Cover Crop Species Selection

Cover crops may consist of many plant species which are grown during the off-season when primary crops are not being grown. In the Midsouth region of the United States where primary crops are usually grown during summer, this means cover crops are cool season species which are planted in the fall and grow through the winter and early spring. Accordingly, cool season cover species generally can be classified into cereal grains, cool season legumes and brassicas or other non-legume broadleaves.

Cereal grains, legumes and brassica plant species possess different characteristics which affect their purpose and the benefits they are capable to producing in an agricultural cover crop system.  Cover crops can improve soil health and organic matter, stabilize soil, reduce erosion, improve nutrient relations, minimize soil compaction and possibly address several other issues. Nevertheless, a blend of cover crop species from different classifications is normally needed to maximize comprehensive results. Thus, it is important to prioritize your goals and realize the benefits and limitations associated with various species, and their relationship with primary crops.

Cereals possess a fibrous root system which can stabilize the soil far better than other options.

Cereal grain species possess fibrous, lateral roots, emerge quickly and usually produce vigorous fall growth. Thus, cereals provide excellent soil stabilization and protection from erosion.  Because of this important purpose, cereals are a primary component of most cover crop systems or blend of species. Cereals produce upright stem growth in the spring and can produce substantial biomass to recycle organic matter back into the soil. However, this spring stem growth has a high carbon content which is slow to decompose and can produce various complications with establishment and growth of primary cash crops planted following a cover crop.

Legumes are unique because they generally have the ability to biologically fix nitrogen through a symbiotic relationship with rhizobia bacteria in root nodules when plants are healthy and properly inoculated. Accordingly, legumes are a primary component of most cover crop systems or blend of species. This supplemental nitrogen benefits plant growth, including companion and subsequent crops, and help improve N relations during the transition from cover crops to the primary crop. For instance, decomposition of plant organic matter produced by cover crops uses nitrogen to facilitate breakdown. Thus, nitrogen produced by legumes may aid this process and help avoid a nitrogen deficit in the soil.  This immobilization process can reduce nitrogen availability for the primary crop because high carbon residue, such as from cereal cover crops, needs more nitrogen to decompose than is present. However, legumes produce less fall growth, compared to cereals, so blends are normally needed to attain sufficient cover to stabilize soil and protect from erosion during the fall and early winter. Legumes are also considerably more vulnerable to winterkill than cereal grains.

Brassica cover crops are usually capable of producing an underground bulb or tuber on their tap root which can relieve soil compaction. They usually produce vigorous fall leaf growth and may produce chemical compounds which inhibit growth of other plant and weed species. Brassicas are generally rather susceptible to winterkill from low temperatures. Accordingly, Brassicas are normally used to serve a complimentary role in cover crop blends of species, or address specialty purposes.

Cereal Cover Crop Species

Cereal Rye is an annual small grain and popular cover crop because it grows very tall and thus can produce more biomass than other cereals. Cereal rye usually grows four to six feet tall, if grown to heading in the spring. Cereal rye foliage is naturally a dull, light green shade, compared to other cereals which are a typical bright green color. Accordingly, this muted tint should not be confused with nutrient deficiency. Cereal rye is the most winterhardy of the cereal species. Fall growth typically has a slight advantage and stem elongation in the spring may initiate quicker compared to other cereals. Cereal rye can be a weed issue if seeds are produced in fields where wheat will be grown for grain in the future.

Wheat is an annual small grain which is readily available, adapted and relatively inexpensive cereal cover crop option. Fall growth of wheat is usually slightly less than other cereals. However, wheat and other cereals have very similar vegetative development in late winter, prior to initiation of stem elongation. Therefore, wheat is a viable option, especially if you intend to terminate cover crop growth prior to stem elongation, which typically occurs in late February to mid March depending upon Mississippi latitude and seasonal weather. Wheat also offers more opportunities for herbicide use, compared to other small grains. Wheat will normally grow two to three feet tall, if grown to heading. Wheat is not quite as winterhardy as cereal rye, but better than oats and normally quite adequate for winter survival in Mississippi.

Oats, including Black oats and common oats, are annual small grains well suited and popular for cover crop use. Oats often have slightly wider leaves and more prostate growth habit during the winter compared to other cereals. Thus, oats may improve canopy coverage of the soil surface compared to other cereals. Oats are normally a little taller than wheat, and grow two and half to four feet tall, if grown to heading. Oats are adapted and planted in the fall in the Midsouth, similar to other winter cereal species. However, oats are the least hardy of the cereal species and thus may be prone to winterkill, especially when topsown.

Triticale is a cross between cereal rye and wheat and thus shows characteristics falling between those typical of these common small grains. Triticale it is moderately tall and typically attains high biomass production and three to four or more feet of height, if grown to heading in the spring. Triticale winterhardiness is high.

Legume Cover Crop Species

Crimson clover can struggle to survive in wet soils, such as in these furrows.

Crimson clover is a common clover species used in the United States for cover crops. Crimson clover is an early maturing legume cover crop which enhances its compatibility with primary summer crops. However, it’s growth and survivability are rather intolerant of saturated soils. Therefore, it is best suited on sandy soils which possess exceptional drainage ability in the rainy Midsouth.

Hairy vetch is a viney legume capable of considerable growth and nitrogen fixation. Similar to Crimson clover, Hairy vetch is  intolerant of saturated soils and therefore, is best suited for sandy, well drained soils. If grown into the spring, Hairy vetch produces long, sprawling vines which can create significant problems with planting equipment, by enwrapping common row cleaning wheels and impeding planting performance.

Berseem clover is a southern adapted annual clover which has consistently produced in our studies. Unlike many legume species, Berseem thrives growing in moist, saturated soils, but has shallow roots and thus, may struggle when planted early, when soils are dry, or on sandy soils. Berseem is a little slower to initiate rapid spring growth than early species, like Crimson and Persian clover.

Persian clover is a southern adapted annual clover that has historically received little consideration as a cover crop, particulary in other regions. Unlike many legume species, Persian clover thrives growing in moist, saturated soils and produces vigorous early spring growth, making it a prime legume cover crop candidate in Mississippi.

Balansa clover is another annual clover that thrives growing in moist, saturated soils and a relative newcomer as a potential legume cover crop. Balansa offers better winterhardiness than other legumes in our studies. Balansa stimulates spring growth a little later than Persian and has shown some consistency issues when grown in agricultural fields.