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AGRICULTURE AND BIOLOGY JOURNAL OF NORTH AMERICA ISSN Print: 2151-7517, ISSN Online: 2151-7525, doi:10.5251/abjna.2010.1.5.1098.1105 © 2010, ScienceHuβ, http://www.scihub.org/ABJNA Sustainable horticultural crop production through intercropping: The case of fruits and vegetable crops: A review *George Ouma and Jeruto,P Department of Botany and Horticulture, Maseno University P. O. BOX 333 – 40105, Maseno *Corresponding author :[email protected] ABSTRACT Sustainable agriculture seeks to at least use nature as the model for designing agricultural systems. Since nature integrates her plants and animals into diverse landscape, a major tenet of sustainable agriculture is efficiency and lack of waste products in nature. When domestication of crops replaced hunting and gathering of food, landscape changed accordingly. By producing a limited selection of crop plants and animals, human kind has substantially reduced the level of biological diversity over much of the earth. There is more cooperation in nature than competition. Cooperation is exemplified by mutually beneficial relationships that occur between species within communities. If left undistributed and unplanted an abandoned crop field will first be colonized by just a few species of organisms but after several years a complex community made up of many wild species develops. Stability is reached by a community when it has reached a high level of diversity. Diverse communities have fewer fluctuations in numbers of a given species and are stable. The practices which promote diversity and stability on the farm are enterprise diversification, crop rotation, use of wind breaks, provision of more habitats for microorganisms, intercropping and integration of crop farming with livestock production. Intercropping is the cultivation of two or more crops at the same time in the same field. Its advantages are risk minimization, increased income and food security, reduction of soil erosion and pest and disease control. This paper discusses the practice of intercropping in horticultural crop production to promote sustainability. Many crop systems are discussed. Keywords: Sustainable, horticultural, diversity, stability and crop. INTRODUCTION: Intercropping is the cultivation of two or more crops at the same time in the same field. Sustainable agriculture seeks to at least use nature as the model for designing agricultural systems. Due to the fact that nature integrates her plants and animals into diverse landscape, a major tenet of sustainable agriculture is efficiency and there are no waste products in nature. When hunting and gathering were replaced by domestication of crops landscape also changed. The level of biological diversity over much of the earth has been substantially reduced when human kind produces a limited selection of crop plants and animals. There is more cooperation in nature than competition. Cooperation is exemplified by mutually beneficial relationships that occur between species within communities. If left undisturbed and unplanted on abandoned crop field will first be colonized by just a few species of organisms but after several years a complex community made up of many wild species develops. Stability is reached by a community when it has reached a high level of diversity. Diverse communities have fewer fluctuations in numbers of a given species and are stable. The practices which promote diversity and stability on the farm are enterprise diversification, crop rotation, use of wind breaks, provision of more habitats for microorganisms, intercropping and integration of crop farming with livestock production (Reddy and Willey,1981;Reddy et al 1992). When two or more crops are growing together, each should have adequate space to maximize cooperation and reduce competition between them. This is accomplished by the following factors namely: spatial arrangement, plant density, maturity dates of the crops grown, plant architecture. The spatial arrangements are: • • Row intercropping – Growing two or more crops together at the same time with at least one crop planted in rows. Strip intercropping – Growing two or more crops together in strips wide Agric. Biol. J. N. Am., 2010, 1(5): 1098-1105 enough to separate crop production using machines, but close enough to interact. • Mixed cropping – Growing two or more crops together in no distinct row arrangement. • Relay intercropping – Plant a second crop into a standing crop at a time when the standing crop is at its reproductive stage but before harvesting (Okigbo, 1979). To optimize plant density, the seedling rate of each crop on the mixture is adjusted below the full rate to reduce competition from overcrowding. The crops will yield well in the mixture (Hiesbick, 1980, Magaguda et al, 1979, Prabhakar et al, 1983). Planning intercrop with staggered maturity dates or developmental periods utilize variations in peak resource demands for nutrients, water and sunlight. This facilitates