Crop-weed interference as influenced by a leguminous or synthetic fertilizer nitrogen source .2. Rotation experiments with crimson clover, field corn, and lambsquarters

Field studies were conducted from 1989 to 1991 to assess the effects of N source (incorporated crimson clover (Trifolium incarnatum L.) residue vs. synthetic fertilizer) on crop-weed interference in a corn (Zea mays L.) silage production system. Two series of experiments were undertaken: in corn and lambsquarters (CLQ) experiments, crimson clover or oat (Avena sativa L.) green manure crops were followed the next year by field corn grown alone or with lambsquarters (Chenopodium album L.) and other weeds; in lambsquarters (LQ) experiments, clover and oat crops were followed by lambsquarters and other weeds grown without corn. In the second year of each rotation sequence, ammonium nitrate fertilizer was applied at several rates to the oat treatment preceding corn or lambsquarters planting, allowing determination of the clover's N fertilizer equivalency value (52 kg N ha(-1)) and subsequent contrast of the clover treatment with a comparable rate of N fertilizer addition (45 kg N ha(-1)). In the 1990 LQ study, lambsquarters drymatter accumulation at final harvest was 34% lower in the clover than the fertilizer treatment. Nitrogen source did not significantly affect lambsquarters growth in the 1991 LQ experiment. Neither corn nor lambsquarters drymatter accumulation differed between fertilizer and clover treatments in the 1990 CLQ experiment. However, at final harvest in the 1991 CLQ experiment, lambsquarters and corn drymatter under weed infested conditions were 65% lower and 131% higher, respectively, in the clover than the fertilizer treatment. Despite the substantially better performance of legume-grown than fertilizer-grown corn under weedy conditions in the 1991 CLQ experiment, corn yield in the legume treatment was still reduced by 59% due to weed interference. Weed control derived from use of a crimson clover N source in these rotation experiments was less consistent and less effective than in previous experiments conducted within a doublecropping system, a result which may stem from the incorporation of winterkilled crimson clover in the rotation experiments as opposed to that of immature, freshly mown crimson clover in the doublecropping study.