Crop-weed interference as influenced by a leguminous or synthetic fertilizer nitrogen source .1. Doublecropping experiments with crimson clover, sweet corn, and lambsquarters

Field experiments were undertaken to assess the effect of N source (incorporated legume residue vs. synthetic fertilizer) on crop-weed interference. In a 2 year study, a doublecropping system was used in which a crimson clover (Trifolium incarnatum L.) green manure was followed by a crop of sweet corn (Zea mays L.) grown alone or with lambsquarters (Chenopodium album L.). Inclusion of several rates of ammonium nitrate fertilizer in the experiment allowed determination of the clover's N equivalency value (55 kg N ha(-1)) and contrast of the clover treatment with a comparable rate of N fertilizer addition (45 kg N ha(-1)). Soil NO3-N concentration in the experiment at one week after corn and lambsquarters planting was 52% lower in the clover than the fertilizer treatment. Differences in nitrate levels between the two treatments tended to decrease at subsequent sampling dates. At two weeks after emergence, drymatter accumulation of lambsquarters was 72% lower in the clover than the fertilizer treatment and remained 39% lower at final harvest. In contrast, sweet corn biomass accumulation in the clover treatment was 31% lower than in the fertilizer treatment at 2 weeks after emergence but recovered to levels attained in the fertilizer treatment as the growing season progressed. As a result of reduced lambsquarters growth, loss of corn drymatter accumulation to weed interference was 8% in the clover treatment as compared to 28% in the fertilizer treatment. Results of a second experiment in which crimson clover was followed by lambsquarters grown alone also showed a weed suppressive effect of the legume N source in comparison to use of fertilizer N. These experiments demonstrate that use of legume green manure has the potential to reduce the need for herbicide as well as synthetic fertilizer applications in subsequent crops.