Cover crop
Encyclopedia
Cover crops are crop
s planted primarily to manage soil fertility, soil quality, water, weed
s, pests, diseases, biodiversity
and wildlife in agroecosystems
(Lu et al. 2000), ecological systems managed and largely shaped by humans across a range of intensities to produce food, feed, or fiber.
Cover crops are of interest in sustainable agriculture
as many of them improve the sustainability of agroecosystem attributes and may also indirectly improve qualities of neighboring natural ecosystems. Farmers choose to grow and manage specific cover crop types based on their own needs and goals, influenced by the biological, environmental, social, cultural, and economic factors of the food system within which farmers operate (Snapp et al. 2005)
Often, green manure crops are grown for a specific period, and then plowed under
before reaching full maturity in order to improve soil fertility and quality.
Green manure crops are commonly leguminous, meaning they are part of the Fabaceae
(pea) family. This family is unique in that all of the species in it set pods, such as bean, lentil, lupin
s and alfalfa. Leguminous cover crops are typically high in nitrogen and can often provide the required quantity of nitrogen for crop production. In conventional farming, this nitrogen is typically applied in chemical fertilizer form. This quality of cover crops is called fertilizer replacement value (Thiessen-Martens et al. 2005).
Another quality unique to leguminous cover crops is that they form symbiotic relationships with the rhizobial
bacteria that reside in legume root nodules. Lupins is nodulated by the soil microorganism Bradyrhizobium
sp. (Lupinus). Bradyrhizobia are encountered as microsymbionts in other leguminous crops (Argyrolobium, Lotus, Ornithopus, Acacia, Lupinus) of Mediterranean origin. These bacteria convert biologically unavailable atmospheric nitrogen gas (N2) to biologically available mineral nitrogen (NH4+) through the process of biological nitrogen fixation
.
Prior to the advent of the Haber-Bosch process, an energy-intensive method developed to carry out industrial nitrogen fixation and create chemical nitrogen fertilizer, most nitrogen introduced to ecosystems arose through biological nitrogen fixation (Galloway et al. 1995). Some scientists believe that widespread biological nitrogen fixation, achieved mainly through the use of cover crops, is the only alternative to industrial nitrogen fixation in the effort to maintain or increase future food production levels (Bohlool et al. 1992, Peoples and Craswell 1992, Giller and Cadisch 1995). Industrial nitrogen fixation has been criticized as an unsustainable source of nitrogen for food production due to its reliance on fossil fuel energy and the environmental impacts associated with chemical nitrogen fertilizer use in agriculture (Jensen and Hauggaard-Nielsen 2003). Such widespread environmental impacts include nitrogen fertilizer losses into waterways, which can lead to eutrophication
(nutrient loading) and ensuing hypoxia (oxygen depletion) of large bodies of water.
An example of this lies in the Mississippi Valley Basin, where years of fertilizer nitrogen loading into the watershed from agricultural production have resulted in a hypoxic "dead zone" off the Gulf of Mexico the size of New Jersey (Rabalais et al. 2002). The ecological complexity of marine life in this zone has been diminishing as a consequence (CENR 2000).
As well as bringing nitrogen into agroecosystems through biological nitrogen fixation, types of cover crops known as “catch crop
s” are used to retain and recycle soil nitrogen already present. The catch crops take up surplus nitrogen remaining from fertilization of the previous crop, preventing it from being lost through leaching
(Morgan et al. 1942), or gaseous denitrification
or volatilization (Thorup-Kristensen et al. 2003).
Catch crops are typically fast-growing annual cereal species adapted to scavenge available nitrogen efficiently from the soil (Ditsch and Alley 1991). The nitrogen tied up in catch crop biomass is released back into the soil once the catch crop is incorporated as a green manure or otherwise begins to decompose.
