An experiment was carried out to evaluate the relative weed species competition with two soybean cultivars of different maturation times, Embrapa-48 (precocious) and Embrapa-62 (median cycle), in Londrina, Parana, Brazil in 1998/99. Four weed species were compared: wild poinsettia (Euphorbia heterophylla), alexander grass (Brachiaria plantaginea), morning glory (Ipomoea grandifolia) and sicklepod (Senna obtusifolia), adjusted to the weed densities of zero, 15, and 30 plants m-2. The experiment was a randomized block design, in sub plots in a 4x3x2 factorial, with four replications. Soybeans and weeds emerged almost simultaneously. Both cultivars responded similarly to competition regarding yield. Productivity of Embrapa-48 was 2,819 kg ha-1 and of Embrapa-62 was 2,565 kg ha-1, with no weed competition. Relative intensity of competition between weed species and soybeans were: B. plantaginea (0.35) < I. grandifolia (0.59) < E. heterophylla (0.61) < S. obtusifolia (1.00). The estimated soybean yield losses (Y%) were accomplished by linear regression/cultivar per plant m-2: B. plantaginea (Y E48 = -1,47; Y E62 = -1,58; I. grandifolia (Y E48 = -2,51; Y E62 = -2,67), E. heterophylla (Y E48= 2,47; Y E62 = -2,83) and S. obtusifolia (Y E48 = -4,52; Y E62 = -4,21). Soybean yield loss equation adjustments to weeds are discussed.
Euphorbia heterophylla; Brachiaria plantaginea; Ipomoea grandifolia; Senna obtusifolia