The effects of different weed competition periods on some soybean yield parameters were studied in field trials with cultivars Santa Rosa and IAC-2 in two soils, "Latossol Roxo" and "Dark-Red Latossol - sandy phase", at Jaboticabal, São Paulo State, Brazil. The experimental design of randomized blocks was used. The cultivars were kept with and without weed competition since the emergence for periods of 0, 10, 20, 30, 40, 50 and 60 days. Based on the results the following conclusions may be drawn: the soybean crop should be kept free of weed competition during the first 30 to 40 days for cv. Santa Rosa and during the first 50 days for cv. IAC-2; for both cultivars yield increase was evident beyond first 20 days weeded in "Latossol Roxo" and 30 days for "Dark-Red Latossol - sandy phase", reaching the maximum value at 50th for Santa Rosa cultivar and around 60th days for 'IAC-2'; in the 1977/78 season (for "Latossol Roxo") weed competition during the first 20 days after emergence caused yield losses to both cultivars, whereas in the 1978/79 season ("Dark-Red Latossol -sandy phase") this period extended to 40 first days, showing the importance of edafic -climatic interferences and different weed species in the competition process; among all parameters related to the grain yield the number of pods per plant was the most affected by weed competition.
Weed competition; competition periods; soybean; grain yield