Conservation of natural resources for sustainable Agriculture

what you should know about…

Crop residue management,

cover crops and crop rotation

The importance of cover crops

Cover crops vs. green manures

Commonly used cover crops

Resistance to decomposition

Cover crop and residue management

Cover crop mixtures

Crop rotation

On-farm production of cover crop seeds

Other information on cover crops

The importance of cover crops in conservation agriculture Cover crops make up a fundamental component of the stability of the conservation agriculture system. They have direct and indirect effects on soil properties as for its capacity to promote an increased biodiversity in the agro-ecosystem. While commercial crops have a market value, cover crops are mainly grown for their effect on soil fertility or as livestock fodder. In regions where smaller amounts of biomass are produced, like dry areas and eroded soils, cover crops are beneficial as they:

protect the soil during fallow periods mobilize and recycle nutrients improve the soil structure and break compacted layers and hard pans permit a rotation in a monoculture can be used to control weeds and pests

Cover crops are grown during fallow periods, between harvest and planting of commercial crops, utilizing the rest moisture in the soil. Their growth is interrupted either before the next crop is sown, or after sowing the next crop, but before competition between the two crops start. Cover crops energize crop production, but they also present some challenges. TABLE 1 Opportunities and challenges of cover crops

Opportunities

Challenges

Protect the soil Maintain nitrogen in organic form (-NH2) to prevent it from leaching Control weed growth Repel soil borne pests Add organic matter to the soil and favour soil fertility and preparation activities Can solve compaction problems Increase soil porosity and internal drainage and thus reducing the chance of flooding Legumes increase available nitrogen

Require a higher level of management Decomposition of cover crops can lead to a deficit of nitrogen at the beginning of the growing period

Cover crops are suitable for:

protecting the soil, when it is not cultivated providing an additional source of organic matter to improve soil structure and create an improved topsoil recycling nutrients and mobilizing them in the soil profile in order to eliminate layers with slow moving nutrients like phosphorus and potassium "biological ploughing" of the soil; the roots of some crops, especially cruciferous crops, like oil radish are pivotal and able to penetrate compacted or very dense layers (plate 1), increasing water percolation capacity of the soil utilizing easily leached nutrients.

Different plants, with diverse rooting systems, exploring different soil depths, which have the ability to absorb different quantities of nutrients and with the production of distinct root exudates (organic acids) result in benefits both for the soil and for the organisms. The presence of a mulch layer (dead soil cover) in conservation agriculture provokes a reduction in evaporation of soil moisture, yet leading to a higher water infiltration in the soil profile. The percentage of rainwater that infiltrates the soil depends on the amount of soil cover provided (figure 1). Whether through the effects of pores left by plant roots, earthworms, insects or other soil organisms, or through an improved soil porosity and higher number of bigger aggregates, in both cases it is accumulated organic matter that creates the favourable conditions (see

FIGURE 1 Effect of amount of soil cover on rainwater runoff and infiltration. (Ruedell, 1994) ����������������

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PLATE 1 The roots of some cover crops are able to break hardpans or compacted layers in the soil. A. Calegari

module Soil health and fertility). The figure clearly shows that on bare soils (cover=0 t ha-1) runoff and thus soil erosion is higher than when the soil is protected with a cover. Crop residues left on the soil surface lead to higher soil aggregation, and higher porosity and higher number of macropores, and thus to a trend in higher infiltration rates. As different cover crops produce different amount of biomass, the density of the residues varies with different crops and thus the ability to increase water infiltration (figure 2). Cover crops contribute to the protection of the soil surface and thus to the maintenance and/or improvement of the physical, chemical and biological characteristics of the soil, including adaptation of effective soil depth through their roots. Vegetative cover is essential for conservation agriculture for the protection of the soil against the impacts of raindrops and to keep the soil shaded and with highest humidity level possible, for utilizing and thus recycling the nutrients, and for its allelopathic en physical effects on weeds leading to a reduction in agrochemical use and thus in production costs. Only this way do the straw residues function as a blanket that dim the pressure put on the soil surface by tractors and harvesting equipment and thus inducing compaction problems.

FIGURE 2 Effect of different cover crops compared to bare soil in a maize production system on rainwater infiltration (Debarba and Amado, 1997). ��������

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'Cover crops' vs. 'green manure' One of the most important components in conservation agriculture is the use of cover crops, as was discussed earlier. In conservation agriculture the terminology 'green manure' is not often used, although generally the same plant species are used for both practices. Green manure is considered as a crop in rotation with the objective of conserving or restoring the productivity of the land through incorporationincorporationincorporationincorporation of the non-decomposed vegetative matter in the soil. In conservation agriculture this is not desired for two reasons: 1. in conservation agriculture systems the soil is not disturbed, or as little as possible,

and more important, 2. when biomass is incorporated in the soil microbial activity increases in order to

decompose the material, resulting in a sudden release of high quantities of nutrients that can not be captured by the seedlings of the following crop and thus disappear from the system.

