Cover Crop Species Selection

Many plant species can be grown to serve as cover crops 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. These cool season cover species generally can be classified into cereal grains, cool season legumes and brassicas or other non-legume broadleaves.

The three groups of cover crop species, including cereal grains, legumes and brassicas each possess different characteristics which affect their purpose and the benefits they are capable of 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 of different types are normally needed to achieve 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, their spring stem growth has a high carbon content which is slow to decompose and can produce various complications with establishment and growth of primary crops, including corn, soybean and cotton, planted following a cover crop.

Legumes are unique because they can biologically fix nitrogen through a symbiotic relationship with rhizobia bacteria in root nodules when plants are healthy and properly inoculated. Accordingly, legumes are an important component of most cover crop blends 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 succeeding primary crop. For instance, decomposition of plant organic matter produced by cover crops uses nitrogen to facilitate breakdown. Thus, nitrogen produced by cover crop legumes can aid this process and help avoid a nitrogen deficit in the soil. This immobilization process can reduce nitrogen availability for the following crop because the decomposing plant residue from other cover crop species need more nitrogen than is present. However, legumes produce modest 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. Our research shows legume species adaptation can vary greatly depending on tolerance to wet soils, maturity and susceptibility to winterkill. Legumes are also considerably more vulnerable to winterkill than cereal grains.

Brassica cover crops are often grown because they can produce 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 freezing 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 cereal crops. 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. Early varieties of cereal rye, such as Elbon, may produce more fall growth and initiate stem elongation earlier in the spring compared to other cereal species. Cereal rye can become 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. Wheat growth in the fall growth may be slightly less than other cereals. However, growth of cereal species are generally very comparable 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 controlling weeds using herbicides, 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 for cover crop use. Oats generally develop somewhat wider leaves and more prostate growth habit during the winter compared to other cereals. Thus, oats may slightly 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 plant species developed as a cross between cereal rye and wheat. Therefore, it  develops characteristics falling between those typical of these two common small grains. Triticale is generally taller than most wheat varieties, and thus should attain more biomass production and grow three to four feet tall, if grown to heading in the spring. Triticale winterhardiness is rather high, or better than wheat and only falling short of cereal rye.

Legume Cover Crop Species

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

Crimson clover is a common species used widely in the United States as a cover crop. The variety Dixie is a standard representative of this species. Crimson clover is an early maturing legume cover crop which enhances its compatibility with primary summer crops. However, it is rather intolerant of saturated soils, which hampers its growth and survivability. Therefore, it is best suited on sandy-textured or exceptionally well drained soils in the rainy Midsouth.

Hairy vetch is a viney, early legume capable of considerable growth and nitrogen fixation. The variety Patagonia is a standard and more winterhardy representative of this species. Similar to Crimson clover, Hairy vetch is relatively intolerant of saturated soils and therefore, is best suited for sandy-textured, well drained soils. If grown into the spring, Hairy vetch produces long, sprawling vines which can create significant problems with planting equipment and impede performance. These extensive vines are likely to enwrap and tangle common row cleaning wheels and other planter parts.

Berseem clover is a southern adapted annual clover which has consistently produced in our field studies. The variety Frosty is a standard representative of this species. 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, during droughty conditions or on sandy soils. Berseem is a little slower to initiate rapid spring growth than early legume species, like Crimson and Persian clover.

Persian clover is a southern adapted annual clover that has historically received little consideration as a cover crop, particularly in other regions. The varieties eNhance and Mihi are standard representatives of this species. Unlike many legume species, Persian also 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. The varieties FIXatioN and Viper are standard representatives of this species. Balansa offers better winterhardiness than other legumes in our studies. Balansa initiates spring growth a little later than Persian and has shown some consistency issues when grown in agricultural fields.