Potential and Advantages of Maize-Legume Intercropping System

Intercropping provides enough scope to include two or more crops simultaneously in same piece of land targeting higher productivity from unit area. Maize, a cereal crop of versatile use, as planted in wide rows offers the opportunity for adoption of intercropping. The intercropping system with maize and legume is beneficial in mul-tifaceted aspects. The success of maize-legume intercropping system largely depends on choice of crops and their maturity, density, and time of planting. Advantage of maize-legume combination of intercropping system is pronounced in the form of higher yield and greater utilization of available resources, benefits in weeds, pests and disease management, fixation of biological nitrogen by legumes and transfer of N to associated maize, insurance against crop failure to small holders, and control of erosion by covering a large extent of ground area. Though maize-legume intercropping system exhibits limitations like less scope of farm mechanization, dependence on more human workforce, and chance of achieving less productivity from maize, the system implies more advantages for small holders in developing countries where human workforce is not a constraint. The chapter has focused on beneficial impacts of maize-legume intercropping system.


Introduction
The cropping system is growing of crops on an area interacting with resources and time and intercropping system is raising of two or more crops simultaneously in the same piece of land [1,2]. This a common practice in developing countries and it is mostly practiced by small and marginal famers. In tropical world, intercropping is prominently visible with food grain cultivation, whereas in temperate countries forage based intercropping is very much common [3]. Intercropping is generally practiced on small farms with limited resources and it has been observed to enhance yields with greater stability in a variety of crop combinations. Moreover, intercropping system is known by less use of inputs, namely, fertilizers, plant protection chemical, and thus healthy, safe, and high quality food under ecologically sound production system. On-farm biodiversity is also promoted by diversification of crops in through mixed cropping, intercropping and agroforestry systems resulting in variation of diet and net return, higher level of production stability, proper utilization of limited resources human labour-force under low levels of technological intervention [3] and all these ultimately lead to achieve production sustainability in agriculture.

Considerations for choosing intercropping system
The success of intercropping depends on different considerations before and during cultivation as because crops grown in mixture may compete spatially and temporally amongst species for available resources. An efficient intercropping system in terms of economic benefits depends on adaptation of planting geometry and choice of compatible and suitable crops. The features of an intercropping system differ with soil and climatic conditions, economic situation and preferences of the farmers. In cereal-legume intercropping system, choice of crop species, density of planting, planting geometry, time of planting and maturity of crops are the key considerations and the success of the system largely depends on these factors.

Choice of crops
Choice of crops is important in intercropping, because severe competition in mixed culture may not be beneficial and even harmful if proper plant species are not chosen. In this way competition amongst plants can be minimized and better utilization of available resources can be assured. The combination of cereal and legume is considered an ideal because cereals can utilize a portion of biologically fixed nitrogen by legumes. In maize based intercropping system, groundnut is chosen as intercrop maize in South East Asia and Africa [5]. Maize can provide shade to associated legumes and the legume species should be to some extent tolerant to shade. Legume species like black gram (Vigna mungo), cowpea (Vigna unguiculata), groundnut (Arachis hypogea) and green gram (Vigna radiata) have much less effect on maize and these are tolerant to maize shade [6, 7]. Cereal-legume intercropping is very common in the continents of Asia, Africa and South America [8], however, in tropical countries, maize based intercropping is practiced with a preference to cowpea [9]. In Central and South America and parts of East Africa intercropping of maize and bean is widely practiced [10]. Maize and dwarf red gram intercropping combination is known as a suitable option in managing cereal component [11].

Maturity of crop
Maturity of crop is another important consideration in adoption of intercropping. Generally, crops grown in intercropping should have different peak period of growth, otherwise there will be competition amongst the crop species for available resources. The complementary effects benefit the system and these are reflected into yield advantage when the component species in intercropping have different growing period for major demands on available resources. Therefore crops with different duration maturity are chosen to get complementary effects. Maize has been recognized as a common crop in cereal-based intercropping and treated as base crop in additive series and dissimilar legumes are preferably considered as intercrop. In maize-based intercropping system choosing short duration legumes as intercrops is an ideal option. For example, in maize + green gram intercropping system, initial growth of maize is slow and it reaches at knee-height stage after 6-7 weeks and peak light demand starts from 55 to 60 days after sowing and by this period green gram sown at the same time will be in reproductive stage or in close to harvest. In this way green gram completes its major growth period and maize starts the same and thus high level of complementarity is observed.

