George Washington, the first US President and successful farmer divided his crops into “crops grown to eat and sell” and “crops grown to replenish the soil”. Cover crops are important members of his second category. In most instances, they are not used for food, feed or industrial purposes. Instead, cover crops remain in the field where they grew. They bridge the gap from the harvest of a grain crop to planting of the next crop.

In the most literal definition, cover crops are crops that are grown to cover and protect the soil surface from the destructive actions of rain and wind. Plant leaves and other parts intercept falling raindrops and dissipate their energy. Because the essential first step of water-caused soil erosion is dislodging of soil particles by raindrops, cover crops greatly reduce soil erosion by nearly eliminating this first step. Cover crops also reduce wind-caused soil erosion, primarily through a reduction of wind speed near the soil surface. This reduces the capability of wind to dislodge and transport soil.

If reducing soil erosion was the only benefit from cover crops, they would have a major impact on agriculture productivity. But they have additional positive effects on crop and soil health. Like all plants, cover crops plants produce roots. As roots grow through the soil they provide channels for water percolation. Carbon-containing compounds from roots serve as food sources for beneficial microbes. As roots die and decompose, they add to soil organic matter.

Cover crop plant material is usually left in the field, so carbon fixed from carbon dioxide in the air and stored in plant tissues adds to soil organic matter. Their roots absorb mineral nutrients, often from deep in the soil profile, that are used in stems, leaves and reproductive structures. As plants decompose these mineral nutrients are released unto the soil surface. In this way, cover crops act as reservoirs for mineral nutrients protecting them from the action of leaching. Some cover crops are members of the legume family and capable of fixing nitrogen from the air. Cover crops suppress growth of some weeds – especially those that are winter annuals. Because cover crops reduce erosion and sequester mineral nutrients they improve water quality. As they add organic matter to soil they improve aspects of soil health including soil structure, biotic activity, and improved water holding capacity.

The abiltiy of a cover crop plant to survive winter temperatures has important implications on the amount of spring biomass that may be produced, spring soil coverage, and if spring termination is required before a grain crop is planted. The United States Department of Agriculture (USDA) has developed a Plant Hardiness Zone Map. Zones are based on normal extreme minimum temperatures. Originally developed to help gardeners and landscapers select adapted plants, the hardiness zones are useful for predicting whether or not a cover crop will survie winter in a particular geographical area. This information is advisory and general becasue winter weather varies among years and varieties within a cover crop species may differ for winter hardiness. Although not always true, plants adapted to more northern zones will likely resume growth earler in spring than plants adapted to more southern zones. Early spring growth may be related to the amout of biomass production before spring termination.

Categories of cover crops with examples:

Plants that are members of Brassica family

Most members of the Brassica family (Brassicaceae) produce large leaves that cover the soil surface and deep taproots. Winter survival of family members ranges from excellent to minimal. Spring soil coverage depends on winter survival, but fall coverage is excellent. Plant parts contain glucosinolate that degrades into sulfur containing compounds. These thiocyanates may have pathogen control properties including nematodes.

Rapeseed/Canola is Brassica napus. The word "rape" comes from a Latin word meaning turnip. Flowers are bright yellow with four petals arranged as a cross. Small seeds (160,000 seeds per pound) are produced in pod-like fruits called siliques. Rapeseed will usually overwinter in Missouri and into USDA zone 4. It produces excellent ground cover with large leaves in both fall and spring.

Canola is a special type of rapeseed developed to reduce the amount of erucic acid in the oil and glucosinolate in the protein meal. Canola could be used as a cover crop, but the grain is too valuable not to harvest. Seed the MU Biofuels Garden for more information about canola

Radish (forage or oilseed radish) is Raphanus sativus. Taproots may reach 6 feet deep and the upper 10 inches may expand to 2 inches in diameter. Although radish usually does not overwinter in Missouri, leaves are large and produce extensive soil coverage in fall and early winter. Radish may not be hardy north of USDA zone 9

Two other mambers of the Brassica family sometimes used as cover crops are turnip (Brassica rapa) and mustard (Sinapsis spp, Brassica spp). Neither mustrad nor turnip will reliably overwinter in Missouri.

