The response of weed communities to changing management practices
The response of weed communities to changing management practices was assessed as part of the Crop Management Study at the Indian Head Experimental Farm from 1988 to 1990. A split plot design with four reps (144 plots), zero, minimum, and conventional tillage as the main plots, two continuous crop rotations and one crop-fallow rotation as the sub-plots, and spring wheat, winter wheat, flax, and field pea as the crops. Each crop in a rotation was present each year. Weeds were sampled during July each year, after the application of post-emergent herbicides, in order to assess the density and composition of the residual weed flora. In each plot (except the 12 fallow plots) the weeds were counted in 20 quadrats, each 0.25 m^2. Multivariate statistical methods were used to determine if weed communities differed among years, tillages, rotations, and crops. Weed densities were generally low for all years (< 10 plants m-2). From 1988 to 1990, the proportional abundance of different weed types changed. Annual broad-leaved weeds accounted for 40% of the total weed abundance during 1988 and 70% during 1990. Redroot pigweed, kochia, Russian thistle, and thyme-leaved spurge were the major weeds in this group and all increased in frequency of occurrence. Winter annuals decreased from 35% of the total abundance to 19% during the same period. Stinkweed and flixweed were the major weeds in this group. The annual grasses, wild oats and green foxtail, accounted for 5% or less of the abundance in all three years. The perennial group, Canada thistle, perennial sow-thistle, and foxtail barley and the volunteer crop group, spring wheat, winter wheat, and flax were less than 11% of the total abundance in each of the three years. The year to year variation in weed abundance was greater than any variation due to tillage system. The highest weed densities were associated with the rotation that included field pea.
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