Plant density and maturity of component crops
Optimum plant stand is synonymous to optimum yield. But in intercropping system two or more crops are accommodated in the same land at the same time and thus there may be reduction in population of crops compared to pure stand of individual species. On the basis of plant density, intercropping may be categorized into two groups, namely, additive series and replacement series. The additive series is comprised of addition of intercrop within fullest population of base crop. Another crop known as intercrop and it is sown into the base crop population by adjusting row spacing or changing planting geometry. Sometimes, paired row planting of maize is done to accommodate greater space for intercrops. But in replacement series of intercropping, there is not the concept of base crop and the crops (two or more) considered are termed as component crops or intercrops. In such type of intercropping, introduction of a component crop is made by replacing another and none of the component crops are sown with 100% population as recommended in their pure stands. It is very clear that certain proportion of population of one crop component is sacrificed and another component is introduced in that place. In many intercropping situations with replacement series, yield advantages are maximized by increasing population density in excess than their recommended population in the sole cropping. Here, the competition is relatively lesser in between component crops as compared to additive series. As maize is widely spaced crop and generally row spacing ranges between 60 to 90 cm and intercrops can easily be raised in uniform rows of planting. The planting geometry, particularly, paired row planting of maize may enhance the efficiency of growth parameters as well as yield of maize and associated legumes by efficient accommodation of crops. Prasad and Brook [12] observed an enhanced LAI per unit area with increase in plant population of maize in maize-soybean intercropping system. Under the major demand for resources at different times of system duration, the long duration cereal crop maize can recover its resource needs in combination with short duration legumes during remaining phase of growth that is after harvest of legumes [13].

Time of planting
Maize is recognized as a very common crop in intercropping system in which legumes can be sown easily. Generally, in maize based intercropping systems, as maize has slower initial growth rate up to knee height stage (6-7 weeks of sowing), if short duration legumes are sown simultaneously can reach into their reproductive stage can start their reproductive period and hence competition for common natural resources do not appear at the same period. Maize has diverse use and if maize is considered as fodder in intercropping, competition does not come into figure because of enhance biomass yield and mixture of grass-legume combination enhances the quality of forage in terms of dietary value. Moreover, maize has higher potential for accumulation of carbohydrate, a source of energy as fodder, from unit area on daily basis. However, legumes can be planted in maize at the same time can also register higher growth attributes because of wider spacing of maize as grain crop.

Advantages of intercropping
Maize and legume intercropping system has advantages in many ways and so preferred by small and marginal farmers. Experimental results showed that maizelegume intercropping can assure higher yield, soil restoration and greater impact of system productivity.

Advantage in improving productivity and soil fertility
In Intercropping, more crops are grown simultaneously in unit area which results not only greater productivity but also utilizes natural resources more efficiently. Management of pests, diseases and weeds is easier because of less incidence which leads to greater yield. Another important aspect of maize-legume intercropping is restoration of soil fertility.

Higher yield and greater resource utilization
Yield is the basic consideration for assessing benefits of intercropping. In maize-legume intercropping maize is treated as based crop without much variation in plant stand of cereal component. In additive series of intercropping, legumes add plant population per unit area and benefits are achieved as total yield of crops, namely maize and legume yields. Further, in a combination of legume and nonlegume, generally non-legume component is benefited by sharing atmospheric nitrogen fixed by legumes. In assessing efficiency of an intercropping system, some competition functions are considered. Of which land equivalent ratio (LER) is a very common index used to measure productivity of intercropping system. Willey and Osiru [14] proposed the concept of the LER and it is defined as the proportionate land area required under pure stand of crop to produce the same productivity as obtained in an intercropping at the same management level. Actually, LER is the summation of ratios of the yield of each crop species involved in intercropping system to its corresponding pure stand yield. Experiments carried out in different countries clearly exhibited higher LER values in maize-legume intercropping system ( Table 1).
The LER indicates the advantage of an intercropping with efficient resource utilization compared to pure stands of respective crops. The value of LER greater than unity (1.0) is indicative of the advantages in intercropping system [2].
The LER indicates on efficiency of using land area, but time factor is not considered for which the crop occupies the land area. To rectify the limitation of the LER, the concept of area time equivalent ratio (ATER) has been developed considering the occupancy of land by the crops for certain periods [23]. Like the LER, values of the ATER more than unity also indicate advantage of intercropping. Different researchers noted beneficial ATER values with maize-legume intercropping systems (Table 2).
However, researchers comment that the LER overestimates and the ATER underestimates the land-use efficiency [27].
Crop equivalent yield is another expression for evaluating the efficiency of intercropping system [25]. Actually, in maize-legume intercropping system, total yields are converted in the form of base crop (maize) equivalent yield by considering the intercrop yield and market price of maize (base crop) and associated intercrops. In maize-legume intercropping system it is termed as maize equivalent yield (MEY) and expressed in kg −ha . If the base crop equivalent yield is obtained higher in intercropping combinations than base crop yield, then intercropping is considered advantageous. Table 3 indicates advantageous MEYs as obtained by the researchers in experiments.
The greater yields in intercropping is recorded when the component crops show complementary effects amongst themselves and use natural resources efficiently than raised as sole crops [28]. The crops with inherent capability can only utilize natural resources efficiently and complementarity plays important role in resource utilization [2]. Further, higher yield of both the crops in maize-cowpea intercropping combination was noted than pure stands [29].
In soils with low nitrogen content, maize legume intercropping performed well [30]. Yield advantage in intercropping is expressed by crops because of greater use of growth resources like light, water, and nutrients and this efficient use is   converted into biomass [2,31]. The combination of maize-cowpea intercropping can assure greater light interception and check evaporation loss of soil moisture than pure stand of maize [32]. Maize and legumes are morphologically dissimilar and their time of peak demand and requirement of light, nutrients and water are different. Therefore, complementary effect between component crops is very common. Jiao et al. [33] noted that maize used strong light and groundnut preferred weak light (because maize provided partial shade) in maize-groundnut intercropping system and the system registered yield advantage. Further, soybean-maize intercropping has been known for efficient utilization of light, nutrients and available soil moisture [2,34]. Soil moisture or water availability to plants is a determining factor in intercropping systems and efficient water use leads to use of other resources. Cereal-legume combination is known to use available water resources more efficiently than pure stands of crops. Scientific investigations showed that maize-legume combination registered greater water use efficiency than that of sole crops and under water stress conditions, it could be one of the best options [35] as soybean as a deep rooted crop having efficiency to use soil moisture from deeper layer (below 1 m) of the soil [36].