The dynamics of residue decomposition depends, amongst others, on the activity of microorganisms but also on soil meso and macro fauna. The macro fauna constitutes mainly of earthworms, beetles, termites, ants, millipedes, spiders, snails and slugs, etc. In conservation agriculture, the incorporation of cover crop and weed residues from the soil surface to deeper layers in the soil is a slow process and depends heavily on the activity of these macroorganisms. The activity of microorganisms is regulated by the activity of these macroorganisms, because they provide them with 'food' and air through their burrows. In this way nutrients are released slowly and can provide the following crop with nutrients. At the same time, the soil is covered for a long time and is protected against the impact of rain and sun. In comparison, when green manure crops are mixed into the soil, a large amount of oxygen and 'food' is worked into the soil that leads to a rapid development of the micro fauna population. This big population can decompose the offered 'food' in a rather quick way but at the moment the 'food' is finished the microorganisms die, because they cannot sustain their way of life. At this moment a large amount of nutrients is released, that if not taken up by plant roots can easily be lost through leaching. The whole time, after

PLATE 2 Soil organisms promote the incorporation of residues into the soil and by doing so, modify the physical properties through the creation of burrows. C. Pruett

incorporation, the soil is not protected by a cover and thus is susceptible to degradation processes, resulting in erosion.

Most commonly used cover crop species There are various crop alternatives to be used as vegetative cover, like grains, legumes and oil crops. All of them are of great benefit to the soil, however some crops emphasize certain benefits, which is useful to keep in mind when planning a rotation scheme. It is important to start the first years of conservation agriculture with (cover) crops that leave a lot of residues on the soil surface, which decompose slowly (because of the high C/N ratio). Grasses and cereals are most appropriate for this stage, also because of their aggressive and abundant rooting system, which need a shorter time to improve the soil. In the following years, when the soil shows a healthier appearance, legumes can be incorporated in the rotation. Leguminous crops enrich the soil with nitrogen and decompose rapidly because of low C/N ratio. Later, when the system is stabilized it is possible to include cover crops with an economic function, like livestock fodder. When a farmer considers using cover crops, it is important to know:

whether it needs to have more benefits which of the available cover crops is the most appropriate when to sow and control the cover crop whether the cover crop needs a lot of water if it is possible to control the cover crop sufficiently, so that it doesn't turn into a weed whether the cover crop provides the same benefits as a rotation with only commercial crops.

In order to be able to successfully integrate cover crops in the current production system, it is crucial to select the plants that are adapted to the different soil and climate conditions and show advantages that fit in the rotation scheme. For this, it is not only necessary to know all the agronomic details of the species, but also all specific conditions of the site where they will be sown (soil and climate) and the anticipated objectives and socio-economic conditions of the farmers. The species that will be used as cover crop need to be tested and validated by the farmers on their land in order to get acquainted with technical details of the plant species. The selection of cover crops should depend on two main criteria, i.e. the presence of high levels of lignin and phenolic acids, which give the residues a higher resistance to decomposition and thus results in soil protection for a longer period. The following aspects need to be considered when using cover crops:

Time of sowing: many species show dormancy or photoperiodism. This means that the production of biomass depends on the period of the year in which the plant is sown. Seeding should be done in the proper season. In order not to jeopardize the following crops, a good planning of the cover crops is necessary. A proper spacing/density of the cover crop is important in order to create a rapid covering of the surface to protect the soil from rain and sun and to suppress the weeds. Soil management: for seeding of the cover crop no land preparation is needed. Cover crops can be sown either using direct seeding or broadcasted over the stubble of the last crop, possibly using a tree trunk, knife-roller, disc harrow used as roller with the discs set at a disc angle close to 0° or chains for putting the seeds into contact with the soil. Some species have the ability to reseed themselves, like hairy vetch.

Seed quality: like in commercial crops, the seeds or planting material of cover crops need to be of high quality and free of pathogens to avoid failure through low quality seeds.

TABLE 2 Agroecological adaptation of most commonly used cover crops

Scientific name English Spanish

Legumes adapted to humid lowlands Centrosema pubescens Centro, butterfly pea Jetirana, bejuco de chivo Phaseolus mungo Black gram Pueraria phaseoloides Tropical kudzu Kudzú tropical

Legumes adapted to fire Centrosema pubescens Centro, butterfly pea Jetirana, bejuco de chivo Desmodium adscendens Glycine wightii Glycine Soja perenne Macroptilium atropurpureum Siratro Siratro