An example of green manure use comes from Nigeria, where the cover crop Mucuna pruriens
(velvet bean) has been found to increase the availability of phosphorus in soil after a farmer applies rock phosphate (Vanlauwe et al. 2000).
levels through the input of cover crop biomass over time. Increased soil organic matter
enhances soil structure
, as well as the water and nutrient holding and buffering capacity of soil (Patrick et al. 1957). It can also lead to increased soil carbon sequestration, which has been promoted as a strategy to help offset the rise in atmospheric carbon dioxide levels (Kuo et al. 1997, Sainju et al. 2002, Lal 2003).
Although cover crops can perform multiple functions in an agroecosystem simultaneously, they are often grown for the sole purpose of preventing soil erosion. Soil erosion is a process that can irreparably reduce the productive capacity of an agroecosystem. Dense cover crop stands physically slow down the velocity of rainfall before it contacts the soil surface, preventing soil splashing and erosive surface runoff
(Romkens et al. 1990). Additionally, vast cover crop root networks help anchor the soil in place and increase soil porosity, creating suitable habitat networks for soil macrofauna (Tomlin et al. 1995).
Soil quality is managed to produce optimum circumstances for crops to flourish. The principal factors of soil quality are soil salination
, pH
, microorganism
balance and the prevention of soil contamination
.
Just before cover crops are killed (by such practices including mowing, tilling, discing, rolling, or herbicide application) they contain a large amount of moisture. When the cover crop is incorporated into the soil, or left on the soil surface, it often increases soil moisture. In agroecosystems where water for crop production is in short supply, cover crops can be used as a mulch to conserve water by shading and cooling the soil surface. This reduces evaporation of soil moisture. In other situations farmers try to dry the soil out as quickly as possible going into the planting season. Here prolonged soil moisture conservation can be problematic.
While cover crops can help to conserve water, in temperate regions (particularly in years with below average precipitation) they can draw down soil water supply in the spring, particularly if climatic growing conditions are good. In these cases, just before crop planting, farmers often face a tradeoff between the benefits of increased cover crop growth and the drawbacks of reduced soil moisture for cash crop production that season.
s during the cover crop growth period, and can prevent most germinated weed seeds from completing their life cycle and reproducing. If the cover crop is left on the soil surface rather than incorporated into the soil as a green manure after its growth is terminated, it can form a nearly impenetrable mat. This drastically reduces light transmittance to weed seeds, which in many cases reduces weed seed germination rates (Teasdale 1993). Furthermore, even when weed seeds germinate, they often run out of stored energy for growth before building the necessary structural capacity to break through the cover crop mulch
layer. This is often termed the cover crop smother effect (Kobayashi et al. 2003).
Some cover crops suppress weeds both during growth and after death (Blackshaw et al. 2001). During growth these cover crops compete vigorously with weeds for available space, light, and nutrients, and after death they smother the next flush of weeds by forming a mulch layer on the soil surface. For example, Blackshaw et al. (2001) found that when using Melilotus officinalis
(yellow sweetclover) as a cover crop in an improved fallow system (where a fallow period is intentionally improved by any number of different management practices, including the planting of cover crops), weed biomass only constituted between 1-12% of total standing biomass at the end of the cover crop growing season. Furthermore, after cover crop termination, the yellow sweetclover residues suppressed weeds to levels 75-97% lower than in fallow (no yellow sweetclover) systems .
In addition to competition-based or physical weed suppression, certain cover crops are known to suppress weeds through allelopathy
(Creamer et al. 1996, Singh et al. 2003). This occurs when certain biochemical cover crop compounds are degraded that happen to be toxic to, or inhibit seed germination of, other plant species. Some well known examples of allelopathic cover crops are Secale cereale (rye
), Vicia villosa (hairy vetch
), Trifolium pratense (red clover
), Sorghum bicolor (sorghum
-sudangrass), and species in the brassicaceae
family, particularly mustard
s (Haramoto and Gallandt 2004). In one study, rye cover crop residues were found to have provided between 80% and 95% control of early season broadleaf weeds when used as a mulch during the production of different cash crops such as soybean
, tobacco
, corn
, and sunflower
(Nagabhushana et al. 2001).
