Annual broadleaf crop frequency and residual weed populations in Saskatchewan Parkland

The development of problematic weed populations is a concern in western Can adian fields where canola and pea are grown in a 4-yr sequence with spring cereal grains. Weed densities were examined at a site near Melfort, Saskatc hewan, Canada, from 1994 to 1997 in seven zero-till managed crop rotations. Four rotations that included canola, pea, or flax in at least 3 of 4 yr (H BF: high broadleaf-crop frequency) were compared with three rotations that included broadleaf crops grown in 2 of 4 yr (LBF: low broadleaf-crop Freque ncy). Spring wheat and barley were the cereal crops in rotation. Residual ( postherbicide application) weed density fur each weed species in a given ye ar was summed across all phases for each rotation to reflect the overall we ed infestation. Four annual broadleaf weed species were most abundant in 19 96 and a second group of three species, having a variety of reproductive st rategies, became progressively less abundant as the study progressed. The d ifference between the HBF and LBF rotations for the density of these specie s varied and was most prominent in years when environmental conditions were conducive for their growth. More frequent applications of ethafluralin, wi th its residual weed control, best explained why wild oat and catchweed bed straw generally were less abundant in the HBF rotations. Of particular inte rest was the 8 plants m(-2) greater density of dandelion and perennial sowt histle in the HBF vs. LBF rotations in the last year of the study It is tho ught that the limited herbicide options for the control of these species co uld present a future problem if they continued to develop in the HBF rotati ons. Differences in herbicide use between the HBF and LBF rotations were co nsidered the primary factor controlling the rotation effects on weed densit y.