Editor's Note: This adapted information pertinent to both the U.S. and Canada originated in the "Complete Guide to Wheat Management" as compiled by the Canada Grains Council.

Weed-crop competition is a complex field of study. The extent of competition is governed by a number of factors including crop species, crop cultivar, crop density, weed species, weed density, the relative time of emergence of the crop and weed, the duration of the weed presence, the efficiency of weed control, and soil and environmental factors.

Of the four major crops, wheat, barley, canola and flax, wheat ranks third in its competitive ability. Barley and canola (once well established) are more competitive, and flax the least . Winter wheat is generally considered to be more competitive than spring wheat, largely due to the fact it has a head start over the weed species. Perennial weeds, such as Canada thistle, are probably of greater significance than annual grassy and broadleaved weeds in winter wheat production.

Competition evidence indicates that semi-dwarf wheats are less competitive than normal-height wheats. Similarly, the long-strawed spring wheat, has been observed to be a better competitor with wild oats than the short-strawed cultivars. The greater shading of the semi-dwarf wheat by the weeds is implicated as a factor responsible for the differential effects of weed competition on yields of the two cultivars. Although there are some exceptions, short-strawed winter wheat cultivars also tend to be poorer competitors with annual weeds than tall cultivars.

Within tall- and short-strawed cultivars, weed competition has been observed to result in differential yield responses. Generally, the higher-yielding cultivars appear to be more severely affected by weed competition than the lower-yielding ones.

Crop and Weed Density
The actual weed density at which wheat yield losses occur is weed species and environmentally dependent. General findings are that the greater the weed density, the greater the yield losses. Several studies have shown this to be the case for wild oat competition in wheat. Irrespective of soil fertility, on previously summer fallowed land, 48 wild oats per square meter significantly reduced wheat yield, and with increasing density above 48 wild oats/ m², yield losses increased.

These results correlate well with other studies. On fertilized stubble land, wild oats at a density of only 12 plants/m² were found to significantly reduce wheat yields, while on unfertilized stubble land, 80 to 120 wild oats/m² were required before yield losses were significant.

Increasing tartary buckwheat density has been shown by a number of researchers to result in increasing wheat yield losses. The density of tartary buckwheat at which a significant yield loss was observed varied considerably from 14 plants/m² to 58 plants/m². This differential probably reflects differences in cultural practices and environmental conditions.

Increasing crop density has been implicated in a number of instances as reducing the competition of weeds. By increasing crop seeding rates, weed growth and therefore competition may be suppressed. Wheat yield losses from wild mustard competition decreased with increased crop seeding rates of 67,135 or 200 kilograms per hectare. The yield of wheat in the weed-free plots was, however, less at the higher seeding rates.

Increasing wheat seeding rates from 22 to 44 or 88 kg/ha decreased the yield-reducing effect of wild oats from 78 percent to 64 percent and 77 percent to 44 percent, respectively. Researchers caution, however, that although increasing wheat density reduced the yield loss due to wild oats, this effect was negligible once the optimum weed-free density was exceeded. Wheat yields declined much more strongly in response to wild oat pressure at low, compared to high, crop densities.