In a recent study released by the Agricultural Research Service
(ARS) scientists examined how rye seeding rates and planting patterns affected cover crop production. http://www.ars.usda.gov/is/pr/2010/100125.htm The results show that planting more pounds per acre of rye increased the cover crop's production as well as decreased the amount of weeds. The same was true when scientists tested seeding rates on legumes and oats; a higher density of seeds planted per acre decreased the amount of weeds and increased the yield of legume and oat production. The planting patterns, which consisted of either traditional rows or grid patterns, did not seem to make a significant impact on the cover crop's production or on the weed production in either cover crop. The ARS scientists concluded that increased seeding rates could be an effective method of weed control.
family, such as mustards, have been widely shown to suppress fungal disease populations through the release of naturally occurring toxic chemicals during the degradation of glucosinolade compounds in their plant cell tissues (Lazzeri and Manici 2001).
in organic strawberry production (Zalom et al. 2001).
Other cover crops are used to attract natural predators of pests by providing elements of their habitat. This is a form of biological control known as habitat augmentation, but achieved with the use of cover crops (Bugg and Waddington 1994). Findings on the relationship between cover crop presence and predator/pest population dynamics have been mixed, pointing toward the need for detailed information on specific cover crop types and management practices to best complement a given integrated pest management
strategy. For example, the predator mite Euseius tularensis (Congdon) is known to help control the pest citrus thrips in Central California citrus orchards. Researchers found that the planting of several different leguminous cover crops (such as bell bean, woollypod vetch, New Zealand white clover, and Austrian winter pea) provided sufficient pollen as a feeding source to cause a seasonal increase in Congdon populations, which with good timing could potentially introduce enough predatory pressure to reduce pest populations of citrus thrips (Grafton-Cardwell et al. 1999).
In one study, researchers compared arthropod and songbird species composition and field use between conventionally and cover cropped cotton fields in the Southern United States. The cover cropped cotton fields were planted to clover, which was left to grow in between cotton rows throughout the early cotton growing season (stripcover cropping). During the migration and breeding season, they found that songbird densities were 7–20 times higher in the cotton fields with integrated clover cover crop than in the conventional cotton fields. Arthropod abundance and biomass was also higher in the clover cover cropped fields throughout much of the songbird breeding season, which was attributed to an increased supply of flower nectar from the clover. The clover cover crop enhanced songbird habitat by providing cover and nesting sites, and an increased food source from higher arthropod populations (Cederbaum et al. 2004).
Crop
Crop may refer to:* Crop, a plant grown and harvested for agricultural use* Crop , part of the alimentary tract of some animals* Crop , a modified whip used in horseback riding or disciplining humans...
s planted primarily to manage soil fertility, soil quality, water, weed
Weed
A weed in a general sense is a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. More specifically, the term is often used to...
s, pests, diseases, biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
and wildlife in agroecosystems
Agroecology
Agroecology is the application of ecological principles to the production of food, fuel, fiber, and pharmaceuticals. The term encompasses a broad range of approaches, and is considered "a science, a movement, [and] a practice."...
(Lu et al. 2000), ecological systems managed and largely shaped by humans across a range of intensities to produce food, feed, or fiber.
Cover crops are of interest in sustainable agriculture
Sustainable agriculture
Sustainable agriculture is the practice of farming using principles of ecology, the study of relationships between organisms and their environment...
as many of them improve the sustainability of agroecosystem attributes and may also indirectly improve qualities of neighboring natural ecosystems. Farmers choose to grow and manage specific cover crop types based on their own needs and goals, influenced by the biological, environmental, social, cultural, and economic factors of the food system within which farmers operate (Snapp et al. 2005)
Soil fertility management
One of the primary uses of cover crops is to increase soil fertility. These types of cover crops are referred to as "green manure." They are used to manage a range of soil macronutrients and micronutrients. Of the various nutrients, the impact that cover crops have on nitrogen management has received the most attention from researchers and farmers, because nitrogen is often the most limiting nutrient in crop production.Often, green manure crops are grown for a specific period, and then plowed under
Tillage
Tillage is the agricultural preparation of the soil by mechanical agitation of various types, such as digging, stirring, and overturning. Examples of human-powered tilling methods using hand tools include shovelling, picking, mattock work, hoeing, and raking...
before reaching full maturity in order to improve soil fertility and quality.
