Harry Schomberg, Agricultural Research Service, Watkinsville, GA
Kip Balkcom, Agricultural Research Service, Auburn, AL
Last modified - January 8, 2009
Cover crops are planted to provide ground cover or green manureGreen manure is young and succulent plant material that is turned into the soil to improve its organic matter and nutrient content. between crops harvested for food, fiber, feed or fuel and usually sold for cash. Growers usually kill cover crops 2-4 weeks prior to planting the next crop, or cover crops may be killed by frost damage. In the southern U.S., cover crops are usually grown during the winter months. In the northern U.S., cover crops are also generally utilized during the winter months, but the growth period is not as long due to much colder winter temperatures. However, cover crops are not limited to the winter months. For example, cover crops are grown ahead of many vegetables planted throughout the year. This use highlights the biggest challenge with any cover crop, which is to find the cover crop (or mixtures of cover crops) that fits into a “window” or “niche” within a specific rotation and accomplishes the grower’s intended purpose.
Cover crops have long been promoted for reducing soil erosion from wind and water, increasing nitrogen availability from nitrogen-fixing legume cover crops, and scavenging residual nitrogen by small grains. Additional benefits of cover crops include increasing surface soil organic matter, improving water infiltration, providing species diversity in cropping systems, enhancing nutrient cycling, and controlling early season weeds (by providing a physical barrier) and inhibiting weed seed germination (by producing allelopathicAllelopathic refers to the suppression of growth of one plant species by another due to the release of toxic substances. chemicals during decomposition).Back to top
Cover crops usually increase production costs because of seed and planting costs along with the time associated with cover crop management. For example, growers who have never used cover crops may need to adjust their schedule of operations to address timely cover crop planting. These growers may have had no need for any additional field operations after harvest. However, costs associated with cover crop management could be offset by eliminating costs for certain inputs, such as nitrogen fertilizer, or for energy intensive tillage operations. As a result, overall production costs could decrease by using cover crops, but costs benefits will vary across individual operations. Growers should also remember that, depending on their particular system and the intended benefit derived from the cover crop, a return on their investment may or may not be noticeable in the short term. Many cover crop benefits associated with improved soil quality are difficult to quantify in short-term monetary terms, but can certainly enhance a grower’s bottom line over multiple growing seasons.
Factors affecting the economics of cover crop use include:
Cover crops can be used to enhance surface residues and maximize biomassBiomass is above and below-ground vegetative material. production. Cover crop residues protect soil from water and wind erosion, enhance soil organic matter, biological activity and soil aggregation, and increase soil water availability for subsequent crops. When used in conjunction with conservation tillage, cover crops contribute to a synergisticSynergistic is when separate elements produce a greater effect when acting together than would be produced if they acted separately. system that enhances soil quality and crop production.
The Soil Quality Test Kit provides several ways to assess some of the beneficial effects of cover crops on soil quality. Soil carbon and infiltration rate are two that can be easily determined to illustrate soil improvements.
Typically, the most popular property monitored to determine how cover crops affect the soil is soil carbon. The beneficial effects of increasing soil carbon by maintaining cover crop residues on the soil surface usually occur near the soil surface (i.e. top 2 inches). These effects are much more noticeable on highly weathered degraded soils compared to more fertile less weathered soils. Growers may notice their soils appear slightly darker on the surface; earthworms may be more prevalent or found where none were before; soils may hold water for longer periods of time; erosion from wind and water has decreased; or soil structure has improved.
Soil samples for soil carbon are usually collected and partitioned by depth increments (0-to-1 and 1-to-3 inches) to determine soil carbon levels prior to using cover crops. After cover crops are used for a period of time (usually after 2 to 3 years, because changes take time), soil samples for soil carbon are collected and partitioned again to observe changes in soil carbon. If possible, an area is excluded from cover crop use to illustrate the effects of cover crops. Ideally, growers should be able to see changes without the use of soil tests.
