Weed Competition in Soybeans (Glycine max L.)

The present investigation was conducted to determine the critical period of weed competition in soybeans at Giza, Egypt. Weed flora in the experimental site at Giza contained a wide range of species during both seasons of study (1990 and 1991). Total weed dry weight increased progressively with time up to 12 WAE (182.0 g/m2). Weed‐free and weedy periods exhibited significant effects on soybeans height, number of pods per plant, stand at harvest, biological and seed yields. Optimum seed yields were produced by weed‐free durations for 6 WAF up to all season (3.229–3.545 t/ha). Whereas, weedy periods for 2, 4, 6, 8, 10 and 12 WAF reduced seed yield by 14.9, 28.0, 35.3, 45.1, 48.1 and 51.1 %, respectively. Weed competition for the entire season caused 56.1 % reduction in soybean seed yield. The obtained results indicate that weed competition during the early stages of growth, adversely affected soybeans. At least 6 WAE of weed‐free period was required to insure optimum soybean growth and yield. However, oil and protein contents were not affected by weed competition.