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While there is some variability in the use of terms among intercom researchers, a certain uniformity seems to have evolved. The following is taken from Andrews and Assam (1976), with modifications and commentary as needed for the purposes of the developments in this chapter. The most general term is multiple cropping, under which is the dichotomous classification of sequential cropping and intercropping:
Multiple cropping. The general term, it is "Growing two or more crops on the same field in a year."
Sequential cropping. The time-dependent form of multiple cropping, it is "Growing two or more crops in sequence on the same field per year.... Crop intensification is only in the time dimension. There is no intercom competition. Farmers manage only one crop at a time in the same field."
Intercropping. The space-dependent form of multiple cropping, it is "Growing two or more crops simultaneously on the same field. Crop intensification is in both time and space dimensions. There is intercom competition during all or part of crop growth. Farmers manage more than one crop at a time in the same field."
Under the general category of intercropping there are four subcategories:
Mixed intercropping. "Growing two or more crops simultaneously with no distinct row arrangement." This is frequently the form taken in indigenous slash-and-burn or fallow agriculture (Rappaport, 1971; Russell, 1968).
Row intercropping. "Growing two or more crops simultaneously where one or more crops are planted in rows." This is the pattern usually encountered in intensive agriculture, where the plow has replaced the machete and fire as the main tool of land preparation.
Strip intercropping. "Growing two or more crops simultaneously in different strips wide enough to permit independent cultivation but narrow enough for the crops to interact agronomically." This form of intercropping is more common in highly modernized systems, especially where the intensive use of machinery is desired.
Relay intercropping. "Growing two or more crops simultaneously during part of the life cycle of Bach." This form of intercropping may actually include the other three as subsets, since its primary categorization variable is time.
In addition to the major categories of intercrops as detailed above, there has also grown an entire lexicon associated with intercropping research. The following list of terms is meant to guide the reader to their use in this chapter. I have tried to incorporate the most genera usage of each term. The terms are:
Monoculture (sole crop). The cultivation of a single species of crop.
Intercom (polyculture). The cultivation of two or more species of crop in such a way that they interact agronomically (biologically) Intercrops can be of four flavors--mixed, row, strip, or relay--as indicated above.
Cropping pattern. The yearly sequence and spatial arrangement of crops or of crops and fallow on a given area.
Cropping system. The cropping patterns used on a farm and their interaction with farm resources, other farm enterprises, and available technology, all of which determine their makeup.
Land equivalent ratio (LER), or relative yield total (RYT). The ratio of the area needed under monoculture to a unit area of intercropping at the same management lever to give an equal amount of yield. ~ -
Competition (interference). The process in which two individual plants or two populations of plants interact in such a way that at least one exerts a negative effect on the other (cf. facilitation).
Facilitation. The process in which two individual plants or two populations of plants interact in such a way that at least one exerts a positive effect on the other. Double facilitation is equivalent to mutualism.
Social & economic disadvantages of multiple cropping systems :
(1) The systems are more complex and little understood.
(2) In some cases yields are lower.
(3) In many actual economic systems, not considered to be economically efficient.
(4) Normally a greater need for hand labour.
(5) Don't offer sufficient stimulus to lower income farmers.
(6) For producers with limited economic resources it may take longer to recover initial investment.
(7) Opposition from prevalent social economic and political systems.
(8) There is a shortage of trained personnel.
(9) General lack of knowledge or understanding.
1) Each of the many possible intercropping patterns is appropriate for a particular range of conditions and inappropriate for others.
2) Each intercropping pattern is usually chosen to alleviate a particular limitation in resources.
3) Intercropping is generally associated with small land holdings.
4) Intercropping systems make it difficult to cultivate between rows
Disadvantages of multiple cropping systems :
(1) Competition between plants for light.
(2) Competition between plants for soil nutrients.
(3) Competition between plants for water.
(4) Possibility for allelopathic influences.
(5) Harvesting of one crop component may cause damage to the others.
(6) Difficult to incorporate a fallow period.
(7) Many times very difficult to mechanize multiple cropping systems.
(8) Increase in evaportranspiration loss of water.
(9) Possible over-extraction of nutrients.
