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Cover Crops

Not too long ago, growing a winter crop to turn under in the spring was an accepted practice. The introduction of inexpensive nitrogen fertilizers, however, encouraged many people to discontinue this practice. Now, farmers are "rediscovering" winter cover crops.

There are many benefits to growing cover crops:

  • reduced erosion
  • improved soil structure and reduced surface crusting
  • increased water-holding capacity of the soil
  • reduced winter weed growth
  • reduced herbicide injury
  • hardpan penetration in the winter, improving soil relations for the crop that follows
  • provide nitrogen if the cover crop is a legume.

Soil improvement through the use of a cover crop is a long-term venture. A cover crop probably will not increase the permanent, stable organic matter content of the soil unless combined with minimum tillage and crop rotation over several years. However, a cover crop will temporarily increase organic material during the early growing season.

Types of Cover Crops

There are two general types of cover crops - leguminous and nonleguminous. Leguminous cover crops, such as vetch and clover, add nitrogen to the soil. Nonleguminous ones, such as wheat and ryegrass, are preferred on erosive soils and early-planted grain crops.

Crimson clover is probably the most commonly used and most desirable of the clovers grown for a cover crop. It matures earlier and produces more nitrogen and dry matter earlier than most other clovers. An excellent crop of crimson clover can produce up to 120 pounds of nitrogen; however, production of 30-50 pounds of nitrogen is common.

Vetches and Austrian winter peas produce tremendous vegetative growth and fix large amounts of nitrogen, but most of this growth occurs too late to permit timely planting of the summer crop after their destruction. The vetches have lost popularity because of this. Vetches grown under ideal conditions may produce up to 150 pounds of nitrogen for the following crop, but 50-70 pounds of nitrogen are realistic. Table 1 illustrates potential nitrogen production and expected nitrogen production of legume cover crops. Peak nitrogen production occurs when the cover crop is in full bloom.

Nonleguminous cover crops (rye, ryegrass, and wheat) have several advantages: establishment of a nonleguminous crop is less expensive than establishing a leguminous cover crop; nonleguminous crops provide longer and better erosion control because of more winter growth and a fibrous root system.

The major disadvantage of nonleguminous cover crops is that they do not fix nitrogen and may even require some nitrogen fertilizer when planted after cotton, corn, or sorghum. These crops are better suited for crops planted early, such as corn and early-maturity soybeans, because early-season destruction does not sacrifice much of their advantages, in contrast to leguminous cover crops.

Table 1. Potential and expected N production from legume cover crops.
Cover crop Potential lb N/Acre
Expected
Red Clover 110 30-50
Crimson Clover 100 30-50
Hairy wetch 150 50-70
Winter Peas 80 30-50
Arrowleaf clover 100 30-50
Source - W. E. Knight and V. H. Watson, Mississippi State University.

Seeding Rates and Methods

Seeding rates vary with the method of seeding. For example, less seed is required when planted with a culti-packer seeder or grain drill into a prepared seedbed than if it is broadcast onto an unprepared seedbed. Table 2 suggests seeding rates for various cover crops. The low seeding rate in the range is for a prepared seedbed, and a high seeding rate is for an unprepared seedbed.

Table 2. Seeding rates for various crops.
Cover Crop lb Seed/Acre
Legumes Crimson clover 12-18
White clover 2-3
Hairy vetch 20-30
Common vetch 40-50
Winter peas 30-40
Grasses Small grains (drilled) 60-90
Small grains (broadcast) 90-120
Small grains (for grazing) 120
Ryegrass 25-35

It is important to plant cover crops early to establish early root growth before cold weather occurs. This helps the crop better survive a hard winter. Plant grasses in early October to mid-November, and plant legumes in mid-September to mid-October.

Planting cover crops into standing cotton or soybean probably requires aerial seeding before leaf drop. Success may be obtained by seeding when the ground is muddy. Seeds stick in the mud and give a better stand distribution than if the seed are flown on when the ground is hard and dry. Seeding on hard, dry soil usually causes seeds to roll into furrows, giving poorer plant distribution and seed placement. The plane should drop the seed from an altitude of 50-60 feet.

When seeding a cover crop after corn or grain sorghum, planting into a prepared seedbed usually gives best results.

Fertilizing Cover Crop

A soil test will accurately determine needs of lime, phosphate, and/or potash for a cover crop. If lime, phosphate, and/or potash are needed, apply in the fall. If a seedbed will be prepared, apply lime and fertilizer before land preparation.

If you are growing a legume cover crop, do not add nitrogen fertilizer. However, treat the seed with the correct nitrogen-fixing bacteria (inoculant).

Some nitrogen fertilizer is needed if the cover crop is nonleguminous, unless it is following soybeans. Following cotton, corn, or grain sorghum, apply 20-30 pounds of nitrogen per acre in the fall to get adequate winter cover.

In mid-February, observe the nonleguminous cover crop to determine if it needs more nitrogen. Nitrogen is not necessary if the grass has a good green color. However, if the grass looks pale green or if more growth is desired, apply 20-30 pounds of nitrogen per acre.

Revised by Dr. Erick Larson, Agronomy Specialist

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Publication 1552
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

Copyright by Mississippi State University. All rights reserved.

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