Intercropping

Intercropping is the practice of growing two or more crops together in the same field. Intercropping may provide benefits such as increased yield or decreased pest infestations. Historically, peasant farmers around the world have intercropped. However, since the advent of industrialized agriculture, intercropping has often been abandoned in favor of monoculture.

Weed Suppression

 * "Intercrops 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 weeds or convert resources to harvestable material more efficiently than sole crops. Weed suppressive intercropping systems include associations of maize-navy beans, sorghum-cowpea, barley-red clover, cassava-bean, pigeon pea-sorghum, cowpea, barley, and red clover."

Measuring Intercropped Yields
Crop yields in intercropped fields can be measured using the Land Equivalent Ratio (LER), "the ratio of the area under sole cropping to the area under intercropping needed to give equal amounts of yield at the same management level. It is the sum of the fractions of the intercropped yields divided by the sole-crop yields." In other words, if one acre of monoculture can produce 150 bushels of corn or 40 bushels of beans, but one acre of intercropped corn and beans produces 90 bushels of corn and 30 bushels of beans, then the LER is calculated as (90/150) + (30/40) = 1.35. Therefore, one acre of intercropped corn and beans produces as much yield as 1.35 acres of monoculture split between corn and beans. Scientists test various crop combinations to calculate their LER in order to determine which crops are the most productive when intercropped.

A common mistake involves judging crop yields for intercropped fields without calculating LER. When one does that, it can appear that crop yields actually decrease with intercropping, not increase. In the example above, a farmer obtained the equivalent of 1.35 acres of corn and beans from one intercropped field. However, if one solely compares an acre of corn alone with an acre of corn intercropped with beans, they may say that 90 bushels of corn (in the intercropped example) is less than 150 bushels of corn grown alone, and therefore intercropping decreases yield. This is inaccurate.

Intercropping in Mexican Milpas
One of the most famous intercropped combinations is corn, beans, and squash, which are planted together in Mexican cornfields, known as milpas. "In a series of studies of the corn-bean-squash polyculture, done in Tabasco, Mexico, it was shown that corn yields could be stimulated as much as 50% beyond monoculture yields when planted with beans and squash using techniques of local farmers and planting on land that had only been managed using local traditional practices. There was significant yield reduction for the two associated crop species, but the total yields for the three crops together were higher than what would have been obtained in an equivalent area planted to monocultures of the three crops." The reason for the yield increases include:
 * "In a polyculture with corn, beans nodulate more and are potentially more active in biological fixation of nitrogen.
 * "Fixed nitrogen is made directly available to the corn through mycorrhizal fungi connections between root systems."
 * "Net gains of nitrogen in the soil have been observed when the crops as associated, despite its removal, with the harvest.
 * "The squash helps control weeds: the thick, broad, horizontal leaves block sunlight, preventing weed germination and growth, while leachates in rain washing the leaves contain allelopathic compounds that can inhibit weeds.
 * "Herbivorous insects are at a disadvantage in the intercrop system because food sources are less concentrated and more difficult to find in the mixture.
 * "The presence of beneficial insects is promoted due to such factors as the availability of more attractive microclimatic conditions and the presence of more diverse pollen and nectar sources."

However, when the same corn-bean-squash polyculture was planted in a field that had been cultivated with mechanical cultivation, synthetic chemical fertilizers, and pesticides for 10 years previously, the yield advantages disappeared. "Apparently, the positive interactions that occurred in the traditional farm field were inhibited by the alteration of the soil ecosystem, which occurred with conventional inputs and practices."

During the Green Revolution, agronomists encouraged farmers to abandon intercropping in favor of monocultures, but peasant farmers, by and large, continued to practice intercropping.


 * "Mangelsdorf knew that many campesinos intercropped corn and beans, and the MAP's scientists too this "ancient custom" into account when they began working on beans, believing that "it was likely to persist for generations to come," but they also said that it lowered yield of both crops. Bean research targeted commercial growers who monocropped and used insecticides. The MAP did not create seeds to solve the problems of peasant farmers."