On less sandy soils effects of cover crops may be determined by measuring infiltration rates. Infiltration rates generally increase with increasing soil carbon at the soil surface due to improved soil physical structure. A comparison between areas with and without cover crops can dramatically demonstrate the effects of the added soil carbon. The Single Ring (Flooded/Ponded) Infiltrometer Method is described in the Soil Quality Test Kit Guide, Section I, Chapter 3, pp. 7 - 8. See Section II, Chapter 2, pp. 55 – 56 for interpretation of results.
Cover crop seed costs vary considerably from year to year and from region to region, but historically, legume cover crops cost about twice as much to establish as small grain covers. The increased cost of the legume seed can be offset by the value of nitrogen legumes provide. Properly managed legume cover crops can be expected to supply at least 50 lb N/acre. On the other hand, a grass cover crop, such as cereal rye, terminated at a late stage of growth may increase the summer crop nitrogen requirements up to 20-30 lb/acre. Residues from a grass crop can limit nitrogen availability to the cash crop through a process called immobilization. This process is the result of microbial nitrogen demand during the decomposition of the grass residue.Back to top
The following table summarizes region specific data across the United States for typical cover crops used, specific benefits of cover crops, and potential concerns for cover crops in the region. The information is assembled from Balkcom, K., H. Schomberg, W. Reeves, A. Clark, L. Baumhardt, H. Collins, J. Delgado, S. Duiker, T. Kaspar, and J. Mitchell. 2007. Managing cover crops in conservation tillage systems. p. 44-61. In A. Clark (ed.) Managing cover crops profitably. 3rd ed. Handbook Series Book 9. Sustainable Agriculture Network.
|Pacific Northwest||Austrian winter pea, brassicas, small grains (wheat, triticale)||Improve water retention, soil structure, and reduce erosion||Cover crop water use and seed establishment in heavy residues|
|Pacific West||Small grain mixtures with and without field pea, vetches alone||Reduce intercrop tillage, suppress winter weeds and pathogens||Cooler soil temperatures with covers and cover crop re-growth|
|Northern Plains||Rye, Austrin winter pea, sweet clover, and sorghum-sudangrass||Improve water retention, reduce wind erosion, and increase soil carbon||Cover crop water use and slow soil warming following cover crops|
|Southern Plains||Wheat, rye, oats||Improves water storage and reduces wind erosion||Water competition with cash crop and value as a forage|
|Midwest||Small grains (rye), red clover, hairy vetch, and sweet clovers||Reduce erosion, suppress weeds, and scavenge nitrates||Short cover crop growing season and establishment time constraints|
|Mid-South||Small grains - rye, wheat, oats, triticale||Reduce erosion, improve soil structure, reduce tillage||Cash crop establishment in heavy soils with high residue amounts|
|Northeast||Diverse - rye wheat, oats, ryegrass, hairy vetch, crimson clover, and Austrian winter pea; some brassicas||Reduce erosion, improve soil, moisture conservation, forage and nutrient management||Cost, time investment for establishment, and water use|
|Southeast||Rye, wheat, oats, crimson clover, and hairy vetch||Increases organic matter, decreases erosion, and improves water storage||Water management at cash crop planting, cost, planter interference from residue|
To enhance many of the benefits associated with cover crops, growers should plant as early in the growing season or “window” as possible and terminate growth as late as possible. These procedures will maximize biomass production and/or nitrogen accumulation. Sowing high quality seed with a high germination rate promotes good stand establishment.
Cover crop selection
Growers who plan to use a cover crop in their operation should take their personal experience into account, as well as the intended use of the cover crop, before deciding on a cover crop species. There are several ideal characteristics that a grower should look for when choosing a cover crop. Reeves (1994) summarized these characteristics as follows. An ideal cover crop should:
Planting cover crops
Planting methods include drilling (no-till and conventional) and broadcasting seed. In conservation systems, planting with a no-till drill is preferred. Following the harvest of the preceding crop, no land preparation is required in no-till systems, facilitating more timely planting. Use of a conventional drill may require some light tillage, prior to planting, which adds another trip across the field. In some cases, however, this trip may be justified to smooth out an extremely rough field. Regardless of the type of drill used, depth of seeding should be monitored and controlled to ensure proper seed placement, and the drill should be calibrated to plant the desired plant population.Broadcasting of seed can be accomplished with a variety of equipment that spreads the seed over a wide area. In some cases, broadcasting is faster than drilling. Aerial seeding (broadcast seeding by airplane), although not as widely adopted, has been successful. Southeastern growers have used aerial seeding prior to cotton defoliation and peanut digging. Leaves from cotton and soil disturbance from peanut digging help to cover the seed which shortens establishment time for the cover crop. Cover crop seeds have been broadcast as a mixture with fall-applied phosphorus and potassium fertilizers, but this can result in chemical deterioration of seed quality. The amount of seed required per land area should be increased 20-30% when broadcasting to ensure an adequate stand. Broadcast seeded cover crops require adequate seed-to-soil contact for germination and establishment which can be accomplished by rolling the soil with a cultipacker or similar piece of equipment. The need for this additional field operation can negate the advantage of broadcast application.