harvesting and separation of grain and reduce competition Akyempong et al 1999, Mead and Willey, 1980). Plant architecture allows one intercrop to capture sunlight that would not otherwise be available to others. This is important to growth and yield of cereal and legume crops (Reddy and Willey, 1981, Gardiner and Craker, 1981). To evaluate the performance of intercrop components, the concept of Land Equivalent Ratio (L. E. R.) is used. It measures the advantages of using intercropping systems on combined yields of both crops (Okigbo, 1979). It provides a standardized basis for comparing systems under different situations and crop combinations (Mead and Willey,1981). The distinct aspects of intercropping are: • • • Detailed planning Timely planting of each crop Adequate fertilization at optimal rate and times • Effective weed, pest and disease control • Efficient harvesting Planning should involve selection of crop species, appropriate cultivars, water availability, plant populations, spacing and labour requirements throughout the season, tillage requirements and predicted profitability of inter crops, timely planting, proper fertilization, pest and disease control (Prabhakar et al,1983,Reddy and Willey,1981). Advantages of intercropping: There has been an increase in grower interest in using intercropping since it could reduce management inputs that result in sustainable systems more efficiently using an even potentially replenishing natural resources used during crop production for long term management of farmland. Intercropping has been practiced in the developing countries of Central America, Asia and Africa (Allen and Obura, 1983, Altier and Liebman1994, Bekunda and Woomer, 1996 1999, Grossman and Qualles1993, Henzel andVallis1977, Jodha, 1979, Prabhakar et al, 1983). The advantages of intercropping are risk minimization, effective use of available resources, efficient use of labour, increased crop productivity, erosion control and food security (Bekunda and Woomer, 1996, 1999, Jodha1979, Owuor et al,2002). There is reduction of insect/mite pest populations due to the diversity of crops grown and reduction of plant diseases because the distance between plants of the same species is increased due to the planting of other crops between them, alteration of more beneficial insects especially when flowering crops are included in the cropping system, increase of total farm production and profitability and reduction of weed population through allelopathy and efficient crop production (Maguguda et al,1979). Further, when the intercrop provides a good soil cover, soil temperature will stay relatively low. This prevents burning of the organic matter in the soil and loss of nutrients. It also provides a microclimate that can be favourable for associated crops. Last but not least, in a maize-bean intercrop, for example climbing beans can use the maize stalks for support (Maguguda et al,1979). Potential problems of Intercropping: Depending on the crops intercropped, competition for water, light and nutrients may result in lower yields. Changes in the spatial arrangement of the crops will reduce competition (Reddy and Willey,1981, Roger and Dennis,1993). A larger distance between plants reduce competition for water. Other problems are difficulties in mechanization and increased labour requirements (Reddy and Willey, 1981, Roger and Dennis, 1993). Crop rotation and Intercropping: Weed population density and biomass production may be markedly reduced using crop rotation (temporal diversification) strategies. Crop rotation resulted in emerged weed densities in test crops that were lower in comparison to monocrop systems. In a good number of cases seed density of weeds in crop rotation was lower 1099 Agric. Biol. J. N. Am., 2010, 1(5): 1098-1105 compared to monocultures of component crops. In intercropping systems where a main crop was intersown with a “smother” crop species, weeds biomass in the intercrop was lower in many cases (Jodha,1979). When intercrops were composed of two or more main crops, weed biomass in the intercrop was lower than in all of the component sole crops ( Jodha,1979). The success of rotation systems for weed suppression appears to be based on the use of crop sequences that create varying patterns of resource competition, allelopathic interference, soil disturbance and mechanical damage to provide a stable and frequently inhospitable environment that prevents the proliferation of a particular weed species. Intercrops may demonstrate weed control advantages over sole crops in two ways. First, greater crop yield and less weed growth may be achieved if intercrops are more effective than sole crops in usurping resources from weeds or suppressing weed growth through allelopathy. Alternatively, intercrops may provide yield advantages without suppressing weed growth below levels observed in component sole crops if intercrops use resources that are not exploitable by weed or convert resources to harvestable material more efficiently than sole crops (Jodha, 1979). For weed suppression to occur in crop rotation and intercropping three areas need to be researched on namely: 1. 