Green manure crops are commonly leguminous, meaning they are part of the Fabaceae
Fabaceae
The Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, is a large and economically important family of flowering plants. The group is the third largest land plant family, behind only the Orchidaceae and Asteraceae, with 730 genera and over 19,400 species...
(pea) family. This family is unique in that all of the species in it set pods, such as bean, lentil, lupin
Lupin
Lupinus, commonly known as Lupins or lupines , is a genus in the legume family . The genus comprises about 280 species , with major centers of diversity in South and western North America , and the Andes and secondary centers in the Mediterranean region and Africa Lupinus, commonly known as Lupins...
s and alfalfa. Leguminous cover crops are typically high in nitrogen and can often provide the required quantity of nitrogen for crop production. In conventional farming, this nitrogen is typically applied in chemical fertilizer form. This quality of cover crops is called fertilizer replacement value (Thiessen-Martens et al. 2005).
Another quality unique to leguminous cover crops is that they form symbiotic relationships with the rhizobial
Rhizobacteria
Rhizobacteria are root-colonizing bacteria that form symbiotic relationships with many plants. The name comes from the Greek rhiza, meaning root. Though parasitic varieties of rhizobacteria exist, the term usually refers to bacteria that form a relationship beneficial for both parties...
bacteria that reside in legume root nodules. Lupins is nodulated by the soil microorganism Bradyrhizobium
Bradyrhizobium
Bradyrhizobium is a genus of Gram-negative soil bacteria, many of which fix nitrogen. Nitrogen fixation is an important part of the nitrogen cycle. Plants cannot use atmospheric nitrogen they must use nitrogen compounds such as nitrates....
sp. (Lupinus). Bradyrhizobia are encountered as microsymbionts in other leguminous crops (Argyrolobium, Lotus, Ornithopus, Acacia, Lupinus) of Mediterranean origin. These bacteria convert biologically unavailable atmospheric nitrogen gas (N2) to biologically available mineral nitrogen (NH4+) through the process of biological nitrogen fixation
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
.
Prior to the advent of the Haber-Bosch process, an energy-intensive method developed to carry out industrial nitrogen fixation and create chemical nitrogen fertilizer, most nitrogen introduced to ecosystems arose through biological nitrogen fixation (Galloway et al. 1995). Some scientists believe that widespread biological nitrogen fixation, achieved mainly through the use of cover crops, is the only alternative to industrial nitrogen fixation in the effort to maintain or increase future food production levels (Bohlool et al. 1992, Peoples and Craswell 1992, Giller and Cadisch 1995). Industrial nitrogen fixation has been criticized as an unsustainable source of nitrogen for food production due to its reliance on fossil fuel energy and the environmental impacts associated with chemical nitrogen fertilizer use in agriculture (Jensen and Hauggaard-Nielsen 2003). Such widespread environmental impacts include nitrogen fertilizer losses into waterways, which can lead to eutrophication
Eutrophication
Eutrophication or more precisely hypertrophication, is the movement of a body of water′s trophic status in the direction of increasing plant biomass, by the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system...
(nutrient loading) and ensuing hypoxia (oxygen depletion) of large bodies of water.
An example of this lies in the Mississippi Valley Basin, where years of fertilizer nitrogen loading into the watershed from agricultural production have resulted in a hypoxic "dead zone" off the Gulf of Mexico the size of New Jersey (Rabalais et al. 2002). The ecological complexity of marine life in this zone has been diminishing as a consequence (CENR 2000).