Legumes adapted to cold conditions Clitoria ternatea butterfly pea Campanilla, zapallito de la reina Desmodium intortum Greenleaf desmodium Pega-pega Desmodium incinatum Glycine wightii Glycine Soja perenne Lotononis bainesii Lotononis Lotononis, Miles lotononis Medicago sativa Lucerne Alfalfa Phaseolus lathyroides Phasey bean Frijol de monte, frijol de los arrozales Trifolium spp. Clover Trébol

Legumes adapted to frequently flooded or inundated areas Lotononis bainesii Lotononis Lotononis, Miles lotononis Phaseolus lathyroides Phasey bean Frijol de monte, frijol de los arrozales Pueraria phaseoloides Tropical kudzu Kudzú tropical Vigna luteola Dalrymplar vigna Vigna umbellata Rice bean

Legumes that tolerate drought Cajanus cajan Pigeon pea Guandul Canavalia brasiliensis Canavalia ensiformis Jack bean, sword bean Canavalia Clitoria ternatea butterfly pea Campanilla, zapallito de la reina Desmanthus virgatus Desmodium uncinatum Silverleaf desmosium Dolichos lablab Lablab bean Frijol caballo, gallinita Galactia striata Frijolillo, Galactia Glycine wightii Glycine Soja perenne Indigofera endecaphylla Indigo Leucaena endecaphylla Macrotyloma axillare Archer axillaris Stylosanthes guyanensis Common stylo, tropical lucerne Alfalfa de Brasil Stylosanthes hamata Caribbean stylo, pencil flower Tebeneque Stylosanthes humilis Townsville stylo, wild lucerne Alfalfa salvaje Stylozobium spp. Mucuna, velvet bean Frijol terciopelo Vigna unguiculata Cowpea Caupi

Legumes adapted to shade Arachis pintoi Horse groundnut Mani forajera Calopogonium mucunoides Calapo Rabo de iguana Canavalia ensiformis Jack bean, sword bean Canavalia Indigofera spp. Indigo Leucaena leucocephala Leucaena Leucena, acacia bella rosa, aroma

blanca Pueraria phaseoloides Tropical kudzu Kudzú tropical Trifolium repens White clover Trébol blanco

Legumes adapted to fertile soils Glycine wightii Glycine Soja perenne Medicago sativa Lucerne Alfalfa Stilozobium deeringianum (= Mucuna pruriens)

Mucuna, Velvet bean Mucuna, Frijol terciopelo

Trifolium spp. Clover Trébol Vicia sativa Common vetch Arveja comun

Legumes adapted to medium fertile soils Centrosema pubescens Centro, butterfly pea Jetirana, bejuco de chivo Galactia striata Frijolillo, Galactia Macroptilium atropurpureum Siratro Siratro Lupinus albus White lupin Lupino blanco Lupinus angustifolius Blue lupin Lupino ázul Lathyrus sativus Grass pea, chickling pea guija Crotalaria juncea Sunn-hemp Crotalaria

Legumes and other species tolerant to low soil fertility Cajanus cajan Pigeon pea Guandul Calopogonium mucunoides Calapo Rabo de iguana Canavalia brasiliensis Canavalia ensiformis Jack bean, sword bean Canavalia Centrosema spp. Centro, butterfly pea Jetirana, bejuco de chivo Desmodium spp. Desmodium Pega-pega Galactia striata Frijolillo, Galactia Indigofera spp. Indigo Leucaena leucocephala Leucaena Leucena Lotus corniculatus Birdsfoot trefoil Lupinus luteus Yellow lupin Lupino amarillo Macroptilium atropurpureum Siratro Siratro Stylosanthes spp. Stylo Stylozobium aterrimum Black mucuna Frijol terciopelo negro Teramnus uncinatus Mani de venado Vicia villosa Hairy vetch Arveja pelluda Vigna unguiculata Cowpea Caupi Zornia diphlla Zornia Zornia, barba de burro Lollium multiflorum Italian ryegrass Ornithopus sativus Pink serradella, bird's foot Secale cereale Rye Centeno Spergula arvensis Corn spurry, spurry Linacilla

Resistance to decomposition One important determining factor of the dynamics of residue decomposition is the biochemical composition of the residues. According to species, their chemical components and the time and way of managing them, differences exist in decomposition rate, as is shown in figure 3. The grain species (oats and wheat) show more resistance to decomposition than common vetch (legume). The latter has a lower C/N ratio and less lignin content and thus is subject to a rapid decomposition. Because one of the main objectives of a cover crop is the protection of the soil, their resistance to decomposition should be one of the criteria to select cover crops. Figure 4 gives an overview of the resistance to decomposition of different grain cover crops. The residues of rye and wheat are most resistant to decomposition, whereas Italian ryegrass is decomposed more quickly. Also for tropical and subtropical cover crops the resistance to decomposition has been researched as is shown in figure 5. Pearl millet followed by pigeon pea, white mucuna and C.rotalaria paulina demonstrated higher resistance and left more soil cover for a longer period than the other crops. Therefore these crops are better suitable as a cover crop, especially during the first years of conservation agriculture than other crops.

