EFFECTS OF COMPETITION WITH WEEDS ON GROWTH, DEVELOPMENT AND YIELD OF SOYBEANS

Two field experiments were carried out to study competition between soybeans (Glycine max (L.) Merr.) and weeds. The crop was kept weed-free or without weed control for increasing periods of time after planting. An analysis of the growth and development of a crop with weed control and of a crop without weed control, was obtained by making observations at the end of each period of time with or without weed control. Uncontrolled weed growth concentrated mainly between the rows. Competition with weeds reduced ground-cover and leaf area index of the crop, leading to a lower weight and yield. Plant density was not affected, but weed competition reduced the number of branches, inflorescences, and pods per plant. Timing of flowering was not influenced. Competition for water was inferred and competition for light seemed likely. Competition for nutrients was weak. Weeding in the period of pod initiation, i.e. around 45 to 70 days after planting, appeared to be essential to avoid competition with weeds. A period of around four weed-free weeks after planting was necessary to avoid yield reduction or too much weed growth at harvest.