YIELD POTENTIAL OF INTERPLANTED ANNUAL FOOD CROPS IN SOUTHEASTERN UNITED-STATES

Intercropping may be 1 way to increase the productivity of land. If so, it is of scientific as well as practical interest to know how the increase is achieved and devise methodologies which will exploit such systems to the fullest. This report compares the productivity of 2 component intercrops with that of their respective mono-cultures under differing N regimes. Two soil conditions were tested in each of 2 yr, a poorly drained Portsmouth fsl (Typic Umbraquult, fine-loamy, mixed thermic) and a well drained Orangeburg sl (Typic Paleudult, fine-loamy, silicious, thermic). Treatment variables were cropping system (intercrops vs. monocultures), row arrangement, and N rates from zero to 270 kg/ha. Corn (Zea mays L.) was used as the overstory crop and soybeans (Glycine max L. Merr.), snapbeans (Phaseolus vulgaris L.), or sweet potatoes (Ipomoea batatas Lam) as the interplanted understory species. All interplanted species were at the same population as their monoculture checks. With adequate N, monoculture corn in conventional rows (97 cm) averaged 9400 kg/ha. Planting monoculture corn in paired 46-cm rows, with 147 cm between row pairs, caused a 12% reduction in yield. An additional reduction of 5 to 10% resulted when soybeans or snapbeans were interplanted into corn in the 147-cm space between row pairs. Monoculture soybeans yielded 2750 kg/ha with only minor variations among sites. Interplanted soybean yields were site and year dependent, however, and varied from 33 to 55% of their respective monocultures. Intercrop soybean yields were inversely related to the amount of N applied to corn. Yields of spring-planted monoculture snapbeans ranged from 7000-15,500 kg of fresh pods/ha. Regardless of yield level, however, interplanted snapbeans yielded 48% of the monoculture check. Fall snapbeans, relay interplanted into maturing corn in mid-August, also produced about 50% of a crop. Land equivalent ratios (LER) of 1.20 to 1.40 thus suggest a 20 to 40% increase in total productivity during one 200-day season by intercropping. Higher production by intercrops was most easily interpreted in terms of total leaf exposure. Monoculture corn had a leaf-area duration (LAD) of 150 days; but when soybeans were interplanted into corn, LAD of the intercrop exceeded 300 days.