FIGURE 3 Reduction of dry matter of different cover crops (Ruedell, 1995)

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FIGURE 4 Resistance to decomposition of seven cover crop species at three different times after management (Roman, 1990). ������������

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Generally, the type of crop and the management afterwards determine the quantity of residues produced, the soil cover generated and the time the cover remains on the surface before being decomposed. TABLE 3 Resistance to decomposition

Difficult to decompose (resistant)

Easily decomposed (fragile)

Cotton Maize Oats

Sorghum Wheat

Legumes Oil crops Potato

Sugar beet Sunflower

FIGURE 5 Resistance of tropical cover crops to decomposition, 73 days after management of the cover crops (Pelá et al., 1999).

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PLATE 3 Soya bean planted on the residues of a previous maize crop. S. Bunning

Cover crop and residue management Conservation agriculture systems start each year with the production and distribution of crop residues or an additional cover crop. Vegetative material adequately managed:

adds organic matter, which improves the quality of the seedbed and increases the water infiltration and retention capacity of the soil fixes carbon by capturing carbon dioxide from the atmosphere and retaining it in the soil buffers the pH of the soil and facilitates the availability of nutrients feeds the carbon cycle of the soil captures the rainfall and thus increases the soil moisture content protects the soil from being eroded reduces the evaporation

Residues badly managed:

provoke an unequal drying of the soil and thus a delay in the warming-up of the seedbed or uneven germination of the crop interfere with sowing and fertilizing activities hinder the emergence of seedlings

Residues are managed during:

harvest land preparation sowing

In conservation agriculture, residues should be manipulated from harvest onwards. It depends on the following cover crop whether or not the residues should be distributed equally over the field or left intact so climbing cover crops like mucuna can use the maize stalks to wind itself. An equal distribution of residues provides homogenous temperature and humidity conditions. In case residues are not distributed more or less equally on the soil surface this may cause the following problems:

bad placement of the seeds at sowing, resulting in an uneven germination a cold and humid surroundings of the seeds favours the development of pests and diseases bringing-about allelopathy

Residue management requires a good knowledge of the characteristics of the residues, a more or less even distribution over the field and modification of seeding equipment. According to Monegat (1991) it is important to choose the precise moment at which the vegetative cover is to be controled, because most of the species used can regenerate if their growth is interrupted prematurely. Alternatively, mature seeds of the cover crop can germinate if the plants are allowed to mature, as may happen with oats, rye, chickpea, Vicias and forage radish. There are, however, species and rotations where cover crops are

Erroneously, it is often thought that Conservation Agriculture can only be successfully implemented if herbicides are applied. Fortunately, the creativity and persistence of many farmers and researchers has led to the current situation in which exist a lot of

knowledge and equipment to manage cover crops WITHOUT the use of herbicides.

purposely brought to maturity to establish a seed bank which will allow the cover crop to grow automatically once the cash crop is harvested. The best moment to control the majority of cover crop species is at full flowering when they have accumulated maximum biomass. In case of legumes, the pods from the first flowering should be already formed but not yet mature. Vicias should have some mature pods. Oats and rye can be best managed at milky stage. Horse radish can be slashed at any growth stage, but in systems of direct sowing and minimum tillage, seeds should be green and physiologically immature to avoid the germination of new plants. Both Crotalaria and pigeon pea need to be controlled before flowering due to high regrowth rate and excessive wood development in the stems. The most adequate form to manage the cover crops depends on the objective of the cover crop and the possibilities of the farmer. In case the dead mulch should cover the soil as long as possible, the best way to manage the biomass is by using a knife-roller, chain, sledge or herbicides. When the decomposition process has to start immediately in order to release the nutrients it is recommendable to slash or mow the cover crop. In some cases it might be necessary to complete this with herbicides. Research shows that the period between slashing/management of the cover crop and seeding of the commercial crop (maize, beans, soya, etc.) defines the production level of the crop (figure 6). This is related to some of the substances that are released during decomposition of the cover crops. These can harm the germination of the seeds, or sometimes even delay the development of subsequent crops. This is called allelopathy. In general, management of leguminous cover crops and horse radish ten days before planting of maize gives highest yield responses. Examples show maize yields that were almost 1.5 tons per hectare higher compared to seeding on the same day.

PLATE 4 A farmer using a sledge to manage a cover crop of mucuna. A. Calegari

PLATE 5 Slashing is a common wayto manage the cover cropin Latin America. FAO

Horse radish shows the least fluctuation in yield in relation to cover crop management. The maize yield on cover of grass species, like oats and Italian ryegrass, increases when more days are left between managing the cover crop and seeding the maize. This is obviously related to a reduction of nitrogen immobilization and allelopathic effects over time and different levels of lignin and hemicelluloses. In case of direct seeding over the cover crop it is recommended to seed 8-12 days after managing the cover crop for cover crops with low to medium C/N ratio (12-22) and 12-20 days for cover crops with high C/N ratio (>24).