Push-Pull Method in Africa
The following describes a system used by over 30,000 farmers in East Africa where it has increased maize yields from less than 1 ton per hectare to 3.5 tons per hectare:
 * "Lepidopteran stemborers and parasitic weeds in the genus Striga are major constraints to efficient production of cereals, the most important staple food crops in Africa. Smallholder farmers are resource constrained and unable to afford expensive chemicals for crop protection. Development of a push–pull approach for integrated pest and weed management is reviewed here. Appropriate plants were discovered that naturally emit signalling chemicals (semiochemicals). Plants highly attractive for egg laying by stemborer pests were selected and employed as trap crops (pull), to draw pests away from the main crop. Of these, Napier grass, Pennisetum purpureum (Schumach), despite its attractiveness, supported minimal survival of the pests’ immature stages. Plants that repelled stemborer pests, notably molasses grass, Melinis minutiflora P. Beauv., and forage legumes in the genus Desmodium, were selected as intercrops (push). Desmodium intercrops suppress Striga hermonthica (Del.) Benth. through an allelopathic mechanism. Their root exudates contain novel flavonoid compounds, which stimulate suicidal germination of S. hermonthica seeds and dramatically inhibit its attachment to host roots. The companion crops provide valuable forage for farm animals while the leguminous intercrops also improve soil fertility and moisture retention. The system is appropriate as it is based on locally available plants, not expensive external inputs, and fits well with traditional mixed cropping systems in Africa."

Related Sourcewatch articles

 * Agroecology
 * Organic Agriculture
 * Food Sovereignty
 * Organic Agriculture in Cuba During the Special Period
 * Rockefeller Foundation
 * Mexican Agricultural Program
 * Green Revolution

External Resources

 * Ong, C.K. R.M. Kho, and K. Onnesstraat. 2004. Ecological interactions in multispecies agroecosystems: concepts and rules. In M. van Noordwijk, G. Cadish, C.K. Ong (eds.), Below-ground Interactions in Tropical Agroecosystems: Concepts and Models with Multiple Plant Components. pp. 1-16. CABI Publishing: Cambridge, MA.
 * Vandermeer, J. 1989: The Ecology of Intercropping. Cambridge University Press: New York.
 * Hart, R.D. 1986. Ecological framework for multiple cropping research. In C. A. Francis (ed.) Multiple Cropping Systems. pp. 40-56. MacMillan: New York.
 * Hart, R.D. 1984. Agroecosystem determinants. In R. Lowrance, B.R. Stinner, and G.J. House (eds.), Agricultural Ecosystems: Unifying Concepts, pp. 105-119. John Wiley and Sons: New York.

External Articles

 * Hauggaard-Nielsen H. and E. S. Jensen. 2005. Facilitative root interactions in intercrops. Plant and Soil 274(1-2): 237-250.
 * Santalla M., J. M. Amurrio, A. P. Rodino, and A. M. de Ron. 2001. Variation in traits affecting nodulation of common bean under intercropping with maize and sole cropping. Euphytica 122(2): 243-255.
 * Maingi, J. M., C. A. Shisanya, N. M. Gitonga and B. Hornetz. 2001. Nitrogen fixation by common bean (Phaseolus vulgaris L.) in pure and mixed stands in semi-arid south-east Kenya. European Journal of Agronomy. 14(1):1-12.
 * Bethlenfalvay, G. J., M. G. Reyes-Solis, S. B. Camel, and R. Ferrera-Cerrato. 1991. Nutrient transfer between the root zones of soybean and maize plants connected by a common mycorrhizal inoculum. Physiologia Plantarum 82:423-432.
 * Boucher, D. and J. Espinosa. 1982. Cropping systems and growth and nodulation responses of beans to nitrogen in Tabasco, Mexico. Tropical Agriculture 59: 279-282.