Fertilizing cover crops
Soil fertility (i.e. phosphorus and potassium) and pH levels should be adequate and within recommended ranges to optimize growth. Nitrogen is generally the most important fertility consideration for a cereal cover crop and is a good option to promote biomass production. It may also be needed to promote early growth of legumes to enhance biomass and nitrogen accumulation for use by a subsequent crop.
Cover crop biomass production is often directly proportional to the benefits associated with the cover crop. In general, a high residue small grain cover crop should contain a minimum of 4000 lb biomass per acre. Application of nitrogen fertilizer may be needed to ensure biomass production reaches this level. Soils with inherent fertility (i.e. soils with > 2.5% organic matter) usually produce cover crops with adequate biomass levels with no additional nitrogen fertilizer. Soils with low fertility may require at least 30 lb N/acre to achieve high residue biomass levels. Unfortunately, due to current nitrogen fertilizer prices, growers are reluctant to apply it to a cereal cover crop. On the other hand, high nitrogen fertilizer prices have increased grower interest in legumes. Many cool-season species grow vigorously in the south. Crimson clover and hairy vetch are the predominate species grown.
Where fields are sown with legumes the first time, they should be inoculated with the species-specific rhizobium inoculant to ensure good nodulation and to promote nitrogen fixation. Even in fields where legumes have been grown before, the small expense of inoculating seed will provide “insurance” that good nodulation will take place.
Cover crop mixtures can enhance the benefits associated with each species by producing a synergistic effect. For example, a legume/grass mixture can provide the benefits of nitrogen fixation from the legume and greater biomass production associated with the grass species. The combined residues may result in nitrogen release that more closely matches the needs of the following crop. Another example may include combining two legume species with different maturities to extend the flowering period thus providing an extended period for enhanced beneficial insect habitat. Cover crop mixtures increase seed costs and require greater management.
Nitrogen availability following a cover crop can be variable and depends on many factors. These factors include:
Termination of cover crops - Timing
Timing of cover crop termination is important because it can influence soil temperature, soil moisture, tillage and planting operations, weed suppression, and N release from legume decomposition. Due to the complex interactions, the decision on timing of termination must be site and situation specific. Assuming adequate biomass is present, a general rule for timing winter cover crop termination is 3-4 weeks prior to the anticipated summer cash crop planting date. There are both positive and negative effects of earlier or later termination.
An early termination date (4-8 weeks prior to anticipated cash crop planting):
A late termination date (<4 weeks prior to anticipated cash crop planting):
In the broadest sense, the previous effects assume that the cover crop chosen for a region is suited to the climatic conditions of the region and the cover crop growing season is sufficient to produce adequate biomass levels. Potential biomass production for any given cover crop in the Southeast is obviously much greater than the Midwest due to warmer winter temperatures found across the Southeast. For this reason, any distinction among possible termination dates across the Midwest is probably irrelevant. The latest possible termination date is required to promote maximum biomass production and thus maximize cover crop benefits. In many instances, the termination date may be the onset of winter in the Midwest. The choice of termination date can also be limited in the Southeast. For example, early planted corn (Feb – Mar) significantly shortens the cover crop growing season and eliminates any real choices for cover crop management with respect to termination.Growers should terminate cover crops early enough to allow residues to dry and become “brittle” which allows planting equipment to cut through the residue much more easily. Recently terminated fresh “wet” cover crop residue is tough and harder to cut and can result in considerable dragging of residue by implements. Allelopathic compounds can be a greater problem when fresh residues become trapped in the seed furrow, a condition known as “hairpinning”. These allelopathic chemicals can inhibit cash crop seed germination, especially small seeded crops. Allowing a three week period between terminating cover crops and planting enables the chemicals to leach out and dissipate before the cash crop is established. Hairpinning can also be a problem with older residues if planting operations are carried out in the morning when residues are still moist from precipitation or dew. In this case, hairpinning can reduce seed to soil contact thus reducing stands.
