The objective of this trial, carried out in Cascavel (PR), Brazil, was to study the weed competition period over some productives parameters of soybean (Glycine max). The experimental design used was the randomized complete block with three replications and the treatments were divided into two groups: in the first the crop was maintained free of weed competition and in the second it one was maintained in weed competition. The periods extensions studied were 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 days after soybeans emergence to two groups. Alexandergrass (Brachiaria plantaginea) was the most important weed of experimental area. The weed community affected most of the parameters in studied crop. Soybeans had to be maintained weed interference free for 20 days and could be in weed interference for 30 days after soybeans emergence to prevent yield loss.
Competition; critical periods of interference; interespecific competition; Brachiaria plantaginea; Glycine max