FIGURE 6 Maize yield related to moment of cover crop management (Ruedell, 1995).

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PLATE 6 Allelopathic effect of oats (residues) on the germination of weeds; in this case Cyperus rotundus. A.J. Bot

Cover crop mixtures Mixed cropping is not a new agricultural practice. For generations, farmers have grown maize and beans in association in tropical areas and in temperate areas legumes are sown into the pastures with the aim to improve the grazing area. Associations of crops have numerous benefits in production systems. Effect on nitrogen availability in proper quantities and adequate moment for subsequent crops and providing soil cover for a longer period are some of the positive impacts of cover crop mixtures. Crop residues can be an effective source of nitrogen for grain crops, like maize, sorghum and others with a high nitrogen demand, provided that the nutrient release from the residues is synchronized with the demand from the crop. Therefore, the ideal crop association is the one that offers enough residues to provide a 'pool' of mineral N from decomposition to attend the commercial crop. The strategy for crop associations is to look for species from different families that have different C/N ratios and lignin contents, and that are able to contemplate with both the supply of nutrients and the provision of soil cover for a long time. Mixtures can be made from cereals/grasses and legumes, cereals/grasses and oil crops or even mixing 2-3 or more species that besides presenting an important impact on the improvement of soil properties produce a C/N ratio that favours a gradual nitrogen mineralization. In general, when residues of grass species are mixed with residues of legumes no immobilization of nitrogen will take place and the gradual mineralization favours the availability of nutrients for absorption by plants. Commonly used mixtures include:

Radish+oats; oats+forrage pea; oats+vetch; oats+clover+vetch; rye+forrage pea+radish; oats+forrage pea+radish for temperate and sub tropical areas Millet+sorghum; millet+Crotalaria juncea; pigeon pea+sorghum; cowpea+millet; sorghum+ Crotalaria juncea for tropical regions

Mixed cropping is another form of crop mixtures. For instance, maize can be mixed with pigeon pea and Crotalaria juncea both planted when the maize plants have reached a height of 30 cm by using the same machine as for planting maize. Another mixture is maize planted with Stilozanthes; in this case both are sown at the same moment. In Latin America very common practices include the mixture of maize with Mucuna or Canavalia in both cases planted about 80-100 days after the maize was planted. After the maize harvest these species accelerate their development, completely overgrowing the maize residues. Subsequent crops can include beans, sorghum or sunflower.

Crop rotation Normally, in agroecosystems the processes of liberalization and absorption of nutrients occur separately in time, resulting in low efficiency of their use. In conservation agriculture systems a new balance is created between all soil properties (chemical, physical and biological) and the whole ecosystem (soil, water and plant) and thus promoting a better balance between mineralization, immobilization, availability and losses, resulting in a higher biological stability of the soil. Crop rotations increase crop yields, add organic matter to the soil and improve soil fertility. Crops differ for the quantity and quality of the residues they produce, and thus for the effect on soil properties. For example, leguminous crops and oil crops produce fewer residues that decompose faster, have a lower C/N ratio and are easier to manage during direct sowing than cereals. Crop rotations can include commercial crops and cover crops. The most ideal rotation in conservation agriculture is one in which cereals and grasses are diversified with legumes, crucifers, Malvaceas or others. This kind of rotation will:

break pest and disease cycles produce different quantities and types of residues facilitate residue management improve nutrient cycles vary sowing dates

While planning a crop rotation in conservation agriculture: alternate a grain crop with a legume or oil crop; alternate a crop that produces a lot of residues with one that produces fewer and determine whether the crop is commercial and cost-effective. Maize, beans, soya, sunflower, groundnut, rice, cotton and wheat are crops that usually show good yield results when grown in rotation. Maize, wheat and rice are recommended to grow in rotation with crops either adapted to cool circumstances like vetch, lupin, field pea and oil radish, or tropical cover crops like Crotalaria juncea, pigeon pea or mucuna. For soya, groundnut and sunflower crop rotation with cereal crops like black and white oats, rye or mixtures of oats+vetch, oats+oil radish, oats+field pea, or short cycle crops like Setaria italica, pearl millet and sorghum are recommended. Sunflower can also be rotated with legumes and other species that improve soil fertility. Most common rotation in tropical Brazil is: Oil radish/maize - black oats/soya - wheat/soya In nematode infested areas the following rotations are suggested:

Black oats or pearl millet/cotton - black oats or pearl millet/soya - pearl millet/soya Black oats or pearl millet/cotton - black oats or pearl millet/soya/oil radish/ maize