10) Leaf, branch, fruit, or water-drop fall from taller elements can damage shorter ones.
11) Higher relative humidity in the air can favor diseases.
12) Possible to favor a proliferation of harmful animals.
Social & economic advantages of multiple cropping systems :
(1) Dependence on only crop is avoided.
(2) Less need to import energy.
(3} Wildlife is favoured
(4) Reduction in the outlay for fertilizers.
(5) There is much greater flexibility of the distribution of labour.
(6) Possible to recover investments in much less time.
(7) Availability of harvest over a much longer period of time .
(8) Can occupy much more labour.
(9) The farmer of little economic resources can produce a large variety of useful products.
10) Permit a gradual change in more destructive farming practices to more appropriate technologies.
11) Promote a return to the land.
12) Components can constitute a type of "savings" for the future.
13) Promote interdisciplinary activities.
When we observe that most natural plant communities consists of a mosaic of individuals of many species, we are perhaps struck by the relative simplicity of crop communities in "advanced" countries. Apart from the generally small populations of weeds, the plants of a present-day crop field have very similar and often identical genetic constitutions. Man has not always grown his food in such a way though. In neolithic times, the first crops were mixtures of cereals with a wide range of weeds. Selection practiced through thousands of generations, seed cleaning, and the development of cultural methods against weeds reduced the heterogeneity of these early crops.
During recent years plant breeders have aimed to produce genetic uniformity within crop varieties, and the use of selective herbicides has simplified the weed flora. To make mechanization more profitable, farms have been amalgamated and hedges and fences have been removed; consequently, individual genetic differences which originally existed between varieties grown in various localities, regions and continents are disappearing as the multitude of locally adapted varieties are replaced by a relatively mall number of widely-adapted, higher-yielding types.
This extensive planting of a few varieties is a naturel consequence of the success of the plant breeders of CIMMYT (International Centre for the Improvement of Maize and Wheat, Mexico) and of IRRI (International Rice Research Institute, Philippines), but it now causes grave concern among crop scientists.' To allow the "miracle varieties" to express their full yield potential, they are given abundant fertilizer and irrigation, which predispose them to disease and pest attack; in addition, the world-wide distribution of the same genetic types provides ideal conditions for the evolution of races of disease and pest organism able to attack them. In view of the danger of widespread epidemics leading to calamitous yield losses, it is now being felt that heterogeneity should be reintroduced into the crop fields in some planned fashion.
This article examines some of the evidence and speculation about the possible advantage of growing mixed -crops. As mixed cropping is still widely practiced under conditions of primitive agriculture, it seems worthy of further consideration.
Types of mixtures grown
When it is believed that some favorable interaction occurs between the plants of certain crops, the crops are often cultured together for most of the period of growth. Thus, in Malaysia a cover of groundnuts is often maintained under rubber for the whole life of the trees (see photograph). Shorter-term mixtures which are sown and harvested together are the grass-legume hay combinations and the dredgecorn (oats and barley) grown in Britain. When there is space temporarily unused in a crop, another rapidly growing crop may be taken by interplanting. In India vegetables are planted between rows of young sugar cane, and in Malaysia tobacco is planted among young rubber (see photograph). By interplanting vegetables in ripening paddy two weeks before the rice is harvested, Taiwan farmers are achieving up to five crops in one year.
Where the sunlight may be very intense, shade trees are planted over many crops. In a remarkable article on traditional mixed cropping methods in India, A. K. T. N. Aider has described how under one system up to three separate age-groups of shade trees are mixed within the same field with each planting of shade tree being followed by its own planting of crop areca. In the desert oases of the Sahara, a similarly three-layered canopy is sometimes produced by planting drought-tolerant date palms to protect the shorter fruit trees, which in turn shade a carpet of vegetables. Shade trees may also serve to support climbing plants such as peppers and betel. Some of the more complex mixtures described by Aiyer have many of the characteristics of natural plant communities; i.e.: stands of individuals of uneven age, and of a wide range of species and growth habit.