As well as bringing nitrogen into agroecosystems through biological nitrogen fixation, types of cover crops known as “catch crop
Catch crop
In agriculture, a catch crop is a fast-growing crop that is grown simultaneously with, or between successive plantings of a main crop.For example, radishes that mature from seed in 25–30 days can be grown between rows of most vegetables, and harvested long before the main crop matures...
s” are used to retain and recycle soil nitrogen already present. The catch crops take up surplus nitrogen remaining from fertilization of the previous crop, preventing it from being lost through leaching
Leaching (agriculture)
In agriculture, leaching refers to the loss of water-soluble plant nutrients from the soil, due to rain and irrigation. Soil structure, crop planting, type and application rates of fertilizers, and other factors are taken into account to avoid excessive nutrient loss.Leaching may also refer to ...
(Morgan et al. 1942), or gaseous denitrification
Denitrification
Denitrification is a microbially facilitated process of nitrate reduction that may ultimately produce molecular nitrogen through a series of intermediate gaseous nitrogen oxide products....
or volatilization (Thorup-Kristensen et al. 2003).
Catch crops are typically fast-growing annual cereal species adapted to scavenge available nitrogen efficiently from the soil (Ditsch and Alley 1991). The nitrogen tied up in catch crop biomass is released back into the soil once the catch crop is incorporated as a green manure or otherwise begins to decompose.
An example of green manure use comes from Nigeria, where the cover crop Mucuna pruriens
Mucuna pruriens
Mucuna pruriens is a tropical legume known as velvet bean or cowitch and by other common names , found in Africa, India and the Caribbean. The plant is infamous for its extreme itchiness produced on contact, particularly with the young foliage and the seed pods...
(velvet bean) has been found to increase the availability of phosphorus in soil after a farmer applies rock phosphate (Vanlauwe et al. 2000).
Soil quality management
Cover crops can also improve soil quality by increasing soil organic matterOrganic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
levels through the input of cover crop biomass over time. Increased soil organic matter
Organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
enhances soil structure
Soil structure
Soil structure is determined by how individual soil granules clump or bind together and aggregate, and therefore, the arrangement of soil pores between them...
, as well as the water and nutrient holding and buffering capacity of soil (Patrick et al. 1957). It can also lead to increased soil carbon sequestration, which has been promoted as a strategy to help offset the rise in atmospheric carbon dioxide levels (Kuo et al. 1997, Sainju et al. 2002, Lal 2003).
Although cover crops can perform multiple functions in an agroecosystem simultaneously, they are often grown for the sole purpose of preventing soil erosion. Soil erosion is a process that can irreparably reduce the productive capacity of an agroecosystem. Dense cover crop stands physically slow down the velocity of rainfall before it contacts the soil surface, preventing soil splashing and erosive surface runoff
Surface runoff
Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...
(Romkens et al. 1990). Additionally, vast cover crop root networks help anchor the soil in place and increase soil porosity, creating suitable habitat networks for soil macrofauna (Tomlin et al. 1995).
Soil quality is managed to produce optimum circumstances for crops to flourish. The principal factors of soil quality are soil salination
Soil salination
Soil salinity is the salt content in the soil.- Causes of soil salinity :Salt-affected soils are caused by excess accumulation of salts, typically most pronounced at the soil surface. Salts can be transported to the soil surface by capillary transport from a salt laden water table and then...
, pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
, microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
balance and the prevention of soil contamination
Soil contamination
Soil contamination or soil pollution is caused by the presence of xenobiotic chemicals or other alteration in the natural soil environment....
.
Water management
By reducing soil erosion, cover crops often also reduce both the rate and quantity of water that drains off the field, which would normally pose environmental risks to waterways and ecosystems downstream (Dabney et al. 2001). Cover crop biomass acts as a physical barrier between rainfall and the soil surface, allowing raindrops to steadily trickle down through the soil profile. Also, as stated above, cover crop root growth results in the formation of soil pores, which in addition to enhancing soil macrofauna habitat provides pathways for water to filter through the soil profile rather than draining off the field as surface flow. With increased water infiltration, the potential for soil water storage and the recharging of aquifers can be improved (Joyce et al. 2002).Just before cover crops are killed (by such practices including mowing, tilling, discing, rolling, or herbicide application) they contain a large amount of moisture. When the cover crop is incorporated into the soil, or left on the soil surface, it often increases soil moisture. In agroecosystems where water for crop production is in short supply, cover crops can be used as a mulch to conserve water by shading and cooling the soil surface. This reduces evaporation of soil moisture. In other situations farmers try to dry the soil out as quickly as possible going into the planting season. Here prolonged soil moisture conservation can be problematic.