In sub-tropical parts the following rotations are recommended:

Lupin/maize - black oats/soya - wheat/soya Vetch/maize - black oats/soya - wheat/millet/soya Black oats/soya - black oats/field pea/maize - oil radish+black oats/soya Black oats/beans - oil radish+black oats/maize - black oats+field pea/soya

In the temperate regions of southern Brazil the following rotations are recommended:

Wheat/soya - vetch/maize - black oats+oil radish/maize Barley/soya - vetch/maize Triticale/soya - vetch/maize

Wheat/soya - black oats grazed+vetch grazed/maize Wheat/soya - vetch/maize or sorghum - oil radish+black oats/beans Wheat/soya - rape (Brassica sp.)/soya or barley/soya - vetch or oil radish/maize Wheat/soya - wheat/soya - white oats/soya - vetch/maize or sorghum

Validations with maize, soya and bean crops in Paraná state of Brazil (Derpsch and Calegari, 1992) show that yields differ, depending on the previous cover crop:

for maize highest yields where obtained on cover crops of white lupin and hairy vetch (6.4 and 6.3 t ha-1 respectively); these were superior to yields obtained after cover crops like wheat, oats, rye, Lathyrus spp. or sunflower. soya yield increased with an average of 770 kg ha-1 to 2.7 t ha-1 after a cover crop of black oats, compared to all other cover crops. Bean yields increased with resp. 85% and 67% after cover crops of black oats and oil radish, compared to fallow.

Farmer validations in the same region prove that cover crops of white lupin and both hairy and common vetch, when sown before a maize crop, supply an equivalent of 90 kg ha-1 of nitrogen (Derpsch and Calegari, 1992; Calegari, 1995). The combination of crop rotation with cover crops and direct seeding gives high maize yields, compared to fallow and conventional tillage systems, as is shown in figure 7. In systems where Lathyrus, serradela, blue lupin or common and hairy vetches are used as a cover crop and direct seeding is applied without the use of chemical fertilizer, the maize yield is even higher than the conventional tillage system in which 90 kg N was applied (see also module Soil health and fertility). Direct seeded systems give a higher maize yield in all cover crops, except for Italian ryegrass, black oats, spurry and fallow.

As sorghum, millet, Crotalaria juncea, short cycle pigeon pea, lablab bean, etc. are extremely fast growing species, they are very useful as cover crop in between two commercial crops, or in situations where water stress is an issue. These crops can make use of the remaining soil moisture to create biomass and will naturally die because of water stress, but the biomass will protect the soil when next rains start and thus increase water infiltration. More water will be available for the next commercial crop and thus more biomass will be created. This can start a conservation agriculture cycle. Other crops than maize, beans and soya can also be grown in rotation with cover crops, for instance vegetables, tobacco, sugarcane and cassava.

FIGURE 7 Maize yield related to previous cover crop and land management (Calegari, 1998) ������������

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�Direct seeding�Conventional tillage

Generally, a rotation of species from different families and with different nutritional needs is recommendable. Nitrogen is usually more consumed by green vegetables; whereas roots, tubers, bulbs and rhizomes need more potassium and legumes extract more phosphorus from the soil. Therefore, in order to reach a balance in the soil it is recommendable to follow the cultivation of cauliflower, broccoli, cabbage, lettuce and spinach with legumes like peas, green beans or dry beans. These will restore and improve the soil so that root and tuber vegetables like carrot, beets, radish, onions, etc. can produce well in a subsequent crop (Calegari and Peñalva, 1999).

PLATE 8 A Costarican farmer grows paprika with a living mulch of horse groundnut (Arachis pintoi) in between the rows. A.J. Bot

PLATE 7 A new tobacco crop on the residues of mucuna cover crop. The aggressive root system of tobacco seedlings often hinders the direct planting of tobacco. V.H. de Freitas

PLATE 9 Small scale vegetable production on cover crop residues. A.J. Bot

As discussed before, cover crops also have a residual effect on subsequent vegetable crops, as is shown in figure 8, where onion yields are compared in relation to different cover crops. As a result, the following rotations are practised by farmers, leaving options according to farmers' interests:

Onion/maize/oats: maize is sown after the onion harvest; during the second weed management in maize crop oats is broadcasted at a density of 80 kg ha-1. The advantage of this system is two commercial harvests per year, but therefore depends on nitrogen applied as chemical fertilizer. Onion/maize/oats+common vetch: as above; recommended density is 50 kg ha-1 of oats and 30 kg ha-1 of common vetch. The advantage of this system is the recycling/provision of part of the nitrogen by the legume. Onion/maize/oil radish: oil radish is broadcasted at a density of 10 kg ha-1 when maize crop is halfway. The rapid decomposition of oil radish is compensated by the slow decomposition of the maize crop that will keep the soil covered for a longer time. Onion/mucuna: mucuna is planted when the onion is harvested at a density of 70 kg ha-1. This system is practically independent of chemical fertilizers.