2. 3. There must be continued attention to the study of weed population dynamics and crop-weed interference in crop rotation and intercropping systems. More information is required on the effects of diversification of cropping systems on weed longevity, weed seedling emergence, weed seed production and dormancy, agents of weed mortality, differential resource consumption by crops and weeds and allelopathic interactions. There needs to be systematic manipulation of specific components of rotation and intercropping systems to isolate and improve those elements (e.g. inter row cultivation, choice of crop genotype) or combinations of elements that may be especially important for weed control (Magaguda et al,1979). The weed-related impacts of combining crop rotation and intercropping strategies should be assessed through careful study of complex farming systems and the design and testing of new integrated approaches.(Magaguda et al,1979) Intercropping and soil fertility: One important reason for intercropping is the improvement and maintenance of fertility. An example of this is when a cereal crop or tuber crop is intercropped with legumes (beans, peas, ground nuts. After the intercrop is harvested, decaying roots and fallen leaves provide nitrogen and other nutrients for the next crop, legumes also fix nitrogen. The crop residues of the legumes can also be used as fodder, by cutting and carrying them to the animals, or by letting the animals graze the residues in the field. The nutrients in the crop residues can then be recycled when manure is used to fertilize crops. Legumes in an intercrop system also provide humus in the soil, due to decaying crop remains resulting in improved soil structure, reducing the need for soil tillage. Water losses, soil erosion and leaching of nutrients are also reduced in intercropping systems due to the improved structure and better soil cover. In intercropping, nitrogen fixation by the legume is not sufficient to maintain soil fertility. If chemical fertilizers are applied, it is not necessary to use nitrogen fertilizer on the cereal crop. Fertilizers are more efficiently used in an intercropping system, due to the increased amount of humus and the different rooting systems of the crops as well as differences in the amount of nutrients taken up (Rehman et al 2006,Rukazambuga et al 2001,Sakala et al, 2000,Trenbath,1976,Trenbath,1979, Willey and Osiru,1972,1979). DIFFERENT INTERCROPPING SYSTEMS: Intercropping of fruit crops: In East Africa fruit crops are usually intercropped with annual crops. For example, banana is intercropped with food and / or fodder crops to increase and use efficiency for small holder farms (Clark and Francis,1985). In one study banana was intercropped with three densities of Grevilla robusta of 208, 313 and 0.25 trees per hectare and it was reported that after 31/2 years the wood volume of Grevilla robusta was highest while banana and bean yields in the intercrop system were unaffected Reddy et a,1992, Roger and Dennis,1993; ). Intercropping banana with sweet potato and beans has been reported to reduce the incidence of weevils and nematodes. (Lima and Lopez1979, Reddy et al1992, Roger and Dennis,1993).In Kenya, an intercrop between Maize,desmodium,alegume and Napier grass has reduced the incidence of Striga hermontheca( Striga) and Maize stemborer(Chilo 1100 Agric. Biol. J. N. Am., 2010, 1(5): 1098-1105 partellu) (Ahmed et al,2008). In India bananas are intercropped with potato which has led to good returns (Okigbo,1979). Mustard has also been intercropped with bananas in India. Other fruit trees have also been intercropped. For example, in an intercrop between citrus mandarin seedlings and cucumber there was a high yield of cucumber fruit and minimal interference in the growth of citrus seedlings possibly due to the low growing nature of the latter (Nataraja and Nairk,1992). In Kenya fruit trees are intercropped with all types of short term crops such as beans, peas, potatoes, maize, millet, exotic and indigenous vegetables when they are still young as a way of attaining food security and income before the trees mature. Vegetable crops: In an intercrop system between cereals, beans and peas, there has been increased yield presumably by the transfer of biologically fixed nitrogen from the roots of legume to the root zone of the companion crop (Akyempong et al, 1999, Gold et al, 1999, Ndungu et al, 2003, Reddy and Willey, 1981;Nanecke et al,1989). This biologically fixed nitrogen can reduce the need for N from commercial fertilizers (Lima and Lopez1979, Hiesbick, 1980). Vegetables such as cabbage, cucumber, radish, snap bean, and broccoli are intercropped into double rows of field maize planted on raised soil beds. Intercropping two vegetables with different architecture and nutritional value such as beet and okra, pepper and