A field experiment was performed at the Agronomy Experimental Station of UFRGS, in Guaíba, RS, Brazil, during the 1979/80 growing season, in order to test four soybean cultivars (Paraná, Prata, Hood e IAS-4) in competition with Euphorbia heterophylla L. established in three densities (0, 12 and 54 plants/m2) during two periods (45 and 115 days after soybean emergence). It was observed that soybean cultivars presented reduction of seed yields under the effect of the pe riods of competition and densities of Euphorbia heterophylla L. The number of seeds and pods per area were considerably reduced in the presence of the weed associated with the longest competition period. The stem diameter and number of nodes were reduced only by competition of the highest Euphorbia density; however, the number of branches and leaf area index, this after a period of 75 days of crop-weed association, were already reduced by a density of 12 plants/m2 of Euphorbia heterophylla L.
Weeds; competition; Euphorbia heterophylla L.; soybean; cultivars