While cover crops can help to conserve water, in temperate regions (particularly in years with below average precipitation) they can draw down soil water supply in the spring, particularly if climatic growing conditions are good. In these cases, just before crop planting, farmers often face a tradeoff between the benefits of increased cover crop growth and the drawbacks of reduced soil moisture for cash crop production that season.
Weed management
Thick cover crop stands often compete well with weedWeed
A weed in a general sense is a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. More specifically, the term is often used to...
s during the cover crop growth period, and can prevent most germinated weed seeds from completing their life cycle and reproducing. If the cover crop is left on the soil surface rather than incorporated into the soil as a green manure after its growth is terminated, it can form a nearly impenetrable mat. This drastically reduces light transmittance to weed seeds, which in many cases reduces weed seed germination rates (Teasdale 1993). Furthermore, even when weed seeds germinate, they often run out of stored energy for growth before building the necessary structural capacity to break through the cover crop mulch
Mulch
In agriculture and gardening, is a protective cover placed over the soil to retain moisture, reduce erosion, provide nutrients, and suppress weed growth and seed germination. Mulching in gardens and landscaping mimics the leaf cover that is found on forest floors....
layer. This is often termed the cover crop smother effect (Kobayashi et al. 2003).
Some cover crops suppress weeds both during growth and after death (Blackshaw et al. 2001). During growth these cover crops compete vigorously with weeds for available space, light, and nutrients, and after death they smother the next flush of weeds by forming a mulch layer on the soil surface. For example, Blackshaw et al. (2001) found that when using Melilotus officinalis
Melilotus officinalis
Melilotus officinalis, known as the yellow sweet clover, yellow melilot, ribbed melilot or common melilot is a species of legume native to Eurasia and introduced in North America, Africa and Australia.-Description:...
(yellow sweetclover) as a cover crop in an improved fallow system (where a fallow period is intentionally improved by any number of different management practices, including the planting of cover crops), weed biomass only constituted between 1-12% of total standing biomass at the end of the cover crop growing season. Furthermore, after cover crop termination, the yellow sweetclover residues suppressed weeds to levels 75-97% lower than in fallow (no yellow sweetclover) systems .
In addition to competition-based or physical weed suppression, certain cover crops are known to suppress weeds through allelopathy
Allelopathy
Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial or detrimental effects on the target organisms...
(Creamer et al. 1996, Singh et al. 2003). This occurs when certain biochemical cover crop compounds are degraded that happen to be toxic to, or inhibit seed germination of, other plant species. Some well known examples of allelopathic cover crops are Secale cereale (rye
Rye
Rye is a grass grown extensively as a grain and as a forage crop. It is a member of the wheat tribe and is closely related to barley and wheat. Rye grain is used for flour, rye bread, rye beer, some whiskeys, some vodkas, and animal fodder...
), Vicia villosa (hairy vetch
Hairy vetch
Vicia villosa, known as the hairy vetch, fodder vetch or winter vetch, is a plant native to Europe and western Asia. It is a legume, grown as a forage crop....
), Trifolium pratense (red clover
Red clover
Trifolium pratense is a species of clover, native to Europe, Western Asia and northwest Africa, but planted and naturalised in many other regions....
), Sorghum bicolor (sorghum
Sorghum
Sorghum is a genus of numerous species of grasses, one of which is raised for grain and many of which are used as fodder plants either cultivated or as part of pasture. The plants are cultivated in warmer climates worldwide. Species are native to tropical and subtropical regions of all continents...