FIGURE 8 Commercial onion bulb (4-8 cm) yield related to different cover crops (E.E. Ituporanga (1989), cited by Calegari et al., 1993) ��������

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10

15

20

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Black oats Canavalia Cowpea Crotalariamucronata

Crotalariaspectabilis

Mucuna(annual)

Mucuna(gray)

Oil radish Fallow +fertilizer

Onion

yield

( t h

a-1 )

PLATE 10 Onions directly planted with a microtractor on the residues of maize. V.H. de Freitas

Another crop with a good response to cover crops is sugarcane. Especially cover crops of Crotalaria spp. showed a 7-9 per cent yield increase compared to a fallow period preceding the sugarcane crop (Caceras and Alcarde, 1995). Also cassava can be mixed or rotated with different cover crops (black oats, vetches, rye, field pea, Canavalia, pigeon pea, Crotalarias, cowpea or even horse groundnut). In this case, cassava is planted in double rows and in between the double rows the cover crop is sown in either 1 or 2 rows. The only crop that is not recommended to rotate with cassava is oil radish in order to prevent problems of root rot (Fusarium spp., Rhizoctonia spp., etc.) (Florentin et al., 2001). For rotation with grass species to be able to integrate livestock and crop production see module Integration of crop-livestock systems. Independent of tillage systems, rotation of crops is always a good practice. In the absence of tillage is even more important to break pest cycles.

Figure 11 Example of crop rotation

On-farm production of cover crop seeds The on-farm production of cover crop seeds is a basic requirement for conservation agriculture systems to allow the farmer produce maximum soil cover. In general, the purchase of seeds of any kind of cover crop for soil cover is not realistic because of the high cost (Monegat, 1991) or unavailability of the seeds. A farmer should start her/his own seed production by obtaining small quantities of one or more species. The production process should act upon the following criteria:

produce only the quantity that is needed at farm level, with a security margin of 10-20%; each year, define and reserve a specific area on the farm with the only aim of producing seeds; produce more than one species, to spread risks.

Seed production with a commercial purpose usually is not feasible because of several reasons:

seed production demands a lot of labour; occupies the area that can be used to grow commercial crops; lack of processing and commercialization infrastructure; low prices; legal restrictions for the production of seeds.

Many farmers face serious problems in producing their own seeds. One of the reasons is that seed production requires more attention than grain production. In general, the following aspects should be considered:

avoid areas with high incidence of weeds; avoid very fertile soils for legumes; reduce plant density in seed production, compared to plants used as cover crop; if the soil is very fertile, or cover crops were not supported, it is better to cut or graze the crop before flowering; proper planting period; in order to avoid pest and disease incidence, it is important to change the area for seed production every year; periodically purchase new seeds, exchange with neighbours or change species.

For plants with winding stems, such as some legumes, a support would avoid the incidence of diseases and thus will result in higher yields and healthier seeds. Means of support can include:

bent down maize stems or harvested sorghum stalks, cut at 1-1.5 m; associated support plants, like pigeon pea, sesbania, lupin, etc.; dried cassava stems; sparse trees; walls and fences.

Like in grain production always use healthy seeds for planting. If a disease incidence is expected, seeds can be treated before planting; something which is not recommendable (cost efficient) when the crop is used for cover only. The complete cycle (from planting to harvest) can vary for the same species and in the same year, because of:

photoperiodism;

climate: temperature and rainfall; soil fertility: poor soil tend to accelerate the growing cycle of several species, like Lathyrus, vetches, field pea, beans, serradella and lupines, but can slow down the development of for instance black oats.

Before storing, the seeds need to be dried, preferably in the sun or in a shed, to a humidity level of 8-12%. A very practical way to determine this level is to try the seeds between the teeth. Seeds need to be cleaned from debris and dust. Also other seeds or when crops are grown together the different seeds need to be separated. Storage at farm level can be done in tins, bottles, paper, cotton or plastic bags, wooden boxes, etc. When seeds are susceptible of insect attack during storage, Eucalyptus leaves or ashes can be mixed with the seeds.

