The critical period of weed control in soyabean (Glycine max (L.) Merr.).
Field studies were conducted during 1990-91 at 3 locations (on loam at Elora, on silt loam at Woodstock and on clay loam at Woodslee) in southern Ontario to determine the critical period of weed control in soyabeans cv. Maple Glen, Pioneer 0877 and Corsoy 79. This period generally consisted of 2 discrete periods, a critical weed-free period and a critical time of weed removal. The most problematic weeds at these 3 sites were Echinochloa crus-galli, Amaranthus retroflexus, Chenopodium album, Setaria glauca [Setaria pumila], Sinapis arvensis, Setaria viridis, Panicum capillare and Abutilon theophrasti. The critical weed-free period was relatively short in duration and consistent across locations and years. A period of weed control lasting up to the 4th-node growth stage (V4), approx. 30 d after emergence (DAE), was adequate to prevent a yield loss of >2.5%. The critical time of weed removal was variable across locations and years and ranged, for example, from the 2nd-node growth stage (V2) to the beginning pod growth stage (R3), approx. 9 to 38 DAE, at a 2.5% yield loss level. A phenologically-based period of most rapid yield loss due to weed interference occurred from beginning bloom stage (R1) to beginning seed stage (R5). The short and consistent critical weed-free period indicates the duration of residual herbicide control necessary in soyabeans and supports the use of non-residual, post-em. herbicides and mechanical weed control.