Yields of crop mixtures in the absence of disease
Mixed cropping is so widespread that it might be thought that a solid scientific basis for it would have already been discovered. This is not sot The question of how the yields of mixtures compare with yields of pure cultures has really only been answered for mixtures of rather similar components such as varieties of grain crops or species of grasses, and then only in relatively short-lived crops. In a series of experiments involving 139 50:50 two-component mixtures of varieties, 64 per cent of the mixtures yielded more grain than the average yield of their components' pure cultures.3 Out of the dry-master yields of over 300 mixtures of grasses or cereals again about 60 per cent were greater than their average pure culture value.4 Perhaps more interesting, in 37 per cent of the 344 comparisons the yield of the mixture exceeded that of the better pure culture. This sort of advantage, termed "overyielding", is of great interest to agriculturalists, but unfortunately the margin by which the mixtures overyielded was usually not large enough for it to be due to anything but inevitable experimental error.
A few instances of apparently real overyielding by mixtures are however known: strong evidence of overyielding has been found in individual mixtures of rice varieties (overyielding by up to 20 per cent on a series of occasions), barley varieties (by up to 24 per cent in a series of treatments), grass varieties (by up to 15 per cent in two treatments), and flax with linseed (up to 31 per cent in a series of separate experiments). Mixtures of leguminous and non-leguminous species also sometimes overyield by up to about 10 per cent, given the right conditions. With the exception of work on legume-nonlegume mixtures, surprisingly little has been clone to follow up, understand and add to this list of cases of observed positive effects.
When advocates of mixed culture realize that the above results are quite exceptional, they often maintain instead that the real benefit from growing mixtures lies in the greater consistency of overall yield from season to season. According to Aiyer, this seems to be the general belief of peasant farmers; I have heard it myself from a Burgundian farmer standing by his mixed field of rye, 2- and 6-rowed barleys, wheat and oats.
The observation that complex communities like tropical rein forests are more stable in their composition from year to year than simple arctic communities has led ecologists to suggest that the difference is due to the greater diversity of plant and animal species in the rein forest. Although the evidence presented by Sir Charles Elton and others appears compelling, experiment and theories suggest that greater diversity and complexity of themselves lead to less stability. The observed relationship in nature seems therefore likely to be due to some other factor, possibly differences in the stability of the physical environment or the time which has been available for the constituent species to convolve stabilizing characteristics.
Another argument for expecting that mixed cultures should product more consistent yields is based on the differing responses of species to weather conditions. For example 'when maize is thriving, potatoes may look distressed (and vice versa` A combination of the two might be expected to produce at least some crop whether the season is sunny or dull. This sort of "insurance mixture of contrasting component might be recommended where the variation between seasons is so great. that no one species or crop variety performs well in all years. A analogous argument applied to space rather than time justifies the use of oats-barley mixtures in Denmark or problem land which has a mosaic or acid patches; oats thrive on the acid spots where barley fails, and barley dominates elsewhere.
Returning to results of experiments with mixtures of rather similar components, it seems that the stability (consistency) of mixture's yield is, like the yield itself, only rarely greater than that. of the more stable component grown pure but usually greater than the average of the stability of the pure components. Some special conditions under which mixtures might be expected to yield more consistently than pure components will be considered later.
Another sort of stability, about which experimenters seem to know' little, is long-term sustainability or production. Since individual specie make differing demands on the sit and have differing requirements for soil nutrients and other resources continued occupancy of an area by the same species is likely to result in deficiencies (quite apart from; the build-up of pests and diseases) A carefully planned mixture o species is sometimes able to alleviate such deficiencies. Thus, the leguminous groundnut grown between rubber trees is able to control erosion and supply "fixed" nitrogen (see below) to the roots of the rubber; without some input of nitrogen, rubber production fall steadily. Similarly, a shrubby Eupatorium species planted under cinchona and tee in Indonesia benefits the plantation crops by providing mulching material. This safeguards long-term production by controlling erosion and weeds, and by encouraging the turnover of nutrients by litter-decomposing organism.