onion has been practised in tropical Asia (Okigbo, 1979). In some areas of Africa, vegetable cropping systems such as maize and cowpea often are practiced (Mead and Willey, 1980). Some factors that will influence or not whether two crops can be successfully intercropped include plant height (Spiejer et al 2006), the size of the leaves and the orientation and distribution of these leaves in the plant canopy (Sogbedji et al 2006). These variables affect the amount of sunlight that passes through the canopies (Grossman and Quales, 1993), which could influence the photosynthetic rates of the leaves within the canopy. In intercropping systems with different canopy heights the crop in the under story needs to be shade tolerant for the plant to be productive (Sogbedji et al, 2006). Land Equivalent Ratio (L. E. R.): This is a method used to determine the effectiveness of intercropping systems. It is the most widely used index for measuring the advantages of intercropping systems on combined yield of both crops. (Mandal and Roy, 1986). It is defined as the relative land area under sole crops required to produce yields achieved in intercropping (Trenbath, 1976). Monetary advantage was calculated to find out the extent of monetary advantage derived due to intercropping (Trenbath, 1979). The L. E. R. emphasized land area without consideration of the time the field is dedicated to production. To correct the deficiency the L. E. R. method was modified (Liebman and Elizabeth,.1993) to include the duration of time the crop was on the land from planting to harvest (ATER). ATER = Area-Time Equivalent Ratio ATER takes into consideration the time taken when the crop was on the land and it is a means of assessing yield advantages using LER as a means of determining productivity (Liebman and Elizabeth,1993). In an intercropping between corn with cow pea and Soya bean productivity was increased on L. E. R. basis by 22 to 32%. There was also an increase in productivity in an intercropping system between wheat (T. cestirum L.) and chick pea (cicer arientimum L). The increase was 29% ( Mandal and Roy,1986) Intercropping barley with lentil with one irrigation and straw mulch produced 28% more productivity, ( Ndungu,1987) when monetary advantage was considered with one irrigation and straw mulch ( Ndungu,1987) For vegetable crops intercropping system to be successful in a geographical location effective cultural practices must be determined, with respect to plant population. It has been reported that in an intercropping between southern pea and sweet corn there was no effect on the number of ears of sweet corn per plant but yields were increased as plant population density increased. This has also been reported previously by other workers (Mashingaidze et al, 2000; Trenbath, 1976, Reddy and Willey, 1981) who observed that in a bean and maize mixture in which the population density of maize was varied increased maize yield was attributed to increasing population of maize as in the study of sweet corn and southern pea (Reddy and Willey, 1981). Plant density and nitrogen fertilizer did not influence the number of ears produced per plant (Reddy and Willey, 1981) Light interception previously has been implicated as a major factor affecting plant growth and yield in a cereal-legume intercrop (Grossman and Quales, 1993). In the intercropping system of southern pea and sweet corn the reduction in component crop light 1101 Agric. Biol. J. N. Am., 2010, 1(5): 1098-1105 intercepted by southern pea and sweet corn as compared to light intercepted by the monocrop system of those crops probably contributed to the reduction in their respective yields (Reddy and Willey, 1981). Intercropped southern pea was more affected by the reduction in light interception than intercropped sweet corn hence had lower yield (Reddy and Willey, 1981). In a previous study (Clark and Francis, 1985), it was reported that growth of maize and beans in an intercrop could be increased if the planting dates of the individual crops were staggered. In the southern pea and sweet corn intercrop their yields were reduced when planting of each was delayed (Reddy and Willey 1981). Maize and legume intercropping has been done for many years in Africa for house hold food security. The yields in most farms have declined due to nutrient depletion. (Clark and Francis,1985, Grossman and Quales,1993, Hiesbick,1980). In such an intercropping system maize gains from the N fixed biologically by beans. Intercropping with different legumes such as groundnuts, cowpea in crop rotation systems has improved yields and so is the staggering of the rows of maize. Combinations such as carrot-cabbage, lettucecabbage, carrot-garlic, tomato-beans, sweet potatopumpkin, maize-tomato, tomato-beans, sweet potatopumpkin and maize-sugarcane have been successful in Kenya and elsewhere. In some countries such as Kenya maize and pigeon pea (cajanus cojan) are intercropped especially in Machakos district. At the beginning of the rains, maize