Does intercropping have a role in modern agriculture?

I ntercropping—growing two or more crops at the same time on a single field—is an ancient practice still used in much of the developing world. For example, in Africa, corn (Zea mays L.), sorghum (Sorghum bicolor (L.) Moench), or millet (Panicum and Pennisetum spp.) are grown with pumpkin (Cucurbita spp.) cowpeas (Vigna unguiculata (L.) Walp), pigeon peas (Cajanus cajan (L.) Millsp.), or beans (Phaseolus spp.). Cocoa (Theobroma cacao L.) is grown with yams (Dioscorea spp.) or cassava (Manihot esculenta Crantz). In the tropical Americas, maize (corn) is grown with beans and squash (Cucurbita spp.). In both Africa and Latin America, beans or peas (Pisum sativum L.) climb tall cornstalks while pumpkins or squash cover the ground below. In these countries, many farmers have limited access to agricultural chemicals and equipment so prevalent in the developed world. Besides, intercropping is much less risky in that if one crop fails another or the others may still be harvested. Before the 1940s in the United States and Europe, growing more than one crop in the same field was common practice (Kass 1978; Andersen 2005), again because there was less risk. But with mechanization and the availability of relatively cheap synthetic fertilizers and pesticides, monocropping—i.e., growing only one crop in a field at a time—became the economically efficient way to go (Horwith 1985). No longer was it necessary to grow a legume with a grain to provide nutrients needed by the latter. Under monocropping, synthetic fertilizer-intensive regime, crop yields increased dramatically. US corn yields increased from 1.9 Mg ha–1 (30 bu ac–1) in the 1940s to 9.7 Mg ha–1 (154 bu ac–1) in 2008. As machines were developed for various single cash crops, intercropping became impractical. These new modern farming methods were also spread to parts of the developing world as high-yielding varieties were developed and fertilized to bring about the Green Revolution that could feed rapidly growing populations. Global fertilizer use increased from 24.5 million Mg (27 million tn) in the late 1950s to 210.5 million Mg (197 million tn) in 2007–2008, according to Food and Agricultural Organization data. Of this composite fertilizer mix, 65% was nitrogen, 19%, phosphorus, and 16% potassium. And worldwide demand for fertilizer is still rising, albeit at a slower rate due to a somewhat increased nutrient use efficiency. But now, fertilizer shortages are developing and costs are escalating. The composite fertilizer price increased 113% between 2000 and 2007, led by gains in nitrogen prices (Huang 2007). The US price of ammonia increased from $250 Mg–1 ($227 tn–1) in 2000 to $474.4 Mg–1 ($523 tn–1) in 2007, while urea (the main solid US fertilizer form) changed from $181.4 Mg–1 ($200 tn–1) to $410.9 Mg–1 ($453 tn–1) (Huang 2007). Meanwhile, environmental problems associated with heavy fertilizer use are becoming well known—e.g., surfaceand groundwater pollution, soil acidification, and ammonia volatilization. And as synthetic fertilizer is a petroleum-based product, prices will continue to increase, while their manufacture contributes to greenhouse gas emissions. Also, the lack of diversity in monoculture fosters weed problems, as well as increased insect pressure. The latter problem is partly because of monoculture’s less diverse insect community that includes fewer or no pest predators (Horwith 1985; Horrigan et al. 2002). In addition, potent insecticides that kill both pests and their natural enemies are currently being used. US synthetic pesticide use increased 33-fold since 1945, but despite this substantial pesticide cost and use, crop yields continue to be threatened by weeds, insects, and disease. Reasons include built-up pesticide resistance, outbreaks of secondary pests, and susceptibility in the plants (Brenner 1991). As these and other problems with monoculture farming become more apparent, “sustainability” is becoming a household word, and interest in intercropping is growing as, possibly, part of the solution. Already double cropping is being widely practiced as an alternative to monoculture. In the Midwest, soybeans are being rotated with corn. While soybean itself is a valuable crop, it fixes nitrogen for the next year’s corn crop. Similarly, across the nation and beyond, grain crops are rotated with doi:10.2489/jswc.64.2.55A