Interactions between mixture components
In closely-planted agricultural crops, individual plants compete strongly for the supplies of plant growth factors (light, water and nutrients). When alternate plants of a pure culture of crop A are replaced by plants of crop B to make a 50:50 mixture, if plants of A compete more strongly than do plants of B for the growth factor in shortest supply, the plants will grow better in the mixture than they do in pure culture; the reverse will be true of plants of B. Uneven sharing of growth factors between the components of mixtures leads usually to roughly equal percentage increases and decreases in per-plant yield of the components as compared with their performances in pure culture. If the farmer is aiming at a particular proportion of the components' products in the yield, he will therefore need to know the likely effect of competition when deciding what proportions to plant. This type of interaction between components results in overall mixture yields Iying somewhere between the yields of the pure cultures of the components (even if the mixture contains several components). The slight tendency for the higher yielding components to be stronger competitors may explain the tendency for the yield of a mixture to be greater than the average pure culture value.
The tendency for mixture yields to be better than might be expected may also be partly due to differences between the components in the way they exploit the site's growth factors. There are so many human examples of fruitful cooperation between unlike partners that well-chosen combinations of plant species could also be expected to perform better than either alone. One well-tried type of combination contains a leguminous species with one or more nonlegumes. Through the presence of symbiotic bacteria in their roots, plants of leguminous species are able to take in, "fix" and utilize nitrogen from the soil air. Because they draw on this usually unavailable source of nitrogen, leguminous species leave most of the soil's nitrogenous compounds available for use by associated species, Also, as the legume's roots die, the fixed nitrogen becomes available in the soil. Hence, on a soil where nitrate is in short supply, a mixture of clover (a legume) with a grass (non-legume) may overyield. Mixtures such as rubber and groundnut (Malaysia), sugarcane and soybean (India), and cereals and field beans (Greece) are expected to exploit this same principle. Importantly, where there is little nitrogen in the soil, the non-legume component in the mixture often has a much greater protein content than in pure culture under such conditions, a mixture of grass and clover will usually be preferred as fodder to pure grass (or to pure clover, which may cause "bloat").
If the components of a mixture differ in the times at which they make demands for soil nutrients or light, the mixture may use site resources more effectively. Experiments with mixtures of early and fate season potatoes at Wageningen, Holland, have shown overyielding by more than half of 54 mixtures, sometimes by up to 50 per cent. Mixtures of flax (early maturing) and linseed (late maturing) overyield for the same reason.
Another sort of difference which is expected to lead to an advantage for the mixture is the occupation of different layers of the soil by the roots of the components. Aiyer suggested that mixtures should be compounded to exploit the whole depth of soil. Although this point has never been tested directly, in an experiment with mixtures of oat species, I found that 5 out of 5 mixtures overyielded on deep soil whereas only 1 overyielded on shallow soil; the root systems of the components of the best yielding mixture were in fact later shown to occupy different depths in deep soil.4
Tall vegetation often greatly alters the microenvironmental conditions below it and use may be made of this in compounding mixtures for use in harsh climates. The use of shade trees for protection from sun (and drying winds) has already been mentioned but trees may also have other effects. Thus, it has been observed that growth begins earlier in spring under trees in Salamanca probably because the soil there is not so cola at night. The removal of trees from groundnut plantations in Senegal (for the sake of "neatness") seems to have caused planting date to be much more critical than previously; the trees presumably used to moderate the microclimate under them. Taller growing components may act usefully as wind-breaks.
Crop mixtures may also have advantages under certain unfavorable conditions such as frost, lodging and weed infestation. In mixtures of wheat varieties in Czechoslovakia, frost-hardy varieties have been found to protect less hardy ones. Similarly, cereal varieties which do not lodge (get beaten flat) in bad weather, may hold up weakerstemmed components. In an experiment in Adelaide, Australia, a wheat variety which lodged in pure culture was prevented from lodging in all 5 of its mixtures with other varieties. Trials at IRRI in the Philippines have shown that if mung bean is grown mixed with maize, the weedsmothering effect of the mung protects the easily-infested maize
There is growing interest at present among ecologists concerning the possible effects of chemicals released by plants of some species on neighbouring plants (allelopathy). Most of the well-studied examples involve substances which inhibit the growth of other plants of either the same sort (autotoxicity) or of other sorts (allotoxicity). In a mixture of individually autotoxic species, the plants of each component will to some extent escape the inhibitory influence of neighbors of its own species and so mixtures could overyield. Thus, it has been suggested that certain desirable forest trees which suffer badly from autotoxicity in Queensland ought perhaps to be cultivated in mixtures.