Termination of cover crops - Method
Terminating cover crops with a non-selective herbicide is commonly used by growers because these herbicides can be applied at any time or growth stage. Most spray equipment allows coverage of a large number of acres in a short period of time.
Lodging of cover crops after chemical termination can negatively affect subsequent tillage or planter operations. One option is to use a mechanical roller (sometimes called a roller-crimper) to lay residues uniformly on the soil surface, parallel to the direction of planting. The residues thus form a dense mat on the soil surface, facilitating planter operation and aiding in early season weed control. Rollers usually consist of a round drum with equally spaced blunt blades mounted perpendicular across the face of the drum. Blunt blades are preferable to sharp blades, because they crimp rather than cut the cover crop. Several types of rollers have been developed, and many growers have modified existing designs to fit their particular situation. Rollers can be front- or rear-mounted on tractors. Front mounted rollers enable another implement to be mounted behind the tractor, which saves a trip across the field.
The roller can be used to kill the cover crop by breaking the stems, thus causing crop drying, but timing of the roller operation is critical if a grower is relying on the roller alone to terminate the cover crop. Rolling should be delayed until the cover crop is flowering or later, or it may not be successful. Growers who do not use herbicides (e.g. organic growers) may want to pursue this alternative.
Early roller designs had limitations primarily related to the speed and width of operation. Evenly spaced straight blades around the roller drum create vibration that is transferred to the tractor (and tractor operator), which dictates slower speeds. Curved or spiral blades on the roller drum enable the roller to stay in constant contact with the ground, thus allowing faster speeds and reduced vibration. Ideally, roller width should match planter width to avoid situations where residues wrap around coulters and disks on the planter. Planting should be in the same direction as crimping. Due to design challenges with weight and transportation between fields, roller/crimpers are usually eight rows or smaller. Inventive growers have designed wider rollers that can be folded for transportation.Rotary mowers can be used to terminate cover crops but can cause problems due to the uneven distribution and uneven size of residue pieces. A flail mower provides a better alternative to a rotary mower because the residues are evenly distributed and are of a more uniform length. Mowing terminates the crop quickly but there is a possibility for cover crop re-growth depending on the species and time of termination. It is also an alternative to the use of herbicides, but is energy intensive. Additionally, in humid climates, mowed residues break down rapidly, negating some of the benefits of keeping the soil surface covered. In drier climates, cover crop residues do not decompose as quickly, but residue is much more likely to be moved off-site by wind and water. Chopping residue into small pieces may adversely affect the performance of tillage and planting equipment. Coulters designed to cut through residue may instead push small pieces of residue into the soil without actually cutting the residue, causing the coulters to drag the residue through the soil.
Cover crops can influence subsequent cash crops either directly, such as a legume providing nitrogen as it decomposes or indirectly by increasing organic matter content, which affects water availability and nutrient cycling. In many cases, cover crops can provide multiple benefits to cash crops simultaneously. As previously stated, cover crop benefits are enhanced as biomass production increases.
Cover crop benefits for subsequent crops include:
Managing Cover Crops Profitably, 3rd Edition. 2007. Sustainable Agriculture Network, Handbook Series Book 9. Available as: http://www.sare.org/publications/covercrops/covercrops.pdf
Reeves, D.W. 1994. Cover Crops and Rotations. In: J.L. Hatfield and B.A. Stewart (Eds) Advances in Soil Science - Crops Residue Management, CRC Press, Boca Raton, FL, 125–172.