Weed management
Intercropping is an effective practice for weed management because enough of ground area is covered by crops which suppress weed growth. Combination of maize and legumes in intercropping is known to reduce weed population and weed biomass compared to pure stands of maize. Research evidences clearly show benefits of intercropping as it provides competitive effect against weeds both spatially and temporally than pure stands of maize. Reduced weed growth in maize-cowpea intercropping system than sole cropping of maize. Chalka and Nepalia [37] mentioned that in maize-legume intercropping systems, maize + cowpea and maize + soybean reduced NPK removal through weeds by 37.4 and 38.0% respectively and the two intercropping combinations registered higher biological yield of maize. Rahimi et al. [38] reported that maize-black gram intercropping combination of either 1:1 or 2:2 recorded lower densities of total weeds compared to pure stand of maize. Shah et al. [39] opined that weed smothering efficiency was higher in intercropping of maize with soybean than the combination of maize with green gram and it was due to the lower availability of space and light leads to reduce the weeds population with maize-soybean intercropping system. Weed biomass is reduced in intercropping as reported by researchers for maize-legume combinations [40,41]. In studies it has been claimed that enhancement of diversity of crop species in intercropping system maintains a highly asymmetric competition over weeds resulting in less weed biomass [42,43]. Weeds compete with crops for available resources and less weed occurrence assures ultimately higher productivity.

Pest and disease management
Intercropping systems can influence the pest and pathogen population dynamics. The population of beneficial insects such as parasites and predators are enhanced in polyculture due to diversity of crops [2] and presence of harmful pests may remain below the economic threshold level. Thus, plant protection becomes easy and use of chemicals for crop protection comes down which ultimately reduces the chemical pollution to agricultural ecology, however, monoculture of maize requires more chemical pesticides [44]. In intercropping system, two or more crop species are cultivated which creates complexity in food and habitat of pests. Further, intercropping of maize with legumes is known to increase population of beneficial insects and decrease the population of bud worm, corn borer, leaf hopper and maize stalk borer [1, 45]. The intercropping system has also an impact against disease management, because in mixture of crops functional diversity is created that checks population increase of pathogen. Some diseases of legume crops like angular leaf spot (Phaeoisariopsis griseola) of beans and ascochyta blight (Mycosphaerella pinodes) were observed with less severity when these were intercropped with maize [46,47] than pure stand of legumes. Reduction of pest-disease incidence not only saves the crop loss and better yield but also assures less use of chemicals for plant protection and thus minimizes the chance of pollution in crop ecology.