Regular overyielding by a certain mixture of rice varieties in India suggests that one variety can sometimes actually stimulate the growth of another. In this case, it was shown that some growth-promoting substance traveled from plants of one component to those of the other. An observation that the presence of certain species of Eucalyptus tree can double the productivity of the pasture in which they stand may be explained by allelopathic stimulation, or, on the other hand by microenvironmental effects. The strongly depressive effects of other Eucalyptus species on pasture growth do not seem to be due t. competition for resources but rather, to allotoxicity. Although the possibility has not been tested, a mixture of allotoxic components would probably yield below the poorer yielding pure culture.
Pests and diseases in crop mixture
A considerable body of traditional fore exists to recommend the "companion" planting of certain crops with other crops. The advantages claimed are usually that the companion plants reduce pest damage in the others. Occasional combinations conferring mutual advantage may be found, such as tomato and asparagus where the tomato will be protected from at least one species of parasitic nematode and the asparagus will be protected from asparagus beetle.
In the tropics where crop pests cause especially serious damage, foresters and planters have long since recognized that individuals of a species in pure culture are often more heavily damaged than individuals of the same species interspersed among individuals of other species. Accordingly, in Brazil the native rubber tree cannot be grown in pure culture although it can survive the level of pest attack suffered in the natural mixed forest.
Agriculturalists are now belatedly coming to recognize the Potentialities of mixed cropping as a powerful and non-polluting means of controlling pests and disease. For instance, recently at the National Institute of Agronomic Research, Paris, the incidence of foot rot in a susceptible wheat variety was found to be halved in a mixture with a resistant variety. Again, in Reconquista, Argentina, it was found that intersowing cotton with maize led to an 80 per cent reduction of pest numbers on the cotton and a doubling of its yield.
To be able to choose combinations which are tolerant of pests and diseases, the grower needs to know something of how an attack in a mixture may differ from one in a pure culture. The presence of two or more kinds of crop has several effects:
1. Fly paper effect. Because many pest and disease organisms tend to be specialised to attack just one or a small group of host species, the individual of other plant species in a mixture constitute a potentially absorptive barrier to movement between those plants which can be attacked. Insect pests usually have a stage in their life cycles where they disperse from their host plants apparently to colonize new ones; at this stage they are often poor at recognizing suitable food plants and steering themselves towards them.
The spores of fungal diseases are passively transported by wind and rain-splash and so are even less likely to find a new host plant. Depending on the proportion of the species in the mixture a fraction of the dispersing individual will be intercepted by non-host plants. Where the dispersing insects or spores cannot "take off" again, they are lost from the population of their species. This loss onto an inert "fly-paper" reduces the rate of build-up of epidemics in mixtures of susceptible and resistant plants.
2. Compensation effects. Where a crop is attacked during vigorous growth, infected plants compete less strongly than healthy plants for growth factors. Plants surrounding a diseased individual therefore yield more than otherwise and compensate to some degree for the longer yields of the attacked plants. A mixture of crops which differ in their susceptibility to a series of diseases may thus produce total yields which are more consistent than those of any of the pure cultures if the various diseases are favored by different types of season.
3. Microenvironmental effects. The presence of companion plants creates a microenvironment for the susceptible crop which differs from that found in pure culture. This different environment may affect the host-parasite relationship in subtle ways:
(a) By acting on the potentially attacked component changing its susceptibility (from that in pure culture). For example, banana crops under shade trees in Malaysia are less attacked by the most damaging of a series of leaf-spot disease (although they are more susceptible to the less damaging ones); the leaves of coffee grown under shade trees in Indonesia provide a less suitable diet for woolly aphids and hence are less attacked.
(b) By acting directly on the attacking organism. For example, where broad-leafed trees grow within stands of spruce, the higher humidity (or possibly longer temperature) of the air is unfavorable to the growth of spruce-bud worm; cocoa under shade trees in Ghana is less attacked by mistletoe because this parasite requires high light intensifies for the establishment of its seedlings; the odor of shallots (more effective than onions) prevents the carrot root fly from finding inter-planted carrots; wheat without awns (sharp projections from the ear, as in barley) is protected from birds by being mixed with an awned variety.
