Soybean and annual weed competition studies were conducted over 2 yr in Balcarce, Argentina, to define the critical period of weed control and to compare yield losses in soybean, when weeds competed in the total cropping area (row plus furrow) or only in the furrow. In the first year, broad-leaf weeds were more abundant than annual grasses, but in the second year, grasses were more abundant. Critical weed-free periods required to prevent yield losses of 2.5 and 10% were long and consistent for both years and for both types of competition, lasting between 50 and 61 d after crop emergence (DAE) or the V8 to V9 and R2 to R3 soybean growth stages, respectively. No differences in predicted yield for both places of competition was found when weeds grew after the V4 or later crop growth stages (35 DAE). The critical periods of weed removal to prevent equivalent yield losses were different in both years. Varying between 1 DAE in 1986–1987 and 30 DAE in 1988–1989, for 2.5% yield losses, corresponding to growth stages between V0 and V3 to V4, respectively. For yield losses lower than 10%, the critical periods of weed removal varied between 15 DAE in 1986–1987 and 35 DAE in 1988–1989, corresponding to the V2 and V4 crop growth stages, respectively. Nonsignificant differences in predicted yield were found between places of competition, if weeds were removed before V4 (35 DAE), maintaining yield losses lower than 20%.

Nomenclature: Soybean, Glycine max (L.) Merr. cv. ‘A3127’.