-sudangrass), and species in the brassicaceae
Brassicaceae
Brassicaceae, a medium sized and economically important family of flowering plants , are informally known as the mustards, mustard flowers, the crucifers or the cabbage family....
family, particularly mustard
Mustard plant
Mustards are several plant species in the genera Brassica and Sinapis whose small mustard seeds are used as a spice and, by grinding and mixing them with water, vinegar or other liquids, are turned into the condiment known as mustard or prepared mustard...
s (Haramoto and Gallandt 2004). In one study, rye cover crop residues were found to have provided between 80% and 95% control of early season broadleaf weeds when used as a mulch during the production of different cash crops such as soybean
Soybean
The soybean or soya bean is a species of legume native to East Asia, widely grown for its edible bean which has numerous uses...
, tobacco
Tobacco
Tobacco is an agricultural product processed from the leaves of plants in the genus Nicotiana. It can be consumed, used as a pesticide and, in the form of nicotine tartrate, used in some medicines...
, corn
Maize
Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...
, and sunflower
Sunflower
Sunflower is an annual plant native to the Americas. It possesses a large inflorescence . The sunflower got its name from its huge, fiery blooms, whose shape and image is often used to depict the sun. The sunflower has a rough, hairy stem, broad, coarsely toothed, rough leaves and circular heads...
(Nagabhushana et al. 2001).
In a recent study released by the Agricultural Research Service
Agricultural Research Service
The Agricultural Research Service is the principal in-house research agency of the United States Department of Agriculture . ARS is one of four agencies in USDA's Research, Education and Economics mission area...
(ARS) scientists examined how rye seeding rates and planting patterns affected cover crop production. http://www.ars.usda.gov/is/pr/2010/100125.htm The results show that planting more pounds per acre of rye increased the cover crop's production as well as decreased the amount of weeds. The same was true when scientists tested seeding rates on legumes and oats; a higher density of seeds planted per acre decreased the amount of weeds and increased the yield of legume and oat production. The planting patterns, which consisted of either traditional rows or grid patterns, did not seem to make a significant impact on the cover crop's production or on the weed production in either cover crop. The ARS scientists concluded that increased seeding rates could be an effective method of weed control.
Disease management
In the same way that allelopathic properties of cover crops can suppress weeds, they can also break disease cycles and reduce populations of bacterial and fungal diseases (Everts 2002), and parasitic nematodes (Potter et al. 1998, Vargas-Ayala et al. 2000). Species in the brassicaceaeBrassicaceae
Brassicaceae, a medium sized and economically important family of flowering plants , are informally known as the mustards, mustard flowers, the crucifers or the cabbage family....
family, such as mustards, have been widely shown to suppress fungal disease populations through the release of naturally occurring toxic chemicals during the degradation of glucosinolade compounds in their plant cell tissues (Lazzeri and Manici 2001).
Pest management
Some cover crops are used as so-called "trap crops", to attract pests away from the crop of value and toward what the pest sees as a more favorable habitat (Shelton and Badenes-Perez 2006). Trap crop areas can be established within crops, within farms, or within landscapes. In many cases the trap crop is grown during the same season as the food crop being produced. The limited area occupied by these trap crops can be treated with a pesticide once pests are drawn to the trap in large enough numbers to reduce the pest populations. In some organic systems, farmers drive over the trap crop with a large vacuum-based implement to physically pull the pests off the plants and out of the field (Kuepper and Thomas 2002). This system has been recommended for use to help control the lygus bugsLygus
The genus Lygus includes over 40 species of plant-feeding insects in the family Miridae. At one time, nearly 200 species were classified as genus Lygus, but most of those have since been reclassified into new or existing genera. The term lygus bug is used for any member of genus Lygus. The more...
in organic strawberry production (Zalom et al. 2001).