(c) By influencing the populations of the natural enemies of the attacking organism. For instance, in the Philippines, the corn-stalk borer is less abundant in maize-groundnut mixtures because spiders which prey upon it are more numerous in such mixtures than in pure maize; citrus under shade has leaves with a thinner cuticle which allows leaf miners within them to be reached more easily by parasitic wasps; blackcurrant bushes planted in Californian vineyards support an alternate host for a parasite of a pest of the vines thus increasing its effectiveness in controlling the pest; buckwheat planted among broad beans is said to attract hover flies which prey on the bean aphids.
That most of the example available show improved control of attacking organisms may be the consequence partly of the greater interest in well-tried, successful mixtures, and partly of the lesser emphasis which negative results naturally receive in reports. Nevertheless, crop mixtures seem to have potential where crops are threatened by pest and diseases.
Multilines and disease
When a crop variety which has a new gene for resistance to some disease begins to be widely used, individual races of the disease organism appear which have a virulence gene which overcomes this resistance. Such races multiply rapidly on the variety. Sometimes, disease races with the necessary virulent gene are very rare at the time of introduction of the new resistance gene and so this resistance gene will protect the new variety for several years before the virulent races have built up to a destructive lever. Such effective resistance genes are called ''strong''. A resistance gene for which corresponding virulent races arc already present in quantity at the time of its introduction will be ineffective and "weak".
Since the Green Revolution is likely to increase the rate at which resistance genes are overwhelmed by virulent races and since in some crops there is already a shortage of new strong genes, we need to find how best to deploy our limited supply of resistance genes. As part of a possible best strategy, the growing of "multiline" varieties has been suggested as a means of bringing the crop into a stable equilibrium with the disease races.'° A multiline variety is a mixture of genetic types (lines) of a crop similar in growth characteristics but which differ in the resistance genes which they carry. Such varieties have already been produced for wheat (in Colombia) and oats (in Iowa, USA).
Since mixed cropping often involves staggered plantings and selective harvesting it tends to be labour intensive. If it is soundly practiced it may require less pesticide, weedkiller and fertilizer, and so be a lowpolluting method of farming.
To appreciate the arguments in favour of multiline varieties (multilines), it is necessary to understand why some resistance genes appear strong and others weak: at the time of its introduction, a gene will seem strong if the disease races carrying the virulence gene to overcome it have been at a competitive disadvantage compared with the races not carrying the virulence gene, as they grew together on the old varieties. In simple terms, on the old varieties, this surplus virulence gene in a disease race conferred a disadvantage which kept it rare. The greater this disadvantage, the stronger the corresponding resistance gene in the crop would appear to be.
If we consider now a multiline made up of four fines where each carries one of a series of new resistance genes, A, B. C and D, each fine will only be attacked by races which have acquired through mutation the virulence gene, a, b, c or d, corresponding to that line's resistance gene. Each of these races will face a strong fly-paper effect which will limit its multiplication and thus the harm it can do. If for some reason one race becomes relatively common, the damage it inflicts o n the corresponding line will be to some extent compensated for by better growth of the other components. However, if further virulence genes are acquired through mutation, races such as ab, bc, a ac, will appear. Being able to attack two components of the multiline instead of only one, the fly-paper effect on them will be weaker, the multiplication rates greater and the opportunity for further mutation greater. When a super-race, abcd, finally produced, all components of the multiline will be susceptible to it .
There are unfortunately some uncertainties in this attractive picture:
1. For some diseases, too few strong resistance genes are available in the crop for a multiline to be made (e.g. in potatoes against fate blight). However, the production of new resistance genes by, say, irradiation, or the use of new techniques of hybridization may make more genes available.
2. High strength in the resistance genes is vital for the success of a multiline but unfortunately strength does not depend only on the resistance gene itself. Given a different set of "old varieties" in the explanation above, the same resistance gene might for example appear weak when introduced.
Since the nature of the fines chosen to carry the various resistance genes will affect the apparent strengths of these genes in the multiline, the choice of fines could be crucial. At present the fines to be used are chosen for their agronomic performance; it seems important to select them also for their ability to maximize the apparent strengths of the resistance genes.
