, a portion of the cover crop residue becomes incorporated into the soil as organic matter. Soil organic matter benefits crop growth by storing and supplying nutrients to plants, improving the physical condition of the soil, and increasing plant-available water content.
Legume type cover crops such as crimson clover and hairy vetch can biologically affix nitrogen from the atmosphere and supply that nitrogen to the soil, thus reducing the need for synthetic nitrogen fertilizer inputs. One research study in Kentucky found that a hairy vetch cover crop can produce a nitrogen fertilizer equivalency of 67 pounds per acre for corn. The nitrogen contribution from legume cover crops typically increases as termination date is delayed. Depending on temperature and moisture conditions, release of nitrogen from the legume into plant-available forms can take several weeks to months.
Weed Suppression Cover crops can reduce weed biomass both during growth due to competition, and following termination by producing a weed-suppressive mulch. Cover crops with rapid establishment and high biomass potential such as cereal rye and annual ryegrass can reduce winter weed biomass and density by as much as 50 to 100 percent prior to spring cash crop planting. Research in Kentucky has determined that if cover crop biomass is high enough, farmers may be able to eliminate either a pre-plant residual or a post-emergence herbicide application. High biomass cover crops can provide a longer window of weed control and reduce the risk of weed resistance development from a post-emergence application of glyphosate.
As with most things not everything is 100% positive. The use of cover-crops does have challenges. Nitrogen management is one of those that is sometimes misunderstood. Cover crops growing in the fall will capture residual fertilizer nitrogen and plant-available nitrogen that is released from soil organic nitrogen stocks. When the cover crop is terminated, it will decompose and release some nitrogen that will be available to the next cash crop, while other nitrogen will be incorporated into microbes, so not 100% of nitrogen is available for the next crop. It is based on the Carbon/Nitrogen ratio, the actual amount depends on the cover crop biomass, the C/N ratio, the termination date of the cover crop and other factors.
Attributes that make a plant a good cover crop (e.g., root biomass production, aboveground biomass production, rapid establishment and growth, etc.) can also make the cover crop a major pest if not controlled properly due to termination challenges. For example, Kentucky farmers utilizing annual ryegrass as a cover crop need to be aware of the spring termination difficulties associated with the cover crop due to its growth potential. Farmers should avoid using this cover crop in wheat rotations due to its capability of becoming a major pest in wheat. If not terminated properly prior to cash crop planting, cover crops can become weeds in the ensuing cash crop as well as produce seeds and establish a seed bank resulting in future weed problems.
Unfavorable soil moisture conditions at planting can also be an unforeseen problem. In dry spring seasons, growing cover crops can deplete the soil profile of water needed for cash crop germination and growth. Therefore, the cover crop should be terminated earlier in dry springs to minimize water loss from the soil.
In wet springs, cover crop residue on the soil surface can reduce the evaporation rate of water from the soil, thus keeping the soils too wet to plant for extended periods of time. Thus causes delayed or reduced crop emergence. Cover crop residue can delay soil warming. Delayed soil warming can make planting more difficult, delay time of planting, delay crop emergence, and ultimately reduce cash crop establishment and yield.
Thick residue coverage formed by a cover crop can cause planter interference, poor furrow closure, and poor seed to soil contact of spring- planted cash crops. Additional equipment on the planter and/or additional passes with other equipment may be needed to combat cover crop residue during spring planting. Early termination of the cover crop at least two weeks prior to planting to minimize residue interference. The right method likely depends on the weather and cover crop biomass each season. Farmers need to ensure that the planter is cutting through the cover crop residue and into the soil, placing the seed at the proper depth, and providing proper furrow closure.
There is additional cost and need for labor for cover crop seeding and planting. If the spring cash crop planting requires additional equipment or additional passes of strip tillage or roller crimpers, these are additional costs and labor. The benefits to long-term soil improvement are difficult to quantify in the short-term, but these values must be incorporated into the economics.
Kentucky farmers were pioneers in no-tillage farming. Despite all the benefits of no-tillage farming, there are still challenges with the practice. Like no-tillage, there are some expected long-term benefits to cover crops; but there are some challenges as well. For more information contact the Pulaski County Extension Service. Information gathered from "AGR 240" Cover Crop Benefits and Challenges in Kentucky Dan Quinn, Hanna Poffenbarger, and Chad Lee, Plant and Soil Sciences.