High weed abundance in organic crops is thought to be a key factor contributing to the greater yield loss in organic as compared with conventional cropping systems. However, even with greater weed densities than conventional systems, some organic systems have yields comparable to conventional systems, suggesting that cropping systems might differ in yield loss due to weed competition. The diversity in soil nutrient resources due to diversity in crop rotations and variable inputs might enhance crop tolerance to weed competition. We assessed the long-term effects of contrasting levels of crop rotations (low, medium and high diversity) on weed density, weed biomass and wheat yield loss in organic and no-till conventional cropping systems using a microplot study within a long-term cropping systems trial at Scott, Saskatchewan, Canada. Weed density and biomass were found to be four times higher in the organic systems than in the conventional systems. Under standard weed management practices, organic had 44% lower yield than the conventional system. Lower yields in organic, even without weed competition, suggest that the lower yields are due to low soil productivity rather than weed competition. No differences in yield loss were observed among the organic and conventional systems or among the diverse crop rotations. We conclude that the organic management practices and/or increased crop rotation diversity did not enhance yield or reduce yield loss due to weed competition, due to the factors associated with lower soil fertility.

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Weed control with herbicides is not possible in several systems including control of wild oat (Avena fatua L.) in tame oat (Avena sativa L.), red rice (Oryza sativa L. var. sylvatica) in rice (Oryza sativa L.), and in organic systems. Competitive crop cultivars can be used to manage weed competition if selective weed control by herbicides is not possible. However, most existing competitive crop cultivars often have low weed‐free yield and poor grain quality. We hypothesize that the progeny of a cross between a competitive forage‐type oat cultivar and a high‐yielding and high grain quality grain oat cultivar will have high yield and good grain quality as well as high competitive ability (CA). The objective of this study was to evaluate progeny lines from a cross between a tall, competitive, forage‐type oat cultivar with a semidwarf, high‐yielding milling oat for their CA against wild oat. A field study was performed in two locations in Saskatoon, SK, Canada, in 2008 and 2009 using seven progeny lines and the two parents. Oat was seeded with and without wild oat at a target crop and weed density of 250 plant m‐2 in a randomized block design with four replicates. All the genotypes were high yielding and did not differ in grain yield or quality either in weed‐free or weedy conditions. Some of the tall (CDC Baler, SA050498, and SA050479) and short (SA050040 and Ronald) genotypes had high average grain yield under both weed‐free and weedy conditions compared with the other genotypes. The tall oat line SA050479 was among the most weed suppressive, which resulted in less wild oat biomass. The seedling total leaf area, crop height, and the seedling third leaf area were negatively correlated with wild oat biomass. This study demonstrates that a cross between a high‐yielding and a highly competitive genotype can result in progeny with high CA, yield, and crop quality.

Effective weed management strategies are limited in organic cropping systems because herbicide use is prohibited. Enhancing crop competitive ability by integrating both cultural and mechanical weed control methods is a key strategy in such instances, but the relative efficacy of different cultural and mechanical strategies and their interactions and additive effects when combined is not well known. The objective of this study was to determine the individual and additive effects of cultural and mechanical methods on weed suppression and crop yield under organic conditions. A study was performed in two organically managed oat (Avena sativa L.) cropping systems in Saskatoon, SK, Canada, in 2008 and 2009. Three cultural practices, two crop genotypes (competitive and less competitive), high and standard crop densities (500 and 250 plants m -2 ), narrow and standard row spacings (11.5 and 23 cm), and two post-emergence tillage levels (harrowing and nonharrowed control) were factorially applied in a randomized block design. Increasing crop density and harrowing increased grain yield by 11 and 13%, respectively. Competitive genotype and high crop density reduced weed biomass by 22 and 52%, respectively. Combining high crop density with postemergence harrowing increased the grain yield by 25%. The combined treatment effects on weed biomass were more profound, as the competitive genotype, increased seeding rate, and post-emergence harrowing decreased weed biomass by 71% compared with standard practices. Integrating cultural and mechanical weed management practices was superior to the use of individual practices because they additively control weeds in an organic cropping system.