PLATE 11 Mucuna uses the bent-over maize stalks to wind itself. A.J. Bot

Other information on cover crops A lot of information is available on the Internet. For those you have access to the World Wide Web the following information can be useful. Centre d'Information et d'Echanges sur les Plantes de Couverture en Afrique (CIEPCACIEPCACIEPCACIEPCA) (Centre for Cover Crops Information and Seed Exchange in Africa) CIEPCA – IITA ; 08 B.P. 0932 Cotonou, République du Bénin Tel: (229) 35 01 88; Fax: (229) 35 05 56; E-mail: ciepca@cgiar.org Their electronic biannual and bilingual (English and French) 'CIEPCA newsletter' and 'Mucuna news' (only in English) are available through their website at: http://ppathw3.cals.cornell.edu/mba_project/CIEPCA/home.html CIATCIATCIATCIAT----UgandaUgandaUgandaUganda is in the process of developing Extension Brochures on Mucuna, Canavalia, Tephrosia and Crotalaria, as announced on the website: http://ppathw3.cals.cornell.edu/mba_project/CIEPCA/exmats/exmat.html The Mucuna flyer is finalised and can be downloaded or printed from: http://ppathw3.cals.cornell.edu/mba_project/CIEPCA/exmats/mucuna.pdf ILEIAILEIAILEIAILEIA: The Centre for Research and Information on Low External Input and Sustainable Agriculture can be accessed through their website: http://www.ileia.org/default.asp Their quarterly newsletter is also available on the Internet and some articles on cover crops are mentioned below:

Between love and despair: http://www.ileia.org/2/11-3/11-3-4.htm\ Aggressive colonizers work for the farmers: experiences from South-East Asia: http://www.ileia.org/2/11-3/11-3-10.htm Green manures, a new chance for small farmers: http://www.ileia.org/2/11-3/11-3-16.htm An Odyssey of discovery: principles of agriculture for the humid tropics: http://www.ileia.org/2/11-3/11-3-18.htm Legumes for sustainable food production in semi-arid savannahs: http://www.ileia.org/2/11-4/11-4-18.htm Edible cover crops: http://www.ileia.org/2/12-2/12-2-30.htm Achieving sustainability in the use of green manures: http://www.ileia.org/2/13-3/13-3-12.htm Monocultures towards sustainability: http://www.ileia.org/2/16-4/04-05.PDF Reducing food poverty with sustainable agriculture: new evidence: http://www.ileia.org/2/17-1/21.PDF

CIMMYTCIMMYTCIMMYTCIMMYT together with IDRC and Rockefeller Foundation has produced a book on the use of mucuna in Latin America: "Cover crops in hillside agriculture. Farmer innovation with Mucuna." The whole document is downloadable at: http://www.idrc.ca/books/focus/841 Cover crop database of UC Davis California at: http://www.sarep.ucdavis.edu/ccrop/

References Caceras, N.Caceras, N.Caceras, N.Caceras, N.T. and J.C. AlcardeT. and J.C. AlcardeT. and J.C. AlcardeT. and J.C. Alcarde. 1995. Adubação verde com leguminosas em rotação

com cana-de-açucar (Saccharum spp.). Revista Stab Vol.13, 5: 16-20. Calegari, ACalegari, ACalegari, ACalegari, A. 1995. Leguminosas para adubação verde de verão no Paraná. IAPAR

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Calegari, A., A. Mondardo, E.A. Bulisani, L.P. Wildner, M.B.B. da Costa, P.B. Calegari, A., A. Mondardo, E.A. Bulisani, L.P. Wildner, M.B.B. da Costa, P.B. Calegari, A., A. Mondardo, E.A. Bulisani, L.P. Wildner, M.B.B. da Costa, P.B. Calegari, A., A. Mondardo, E.A. Bulisani, L.P. Wildner, M.B.B. da Costa, P.B. Alcântara, S. Miyasaka and Alcântara, S. Miyasaka and Alcântara, S. Miyasaka and Alcântara, S. Miyasaka and T.J.C. Amado.T.J.C. Amado.T.J.C. Amado.T.J.C. Amado. 1993. Adubação verde no sul do Brasil. AS-PTA, Rio de Janeiro. Second Edition, 346 pp.

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Debarba, L. and T.J.C. Amado.Debarba, L. and T.J.C. Amado.Debarba, L. and T.J.C. Amado.Debarba, L. and T.J.C. Amado. 1997. Desenvolvimento de sistemas de produção de milho no sul do brasil com características de sustentabilidade. Revista Brasileira de Ciência do Solo 21, p.473-480.

DerpDerpDerpDerpsch, R. and A. Calegarisch, R. and A. Calegarisch, R. and A. Calegarisch, R. and A. Calegari. 1992. Plantas para adubação verde de inverno. IAPAR Circular 73. 80pp.

Florentin, M.A., M. Peñalva and A. CalegariFlorentin, M.A., M. Peñalva and A. CalegariFlorentin, M.A., M. Peñalva and A. CalegariFlorentin, M.A., M. Peñalva and A. Calegari. 2001. Abonos verdes y rotación de cultivos en siembra directa. Pequeñas propriedades. Proyecto Conservación de suelos, MAG-GTZ, Paraguay. 84pp.

Monegat, C.Monegat, C.Monegat, C.Monegat, C. 1991. Plantas de cobertura do solo. Características e manejo em pequenas propriedades. Chapecó. 337pp.

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