Other cover crops are used to attract natural predators of pests by providing elements of their habitat. This is a form of biological control known as habitat augmentation, but achieved with the use of cover crops (Bugg and Waddington 1994). Findings on the relationship between cover crop presence and predator/pest population dynamics have been mixed, pointing toward the need for detailed information on specific cover crop types and management practices to best complement a given integrated pest management
Integrated Pest Management
Integrated pest management is an ecological approach to agricultural pest control that integrates pesticides/herbicides into a management system incorporating a range of practices for economic control of a pest...
strategy. For example, the predator mite Euseius tularensis (Congdon) is known to help control the pest citrus thrips in Central California citrus orchards. Researchers found that the planting of several different leguminous cover crops (such as bell bean, woollypod vetch, New Zealand white clover, and Austrian winter pea) provided sufficient pollen as a feeding source to cause a seasonal increase in Congdon populations, which with good timing could potentially introduce enough predatory pressure to reduce pest populations of citrus thrips (Grafton-Cardwell et al. 1999).
Diversity and wildlife
Although cover crops are normally used to serve one of the above discussed purposes, they often simultaneously improve farm habitat for wildlife. The use of cover crops adds at least one more dimension of plant diversity to a cash crop rotation. Since the cover crop is typically not a crop of value, its management is usually less intensive, providing a window of “soft” human influence on the farm. This relatively “hands-off” management, combined with the increased on-farm heterogeneity created by the establishment of cover crops, increases the likelihood that a more complex trophic structure will develop to support a higher level of wildlife diversity (Freemark and Kirk 2001).In one study, researchers compared arthropod and songbird species composition and field use between conventionally and cover cropped cotton fields in the Southern United States. The cover cropped cotton fields were planted to clover, which was left to grow in between cotton rows throughout the early cotton growing season (stripcover cropping). During the migration and breeding season, they found that songbird densities were 7–20 times higher in the cotton fields with integrated clover cover crop than in the conventional cotton fields. Arthropod abundance and biomass was also higher in the clover cover cropped fields throughout much of the songbird breeding season, which was attributed to an increased supply of flower nectar from the clover. The clover cover crop enhanced songbird habitat by providing cover and nesting sites, and an increased food source from higher arthropod populations (Cederbaum et al. 2004).
See also
- AgroecologyAgroecologyAgroecology is the application of ecological principles to the production of food, fuel, fiber, and pharmaceuticals. The term encompasses a broad range of approaches, and is considered "a science, a movement, [and] a practice."...
- AllelopathyAllelopathyAllelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial or detrimental effects on the target organisms...
- Biological control
- Green manureGreen manureIn agriculture, a green manure is a type of cover crop grown primarily to add nutrients and organic matter to the soil. Typically, a green manure crop is grown for a specific period of time , and then plowed under and incorporated into the soil while green or shortly after flowering...
- Nitrogen cycleNitrogen cycleThe nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can be carried out by both biological and non-biological processes. Important processes in the nitrogen cycle include fixation, mineralization, nitrification, and denitrification...
- Nitrogen fixationNitrogen fixationNitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
- Organic matterOrganic matterOrganic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
- Soil contaminationSoil contaminationSoil contamination or soil pollution is caused by the presence of xenobiotic chemicals or other alteration in the natural soil environment....
Further reading
- Midwest Cover Crops Council. http://www.mccc.msu.edu Resources for growers, researchers, and educators.
- Hartwig, N. L., and H. U. Ammon. 2002. 50th Anniversary - Invited article - Cover crops and living mulches. Weed Science 50:688-699.
- Sullivan, P. 2003. Overview of cover crops and green manures. ATTRA, Fayetteville, AR. http://attra.ncat.org/attra-pub/PDF/covercrop.pdf
- University of California Sustainable Agriculture Research and Education Program. UCSAREP cover crop resource page. http://www.sarep.ucdavis.edu/ccrop/index.htm
External links
- http://www.tumbledownfarm.com/drupal/Cyclopedia_of_American_Agriculture/Crops/Cover_Crops "Cover Crops," Cyclopedia of American Agriculture, v. 2, ed. by L. H. Bailey (1911). A short encyclopedia article, early primary source on varieties and uses of cover crops.