Mixed and Intercropping

One of the features of modern agriculture is an ever increasing tendency towards the use of monocultures (Theunissen and Den Ouden, 1980). These have allowed agriculture to become intensified and more easily manageable with a high input mechanized approach to farming. In tropical countries, the use of monocultures is practised only on larger farms or estates, while the more traditional approaches to farming utilize polycultures. The polycultures include mixed intercropping (no distinct row arrangement), row intercropping (one or more crops planted in rows), strip intercropping (crops grown in different strips, wide enough to permit independent cultivation), relay intercropping (two or more grown simultaneously for part of the life cycle of each, a second crop being planted before the harvest of the first) (Andrews and Kassam, 1976) and alley intercropping (annual crops are grown in strips between trees) (ICRISAT, 1989). Most of the food consumed in tropical Asia, Latin America and Africa is produced in such systems which often more readily meet the needs of the smaller scale farmer than does the monocrop (Perrin and Phillips, 1978). A mixed or intercropping regime can provide a greater total land productivity as well as insurance against the failure or unstable market value of any single crop. In addition crops in intercropping systems may improve soil fertility and the availability of alternative sources of nutritious products (Risch et al., 1983) as well as reducing the incidence of insect pest attack (Tingey and Lamont, 1988) and thereby maintaining lower pest control costs. Intercropping has been studied sufficiently for there now to be a considerable body of evidence to show that it can be used to reduce the incidence of pest insects (e.g. Theunissen and Den Ouden, 1980; Tukahirwa and Coaker, 1982; Uvah and Coaker, 1984; Tingey and Lamont, 1988; Edwards et al., 1992; Khan et al., 1997). For this reason, policy makers in tropical countries continue to give attention to improving production in traditional intercrops, rather than replacing them with capital and energy intensive technology (Perrin and Phillips, 1978). In more intensive systems, monocrops are maintained because with high inputs, they are economic despite susceptibility to pests. For these reasons, it is unlikely that intercrops will play any major role within the framework of intensive crop production, except perhaps in smaller scale farming enterprises and horticulture, and as a result most intercrop studies will remain pertinent to situations in tropical countries (Coaker, 1990).

One of the major problems has been predicting which cropping systems will reduce pest abundance, since not all combinations of crops will produce the desired effect and blind adherence to the principle that a more diversified system will reduce pest infestation is clearly inadequate and often totally wrong (Gurr et al., 1998). In an examination of 150 studies involving a total of 198 plant damaging insect species, 53% were found to be less abundant in a more diversified system, 18% were more abundant, 9% showed no difference and 20% showed a variable response (Risch et al., 1983). Clearly the majority of the species were less abundant in more diversified systems but 38% were either more abundant or produced a variable response. This indicates the need for caution and a greater understanding of the mechanisms involved to explain how, where and when such exceptions are likely to occur. It will only be through detailed ecological studies that such an understanding can be gained and an appropriate predictive theory developed. This means a greater emphasis has to be placed on detailed ecological experiments rather than on purely descriptive studies of relative insect abundance under different cropping systems. Since there are numerous possible intercrops and a multitude of conditions under which such systems could be utilized, there is little likelihood that each of these could be investigated (Herzog and Funderburk, 1986). Hence, it is necessary to understand the underlying mechanisms to allow extrapolation to various other cropping systems and situations (Gurr et al., 1998).

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