DOWNY BROME (BROMUS-TECTORUM) DENSITY AND RELATIVE-TIME OF EMERGENCE AFFECTS INTERFERENCE IN WINTER-WHEAT (TRITICUM-AESTIVUM)

Field experiments over 3 yr at Lethbridge, Alberta, determined the effect of various downy brome densities and times of its emergence on winter wheat biomass and seed yield. Downy brome reduced wheat biomass up to 59% and seed yield up to 68%. Time of downy brome emergence relative to wheat affected the magnitude of these yield reductions more than the density of downy brome. At comparable densities, downy brome caused 2- to 5-fold greater reductions in yield when it emerged within 3 wk after winter wheat than when it emerged 6 wk after wheat or in early spring. Late-emerging downy brome caused significant wheat yield or biomass losses only at densities of 200 to 400 plants m(-2) Late-emerging downy brome plants were strongly shaded (70 to 90%) by winter wheat throughout much of the growing season.