3. While it is known that the presence of non-virulent spores on a leaf can sometimes prevent infection by virulent ones ("cross-protection"), the presence of virulent spores can sometimes make non-virulent ones cause infections ("potentiation"). While cross-protection could increase the resistance of a multiline to attack, potentiation could decrease it. Too little is known about these phenomena to yet say which is the commoner.
4. Although experiments have measured disadvantages to races carrying many virulence genes (potential super-races) when mixed with races carrying few virulence genes, it is not clear yet whether this disadvantage is strong enough to balance the advantage to the former of their wider host range. Mathematical models now being developed may be able to show whether any equilibrium will be at a low enough lever of infection for equilibrium is not wanted if it leaves our crops regularly devastated!
5. While a multiline can be reconstituted each year to meet shifts in the relative frequency of the attacking races, an optimized set of rules needs to be worked out to replace the rule of thumb used so far in Colombia and Iowa. There are opportunities for the application of control engineering techniques in disease and pest management.
Crop mixtures and the future
This article has shown that in a series of cases mixed cropping has biological advantages over the use of pure cultures; multilines may also have advantages. However, in real life, it usually is not biological but economic advantage which decides what farming and cropping systems are actually used. Since mixed cropping often involves staggered plantings and selective harvesting it tends to be labour intensive. If it is soundly practice, it may require less pesticide, weedkiller and fertilizer, and so be a lowpolluting method of farming. Where there is rural unemployment, where capital is in short supply and where production must b e sustainable without expensive fossil fuels and pollution control, mixed cropping is a possible solution. Thus in Nigeria and Malaysia, rubber planters are being advised to interplant their rubber with cash crops to raise capital for replacing old stands. The jobs so created can help to slow or reverse the drift to the towns.
It has been emphasized already that for mixed cropping to be biologically advantageous, the mixture components need to be chosen with care. Unfortunately, the interactions among the plants, animals and microorganisms in a crop are so subtle and specific to particular locations that present knowledge only provides a rough guide as to what new combinations of crops and varieties should be tried. If then the possible advantages of mixed cropping are to be exploited, local experimentation will be needed, using a range of possible components and a series of seasons. In the search for "ecological combining ability'', the traditional combinations should perhaps be evaluated first, as at the International Rice Research Institute in the Philippines. Better, more compatible components for mixtures are being actively sought in many research centres. Examples where ranges of types are under test include trials of shade trees for cocoa (Sarawak) and for tee (India), of intercrops for rubber (Malaysia), of grasses for hay mixtures, (UK, USA, USSR) and even of strains of nitrogen-fixing bacteria for introduction into the legume component of grass-legume mixtures ( Australia). If the difficulties of managing diversity in the crop field can be overcome, diversity in this and other forms will help to safeguard our crops against pests and epidemic disease. Unless the Green Revolution changes course, much of the world's green could turn to rust-red almost overnight.
I. Allaby, M. (I 972 ) Miracle rice and miracle locust Ecologist 3, 180-186.
2. Aiyer, A. K Y. N. (1949 ) Mixed cropping in India. Ind. J. Agr, Sci. 19, 439-454.
3. Clay, R. E. (1967) A comparison of the performance of homogeneous and heterogeneous barley populations. PhD. Thesis, Univ. of California.
4.Trelibath, B. R. (1974) Biomass productivity of mixtures. Adv.. Agron. 26, 177-210.
5.Elton, C. S. (1968 ) The ecology of invasions by animals and plants. Methuen London.
6. May, R. M. (197 3 ) Stability and complexity in model ecosystems. Princeton UniversitY Press.
7. Marshall, D. R., & Brown, A. H. D. (1974 ) Stability of performance of mixtures and multilines. Euphytica 22, 405-412
8.Sibma, L Unpublished data.
9.Philbrick, H. and Grege, R. B. (1966) Companion plants, and how to use them. Robinson and Watkins, London.
10.Van der Plank, J. E. (1968 ) Disease resistance in plants. Academic Press. New York.
11.Harper J. L. (1967) A Darwinian approach to plant ecology. J. Ecol. 55,
Copyright © 1997 Ecological Agriculture Projects. Reprinted with permission. All rights reserved.
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