N-fixation by legumes and transfer to associated non-legume
Legumes are known to fix atmospheric nitrogen biologically. The biological nitrogen fixation (BNF) is a process where some bacteria convert atmospheric N 2 into ammonia (NH 3 ) and making it available to plants. In maize-legume intercropping system, both the crops acquire N from the common soil pool and compete and thus deficit of mineral N may occur in the rhizosphere which promotes legume to fix atmospheric N [48,49]. Maize is an exhaustive crop and legumes are soil replenishing crops and decomposition of legumes residue improves soil fertility. In the soils with poor available nitrogen status, the biologically fixed nitrogen plays an important role. Under the situation of limited supply of nitrogenous fertilizer also intercropping legume and non-legume may a suitable option of nutrient management. Further, chemical N fertilizers are responsible for degradation of ecosystem in the form of nitrate pollution and legumes grown as intercrops help in environmental sustainability [50]. In maize-soybean intercropping system, soybean supplements nitrogen to cereal component [51]. Maize grown as forage in intercropping with legumes is known to improve quality parameters of forage like higher crude protein and mineral content and digestibility [48,52]. Biologically fixed N by pigeon pea was transferred to associated maize and N content and uptake by maize was improved in maize-pigeon pea intercropping system [53]. The associated non-legume crop (maize) gets benefit of fixed N by legumes [1]. Thus, maize-legume intercropping system is beneficial in terms of N economy too. Leaf defoliation of legumes is known to increase productivity of maize-soybean intercropping system [22].

Erosion control
Intercropping is advantageous in terms of erosion control because of coverage of more ground area than monocropping of cereals. The striking actions of rain drops can erode the bare or uncovered soil, but the coverage of soil by legumes can check it. In maize-cowpea intercropping combination, ground area is mostly covered, thus soil erosion is reduced [54]. Taller crop like maize also plays a vital role as wind break and protects the crops with shorter canopy (like legumes) as well as erosion caused by wind [45].

Insurance against crop failure to small holders
Intercropping is a common practice of small and marginal farmers in developing countries of Asia and Africa and in risky and fragile ecological conditions which is known as a suitable practice to provide natural insurance and thus provides a profitable shape to farm economy. Under moisture stress conditions, more of ground area is covered under maize-legume intercropping than sole cropping of maize which leads to less evaporation loss of soil moisture. Under extreme conditions, may be due to either biotic or abiotic factors, a crop may fail, but there will be less chance of failure of more crops grown in intercropping, which are morphologically dissimilar and if so happened some yield and return will be earned to save small holders' economic interest. Thus stability in yield and return are achieved due to creation of crop diversity in the intercropping systems. In economic point of view, it may be stated that small farmers may face problem of seasonal price variability of commodities which often can destabilize net realization, but diversification in the form of intercropping can stabilize farm income to a great extent. Experimental results indicated superiority of intercropping maize-beans in soil fertility restoration and income enhancement than monocropping of the component crops [55]. Yield enhancement of crops is another basis to strengthen the economy of small and marginal farmers adopting intercropping system [56]. Though intercropping of maizegrain legumes is labour and cost intensive, small holders of central Mozambique prefer it because of reduced risk of crop failure and enhanced productivity [57].

Sustainability of the system
Intercropping is now in the centre of attention targeting sustainability in agriculture. The negative impacts of industrialized and modern agriculture have already been realized and issues are very crucial in crop ecology to achieve sustainability. On the other side, maize-legume intercropping has enough potential in the form of more yields from limited resource, proper utilization of resources, and restoration of soil fertility, efficient pest management and creation of above and below ground diversity. In the moisture stress or resource poor conditions, intercropping provides natural insurance against crop failure caused by biotic and abiotic factors and thus ascertains economic stability of small holders. Considering the multiple advantages, it can be stated that maize-legume intercropping system is one of the suitable options for achieving production sustainability for small holders.

Limitations of intercropping
Despite a number of benefits of maize-legume intercropping over monocropping, sometimes intercropping may exhibit some limitations especially in terms of agronomic management. In the field where farm mechanizations have been adopted, intercultural operations and harvest become difficult with two dissimilar crops. However, there is no problem where the intercrops are harvested for forage or grazed [13]. It may be mentioned that where human workforce is sufficient, particularly in developing countries, there is no need for investment in costly machines for agronomic management and harvest of crops in intercropping and in this regard intercropping does not express any disadvantages. Intercropping may cause yield loss of the base crop (maize) compared to its sole stand, but MEY become more and thus intercropping may be considered more productive than monoculture. Further in intercropping, crowded crop canopy may create a microclimate which may be congenial to spread of fungal pathogens, but in maize-legumes intercropping combination, such incidences are not common.

Conclusion
Considering the importance of maize in cereal basket of the world, production sustainability is a prime concern. Maize is an exhaustive crop by nature that requires enough nutrient inputs to achieve target yield. Under small holders' practices in poor soil and fragile ecological conditions continuous growing of maize may create further depletion of soil nutrients causing a threat to production sustainability. In this regard, maize-legume intercropping system is considered a suitable option as it has enough potential to replenish the soil nutrients, produce more yield and economic benefit by utilizing limited resource, check damage caused by pests, diseases and weeds to a large extent, control soil erosion by covering ground and provide natural insurance to small holders under risky conditions against crop failure. Thus, in true sense, maize-legume intercropping system can boost yield as well as production sustainability of the system as a whole.
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