Intercropping: Farming for the future?
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CTA. 1988. Intercropping: Farming for the future? . Spore 15. CTA, Wageningen, The Netherlands.
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Intercropping: Farming for the future? Until recently, intercropping has received relatively little attention from agricultural scientists, even though it has been practised for centuries by millions of farmers in the tropics and is a system which...
Until recently, intercropping has received relatively little attention from agricultural scientists, even though it has been practised for centuries by millions of farmers in the tropics and is a system which can be particularly beneficial to farming families short of cash or credit Intercropping involves the growing of two or more crops in the same field - in sequence, in combination, or both. This includes the practice of relay cropping - when a second crop is planted between the rows of a first crop which is nearing maturity. Crop and animal production may also be mixed in even more complex, ecologically sound systems designed to maximize production. Studies of agricultural systems throughout the tropics indicate that farmers prefer, where possible, to plant at least two crops in the same field. This helps to minimize the risks of crop failure, which can prove disastrous if the farmer devotes all his resources to growing a single crop In Africa, for example, 98% of the continent's most important legume crop - cowpea - is grown in combination with other crops. In Nigeria,over 80% of the cropland used is given over to polyculture (mixed cropping). A Nigerian farmer may typically grow up to eight crops,including bananas, beans, cassava, melon and yams, in a variety of mixtures. In India, more than 80 crops are utilized in mixed-crop combinations. Over the last 15 years, agricultural scientists have turned their attention to intercropping in order to determine the reasons for its success. Species compositions have been studied together with spatial and temporal arrangements of crops, with a view to finding ways of improving mixed-cropping procedures and tailoring them to specific purposes and conditions. It was soon discovered that, in many instances, crop mixtures produce more than individual crops grown alone. Where the crop species complement each other with regard to such parameters as growth rhythms, rooting depths, nutrient uptake and use of water and light, production is maximized. In Trinidad, for example, intercropping maize and pigeon pea increases the efficiency of nutrient uptake, primarily as a result of differences in nutritional requirements and root depths between the two crops. The polyculture makes use of up to twice the level of nutrients from the soil than does the monoculture Optimizing resources Investigations to establish how well crop mixtures utilize available resources have included an analysis of the land equivalent ratio (LER). This is the amount of land planted in monoculture that would be needed to achieve the same yield produced by a mixture. Researchers in Mexico found that 1.73 ha of land would have to be planted with maize to produce the same amount of food as one hectare planted with a mixture of maize, beans and squash. Measured in terms of total biomass produced, the ratio increased (1.78 ha). In the majority of cases where comparisons have been made, polycultures outperformed monocultures. Efficiency of use of local resources is one of the major advantages of intercropping. Short-term crops planted with others that take a year or more to mature (such as sugar cane or cassava) can use land, water and light while the main crop is still small. Planting both early and late maturing crops together helps spread the harvest throughout the year. This can be particularly valuable where a crop does not store well or storage facilities are inadequate or lacking. In Mauritius maize groundnut and vegetables are grown and harvested in-between sugar cane before the cane grows high enough to shade the row spaces. Similarly, tall-growing crops can be combined with short-growing, shade-tolerant crops. Furthermore, some crops, such as maize, respond to increases in row spacing by raising the yield per plant but do not increase their leaf area. Consequently, crops such as sweet potato can be raised between the maize rows, thereby increasing and diversifying production with no additional innut. Intercropping is not confined to annual crops. Among other examples, Pacific islanders grow cocoa under coconut and in East Africa bananas are interplanted with coffee, the bananas providing shade and material for mulching. Temporal and spatial variants in crop preference have traditionally been exploited by tropical farmers to the full. Multi-storey crop combinations - such as coconut-black pepper-cocoa-pineapple - are now becoming more commonplace in the effort to produce as much as possible from a unit of land, preferably without the need for expensive inputs. Another advantage of intercropping lies in the value of increased crop residues, measured in terms of what nutrients they return to the soil and how they improve filth. In rubber-tea intercrops, plant debris was found to provide soil with the equivalent ot 813 kg/ha of ammonium sulphate (nitrogen fertilizer), 65 kg/ha of calcium superphosphate and 146 kg/ha of potassim sulphate. Livestock fed on crop residues also produce additional fertilizer in the form of dung, providing a further reason for nlanting mixed crops. In Cameroon, mixtures of maize and groundnuts yielded more food than monocultures, and did so irrespective of whether fertilizers were used or not. Comparisons of productivity with and without fertilizers showed increased yields of 21% and 11% respectively for unfertilized and fertilized mixtures than for monocultures receiving the same treatment. LER's ranged from 1.24 in the unfertilized mixtures to 1.49 in the fertilized areas. Intercropping can provide several other benefits to help improve farm performance. Rubber-tea intercrops not only have a good rate of return of nutrients to the soil, they can raise ambient temperature by 20 C - limiting the chances of frost damage to the rubber trees - and reduce soil erosion by up to 70%. The increased rooting of mixed crops, usually extending to deeper levels than would be seen with single crops, helps to hold soils together. Moreover, by choosing a mixture