[Maize, soybean and cowpea production when planted in association with grasses in the humid tropics]

Two experiments were carried out in the humid tropics of Costa Rica to evaluate the effect of competition between grasses and annual grain crops when grown simultaneously and in alternate rows, as an strategy to cover the cost of reclaiming degraded pastures dominated by native grasses (mainly Axonopus compressus). The present study only reports the results obtained with annual crops (see Perez et al., 1993 and Duarte et al., 1994 for the data on grass yield). In experiment 1, soybean cv. IAC-8 and Jupiter were sown at a density of 200,000 plants/ha (0.5 x 0.1 m) in monoculture and intercropped with Brachiaria brizantha cv. Marandu or B. dictyoneura CIAT 6133, both at a density of 40,000 plants/ha (0.5 x 0.5 m). To define the growth pattern of all crops, total aerial biomass and leaf area were measured at five phenological stages. A randomized complete block design, with six treatments (2 soybean genotypes x 3 cropping systems) and three replications in time was used. Grain yields for soybean var. IAC-8 was 2.71 t/ha when grown in monoculture, but decreased by 12.7 percent and 52.0 percent when associated with B. dictyoneura and B. brizantha, respectively. In experiment 2, maize cv. Tuxpeno or cowpea (Vigna unguiculata) cv. Chiricanowere intercropped with either B. dictyoneura CIAT 6133 and B. brizantha cv. Marandu or dwarf elephant grass (Pennisetum purpureum cv. Mott). Planting densities (plants/ha) were: maize, 40,000 (1.0 x 0.5 m, 2 plants/site); cowpea, 160,000 (0.5 x 0.25 m, 2 plants/hole); dwarf elephant grass, 20,000 (1.0 x 0.5 m); and Brachiaria, 40,000 (0.5 x 0.5 m). The crop variables measured and the experiment design used were similar to those described for experiment 1, except that four replicates were considered in this experiment. Independent statistical analysis was also run for maize and cowpea data. In both crops, grain yields and total aerial biomass were not affected by any of the intercropping systems studied. Dry grain yields (14 percent moisture) were 5.95 and 2.56 t/ha for maize and cowpea, respectively. The highest decreases in grain yield, due to the association with B. brizantha, were only 7 percent and 6 percent for maize and cowpea, respectively. Although the grain yield required to cover all costs of degraded pasture reclamation were widely overcome by all cropping systems inciuding corn and cowpea (the break even points were: 3.75 and 1.44 t/ha, respectively); only those with corn as the financing crop seem feasible, since the detrimental effects on grass growth were quite lower for this crop than that exert by soybeans or cowpea.