ECONOMIC LOSSES CAUSED BY WEED COMPETITION IN MANITOBA GRAIN FIELDS. I. WEED SPECIES, THEIR RELATIVE ABUNDANCE AND THEIR EFFECT ON CROP YIELDS

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Abstract
Economic losses caused by weed competition in Manitoba grain crops were studied on 142 farm fields during a 3-year period, 1956 to 1958 inclusive. Fields were located at intervals of 6 miles along main roads and were all within a 60-mile radius of Winnipeg. Immediately after seeding, ten paired plots, each 4 feet square, were staked in each field. One member of each pair of plots was kept free of weeds by hand pulling at weekly intervals. The species of weeds present and the relative numbers were recorded. As the crops reached maturity a square-yard sample from each plot was harvested for yield determinations.Twenty-eight species of weeds were found growing in the fields. Weed counts ranged from 0 in the cleanest field to 2,143 weeds per square yard in the weediest field, with an over-all recorded average of 224 weeds per square yard. Reductions in crop yields due to weed competition in individual fields ranged from no apparent decrease to 61.5 per cent. The average reduction in crop yields for all fields over the 3-year period was 15.25 per cent. This represents an average annual loss in Manitoba of 28,657,070 bushels of grain, valued at approximately $32,379,537.