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Traditional weed control practices in Mexico use legumes as cover crops or manures. Legumes used in these practices play a dual role in agroecosystems by protecting the soil from erosion and by enriching it with organic matter and N through Rhizobium symbiosis. Farmers in the tropical regions of Mexico use Mucuna spp., Canavalia spp. and other legumes to control weeds in their fields. We conducted in vitro bioassays and greenhouse experiments to evaluate the toxic effect of four legumes velvetbean [Mucuna deeringiana (Bort) Merr.], jackbean [Canavalia ensiformis (L.) DC.], jumbiebean [Leucaena leucocephala (Lam.) de Wit], and wild tamarind [Lysiloma latisiliquum (L.) Benth.] on weed growth, and on the survival of insects and nematodes. The aqueous leachates (1%) of the four legumes were tested on three test plants by seed germination and radicle growth bioassays in petri dishes. The aqueous leachates of all four legumes exhibited strong phytotoxic effect on the radicle growth of the test plants. The effects of velvetbean and jackbean leachates were also evaluated on the survival of phytopathogenic nematodes. Both leachates had nematotoxic effects. Greenhouse experiments were conducted to evaluate the effect of the four legumes dried leaves incorporated as mulches to potting soil, on the number and biomass of weeds, and on the biomass of tomato (Lycopersicon esculentum L.) plants. The decomposition of velvetbean and jackbean leaves in potting soil significantly reduced (>50%) the development of phytopathogenic nematodes in the roots of tomato. A 5-yr field experiment conducted at the University of Yucatan (UADY) evaluated the effect of velvetbean and jackbean used as living cover crops, and jumbiebean and wild tamarind used as dead mulches incorporated on soil surface, on weed growth, and corn (Zea mays L.) yield. The experimental field was treated with the traditional slash and burn system in February 1994. In July 1994 the experiment was performed using the local agricultural practices in a complete randomized block design with three repetitions. The treatments were: corn+velvetbean, corn+jackbean, corn+jumbiebean, corn+wild tamarind, corn+Paraquat (1,1'-dimethyl-4,4'-bipyridinium ion) herbicide, corn weeded by hand, and plots without corn weeded by hand. The number, biomass, diversity, and relative importance of weeds, as well as corn yield, were evaluated. In addition, taxonomic composition of weeds was determined. All legumes reduced weed growth with velvetbean (as living cover crop) producing the largest weed biomass reduction (68%). These legumes also improved the yield of corn during the first 2 yr of the experiment. For better management of natural resources, the use of legumes as biological tools in agriculture to control weeds and improve soil conditions should be encouraged through coordinated efforts between farmers, academic, and governmental institutions.

Evaluación de tres leguminosas como coberturas asociadas con maíz en el trópico subhúmedo. El objetivo fue evaluar el potencial de tres leguminosas como coberturas para mejorar el sistema de producción de maíz. Se evaluaron cuatro tratamientos: 1) maíz solo; 2) maíz y Phaseolus lunatus de ciclo corto de semilla blanca 3) maíz y Phaseolus lunatus de ciclo largo de semilla blanca y 4) maíz y frijol terciopelo (Mucuna sp.) en un diseño experimental de bloques al azar, con cuatro réplicas, durante cuatro ciclos de cultivo (1999-2002) en Yucatán, México. Se midió la biomasa de cultivos, pH, N total, mineralización potencial anaerobia del nitrógeno, evolución del CO2, potasio (K), fósforo (P) y materia orgánica del suelo, biomasa y frecuencia de arvenses. La cantidad de rastrojo en los tratamientos con leguminosas fue: 3436, 3425 y 4018 kg MS /ha para el primero, segundo y tercer año, respectivamente. El nitrógeno aportado por los rastrojos al maíz fue 25,8; 31,9; 52,8 y 43,4 kg/ha para los tratamientos 1, 2, 3 y 4, respectivamente. El rendimiento de grano de maíz fue en promedio 841, 843 y 460 kg MS /ha para el primero, segundo y tercer ciclo, respectivamente; sin diferencias entre tratamientos. En el tercer ciclo, el frijol terciopelo disminuyó la biomasa de arvenses de 126 a 58 g MS /m². Las coberturas ejercieron poca influencia en las variables de suelo y rendimiento de grano de maíz. Evaluation of three legumes as cover crops with maize under subhumid tropic. The objective of this work was to evaluate the potential of three legumes as cover crops to improve maize production systems. Four treatments were evaluated: 1) control, maize in monoculture; 2) maize - short-cycle, white seed lima bean (Phaseolus lunatus); 3) maize - long cycle, white seed lima bean Phaseolus lunatus), and 4) maize - velvet bean (Mucuna sp.), in a randomized complete blocks design with four replications during four growing cycles (1999-2002), in Yucatán, México. We measured: crop biomass, pH, total N, potential anaerobic mineralization of N, CO2 evolution, potassium (K), phosphorus (P), and soil organic matter, and biomass and frequency of weeds. Legume stubble residues accumulated on maize crops were: 3436, 3425 and 4018 kg DM/ha for the first, second and third year, respectively. Nitrogen contributed by stubble material was 25.8, 31.9, 52.8, and 43.4 kg/ha in treatments 1, 2, 3, and 4, respectively. Maize grain yield was 841, 843, and 460 kg DM/ha in the first, second and third year, respectively, without significant differences between treatments. During the third year, velvet bean caused a reduction in weed biomass from 126 to 58 g DM/m². Cover crops had little influence on soil variables and maize grain yield.