and pigeon pea are planted in rows. After 5 months the maize is harvested at the time the pigeon pea starts to flower and fruit. Pigeon pea being deep rooted improves soil fertility through nitrogen fixation and is also used for food security, shade and firewood. In South Africa, Pigeon Pea is intercropped with Maize and are planted at the same time but early development of pigeon pea is slow and Maize is harvested before the long duration pigeon pea begins to form substantial biomass. After the Maize is harvested pigeon pea grows for several more months on residual soil moisture and produces a complete canopy cover and produces good yields. Maize is planted at the same spacing as the monocrop and yields of Maize planted as an intecrop are similar to those of sole Maize. Combining Maize and pigeon pea reduces N and P fertilizer needs in subsequent years( Spiejer et al,1993) Inputs of N through fallen pigeon leaves contributes 75-90 kgNper ha which immensely benefits a subsequent crop of Maize ( Sanchez et al,1996). Pigeon Pea is also able to access scarce soluble P and can efficiently utilize residual P remaining in the soil from fertilizer applied to Maize( Bahl and Pasricha,1998).The use of pigeon also causes significant pest and disease damage( Allen and Obura,1983, Sogbedji et al, 2006). Other countries practicing Maize-Pigeon pea intercropping are South Malawi, Parts of Mozambique and Tanzania, Benin and Southern Nigeria (Sileshi and Mafongoya,2003)In Kenya sweet potato (ipomea batatus) is intercropped with maize and bananas. The leaves and stems of sweet potato are used as cattle feed while its tubers are eaten. After a single weeding and fertilization, sweet potato are planted in between maize. The advantages of the sweet potatomaize-banana intercrop are a diversification of crops and more efficient use of land. The permanent soil cover by the sweet potato reduces weed costs and soil erosion. Innovations in Maize-Legume intercropping allow farmers to grow a wide range of food legumes as understory intercrops with Maize. Maize can be planted at its recommended population, but every other row is shifted to provide a wider alternate inter_row to the legume or strip-cropped by lowering Maize population but maintaining similar yields. This approach permits more advantageous intercropping with high value food legumes such as groundnuts, green gram, soybean, etc, which are not usually intercropped with Maize because of excessive shedding( Woomer et al, 2004). This Intercropping system is called MBILI( Better Interactions For Legume Intecrops) has been reported( Sileshi and Mafongoya,2003).This system also involves the application of Rock Phosphate( Buresh et al,1997) and application of fortified manure compost( Ndungu et al,2003) to the Maize legume intercrop, it is aimed at increasing legume productivity but the Maize yields have also increased. In Western Kenya the widespread recommendation is to plant seed of Maize at a spacing of 75cm between and 30cm within, rows resulting in a plant population of about 44000 plants per hectare from 75kg of seed. Beans are then planted between the 75cm Maize rows. With MBILI, the row arrangement is changed. Maize rows are spaced as 50cm pairs that are 100cm apart (the gap).Two rows of legumes are planted within the gap, 33cm row spacing. By staggering the rows of Maize into a two –by-two arrangement, the same plant populations as conventional intercrops is maintained but the legumes are better able to compete with the 1102 Agric. Biol. J. N. Am., 2010, 1(5): 1098-1105 Maize.The rows should be run on an East-West direction on a level land. The results have been high returns to benefits cost ratio.Similar results have been observed with Sorghum –Legume intercropping in Uganda.( Owuor et al,2002) Sorghum and Millet have also been intercropped with Cowpea in West Africa where Cowpea is used for human food and fodder and the breeding work at IITA( International Institute for Tropical Agriculture) is aimed at increasing Cowpea biomass to produce reasonable quantities of both grain and fodder with minimal application of pesticides.(Sileshi and Mafongoya,2003) Finally, Maize and Pumpkin have been intercropped in Zimbabwe and reported to reduce the number of weedings as compared to Maize sole cropping( Mashingaidze et al,2000).High levels of intercropping population reduced Pumpkin vine length and harvestable leaf area per plant. This agrees with ( Olaniyan et al, 2006)who indicated that spacing should be increased in Pumpkin for the best vining habit.Naturally,Pumpkin has a prostrate growth habit and requires a lot of free space which was lacking in the intercropping between it and Maize resulting in reduced vine length as reported (Mashingaidze et al,2000) However, leaf yields and partial Leaf Area Index increased with high levels of Pumpkin intercropping populations signifying the importance of intercropping populations in determining yields. 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