of crop species, farmers can ensure that leaf coverage of their land is expanded to try and use what solar radiation and water are available to the full. The change from monoculture to polyculture marks a notable shift in water balance - from evaporation from the soil to transpiration from the leaves. However, the overall water loss (evapotranspiration) is little changed and so water and irrigation requirements of monocultures and polycultures do not differ significantly and production can be increased without the need for improving water supplies. Fewer pests and diseases Farmers who intercrop are able to introduce a means of controlling crop pests - at virtually no cost. The effect of viruses, fungi and nematodes can all be diminished just by planting more than one crop. A study of plant-feeding insects has found that 60% of the species tested were less abundant in mixtures than in monocultures. What is more, this simple method of biological control works even on a small-scale. Several disease-resisting crop combinations have already been identified. Cassava and bean mixes reduce both incidence and severity of powdery mildew on cassava and angular leaf spot on beans. Cowpea viruses have also been found to be diminished when cowpeas are grown with cassava or plantain. Other beneficial combinations include, maize - sunflower - oats sesame, potato - mustards, and okra - tomato - ginger - mungbean grown under coconut trees. Cowpeas appear to be protected from insect attack by interplanting with sorghum or other cereals, and okra protects cotton from flea beetles. Even simple physical characteristics of individual crops, when combined, can provide protection, disrupting the visual or olfactory processes by which insect pests home in on their food or deterring colonization by weeds. Laboratory experiments showed that merely by interplanting kale with tomato and tobacco, leaf damage from cabbage-eating beetles was reduced by 75%. Throughout the tropics maize is frequently planted with another crop. In the Congo basin it is cucurbits, in the Philippines sweet potato and in Colombia beans. In each case the maize has to compete with fewer weeds. The combined vegetation leaves little room for weeds to either invade or survive, and the efficiency with which the crop mix uses light, water and nutrients leaves precious little for the weeds. Using this knowledge, some crops are now being used as 'barrier' crops to protect another. For example, soybeans planted around pigeon pea prevents the passage of hairy caterpillars which thrive on the peas. Mixed-crop systems meet more of the annual needs of subsistence farming families than monocrops and with less risk. In areas where hunger and malnutrition are endemic the evidence is hard to ignore. Trials of sorghum/pigeon pea mixtures showed that, for a given 'disaster' level, sole pigeon pea crops would fail one year in five, sole sorghum one year in eight, but that intercrops would fail one year in 36 Technique for the future Intercropping offers definite potential for the future. But, as recently as 1985, reviews of the scientific literature devoted to intercropping were bemoaning the lack of published information on the subject. The direction and scope of institutionalized agricultural research has been governed by the desire to maximize production per unit of land and per unit of labour. This has resulted in an historical bias toward advancement and development of agricultural machinery and chemicals in monoculture systems. But now many of the international research centres are following research programmes on multiple cropping systems with the overall aim of improving low- or no-input agriculture. The International Rice Research Institute (IRRI) has developed a strain of rice that matures in 100 days. In Nigeria, the International Institute for Tropical Agriculture (IITA) has produced a variety of cowpea that matures in 65 days, and trials are underway to investigate the possibilities of combining the two. The cowpeas, which are also resistant to several major diseases, have already been found to yield well when grown with maize - and to flower and begin to pod before the maize gets too tall. Some High Yielding Varieties (HYV) bred for use in high-input intensive farming programmes fare well in mixed-crop systems as well, especially the less competitive varieties. But new varieties will need to be tested in typical mixtures and specific environments. Over the centuries some mistakes with crop mixture choices or husbandry have been made. In Trinidad, for example, interplanting of coffee and cocoa led to coffee being pruned in the same way as cocoa, which is incorrect and counter-productive. Mistakes will continue to be identified or made but the drawbacks associated with intercropping have so far - been few and far between. Weeding cannot be mechanized because of the close proximity of crops of different physical characteristics and demands. But this is not a major disadvantage because intercropping tends to prevent weed invasion and growth. Nevertheless, any weeding that has to be done has to be carried out carefully and this is thus time consuming. The problem is compounded because, at present, there are very few herbicides that can be used on mixed crops. Similarly, harvesting has to be carried out carefully so as not to damage the standing crops. The main problem of intercropping identified thus far is that it is labour intensive. The usual model for agricultural research has tended to be one of Technology Transfer, which favours resource-rich farmers who can afford expensive agricultural inputs or capital investments, and which, historically, concentrated on agricultural production in temperate climates. The exploration of tropical-zone farmers' choices of technology, cropping patterns and other factors of their food production has only recently begun in earnest The emerging model is one which relates agriculture to the world's differing ecosystems. This involves a holistic and interdisciplinary appraisal of farming families' needs, resources and problems, where scientists and research stations have a consultancy role. The new model, incorporating the socio-economic and agro-ecological conditions of tropical farmers, and developed with a view to long-term viability, is envisaged to have a much better chance of alleviating rural poverty than any that have gone before.
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