Plants that are members of the Legume family

Several members of the Legume family (Fabaceae) that are usually considered forage crops are used as cover crops. Becasue cover crops are almost always terminated before planting a grain crop, legume cover crops are usually annuals. But, perennial legume species can be used. Legume plants form an association with bacteria that is capable of changing atmospheric nitrogen gas from plant unavailable to available forms through nitrogen fixation. So, legume cover crops are often used in front of grain crops that require substantial amounts of nitrogen fertilizer such as corn or grain sorghum. The amount of nitrogen fixed depends on soil temperature and plant leaf area. Photosynthesis provides the energy to drive nitrogn fixation. Fall growth and winter survival differ amoung the legume species. Their abilty to protect soil from erosion and the amount of nitrogen fixation is highly related to the amount of vegetation present in fall and/or spring.

Hairy vetch is Vicia villosa. It is an annual plant, but will overwinter in Missouri and through USDA zone 4. Fall growth is slow, so winter cover is sparse. In spring, nearly prostrate growth produces abundant vegetation. Leaves possess tendrils and plants will cling to and grow over supporting material such as corn stalks. Purple flowers (sometimes white) are abundant, borne in clusters ( raceme), and have a shape typical of most legume plants. Most seed sources contain a high percentage of hard seed, so late germination in succeeding crops is possible.

Austrian winter pea is Pisum sativum subspecies Arvense is a type of field pea. It is an annual plant, but will usually overwinter in Missouri. Winter hardiness is much less likely north of Hardiness Zone 6. Fall growth is slow, so winter cover is sparse. Leaves produce tendrils, so plants will cling to supporting material such as corn stalks.

Several clover species are used as cover crops. Berseem clover (Trifolium alexandrinum) is a fast growing annual that is the least winter hardy of all the annual clovers (plants, flowers, leaf, stipule). Crimson clover (Trifolium incarnatum) is commonly used in roadside mixes in the South. It is becoming increasing popular as a cover crop in the Midwest (plant, leaf, stipule, flowers). Neither berseem clover nor crimson clover will reliably overwinter in Missouri. Crimson clover may survive some winters in southern Missouri.

Other broad-leafed plants

An example of a broad-leafed plant used as a cover crop but in a family other than Brassica or Legume is buckwheat. Buckwheat (leaves, flowers) is a short-season annual species that grows rapidly. If planted in the fall, it will not survive winter, but buckwheat can be planted in spring. In either season, buckwheat grows fast and produces quick cover.

Plants that are members of the grass family

Many plants from the grass family can be used for cover crops, but the most common are annual forage crops and small grain crops. See the MU Small Grain Crops Garden for more information about small grain crops used for grain.

Annual (Italian) ryegrass (Lolium multiflorum) is an annual grass sometimes used in forage and turfgrass settings. Young leaves are rolled into a cylinder as they emerge on the plant. Leaf blades have a prominent mid-vein and are shiny underneath. Auricles are long and clasping and the ligule is short and membranous. The blade tip is sharp-pointed. Plants produce neither stolons nor rhizomes. The inflorescence is 5 to 15 inches long. Spikelts are arranged in an alternating pattern along the central stalk. Florets produce small awns.

Annual ryegrass establishes easily with abundant fall growth and an extensive root system. It produces abundant seed. This can make termination and control in the spring difficult. Volunteer seedlings in the suceeding grain crop are likely unless the cover crop is terminated before inflorescences are visible.

This species differs from species known as annual ryegrass in Australia (Lolium ridgidum), which is a noxious weed in that country. Biotypes of annual (Italian) ryegrass resistant to glyphosate have been identified in Arkansas and other states.

Small grain crops such as rye, oat, wheat, and barley are popular cover crops. In fact, rye is the most commonly planted cover crop in the US, and acrage as a cover crop may exceed the number of acres harvested as a grain crop. Winter types of all four of these small grain crops exist, but they differ widely in their winter hardiness. Rye is by far the most hardy and will survive winters into USDA zone 3. Oat is the least hardy and often does not survive winter in Missouri.

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