No abstract

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.

Field pea (Pisum sativum L.) is an important organic crop due to its contribution to soil fertility and other rotational benefits. Leafed (wild‐type) pea cultivars tend to be more weed suppressive, but their poor standing ability limits yield compared with semi‐leafless cultivars. Growing mixtures of leafed and semi‐leafless cultivars may improve weed suppression and yield compared with monocultures of the same cultivars by altering canopy morphology. To test this hypothesis, replicated field experiments were conducted under weedy, organic conditions in Saskatchewan, Canada, in 2011 and 2012. Mixtures of a leafed and semi‐leafless cultivar, CDC Sonata and CDC Dakota, were sown in ratios of 0:100, 25:75, 50:50, 75:25, and 100:0 leafed to semi‐leafless pea, at target seeding rates of 88 and 132 plants m−2. Mixtures that included 50% or more semi‐leafless pea had similar lodging resistance and weed biomass suppression to the agronomically superior semi‐leafless cultivar grown alone. The strong competitive ability of the semi‐leafless cultivar was unexpected based on previous accounts. The combined yield of the two cultivars grown in a 75% semi‐leafless mixture exceeded the seed and biomass yield of either single cultivar by at least 18 and 12%, respectively. Yield enhancement was attributed to the leafed cultivar, whose seed yield increased by more than two and a half times in this mixture relative to its monoculture. Results suggest that breeding of leafed cultivars specifically for mixture with semi‐leafless pea may be a future source of yield gains in organic and low‐input systems.

Concern over the development of herbicide-resistant weeds has led to interest in integrated weed management systems that reduce selection pressure by utilizing mechanical and cultural weed control practices in addition to herbicides. Increasing crop seeding rate increases crop competitive ability and thus can enhance herbicide efficacy. However, it is unknown how increasing the seeding rate affects an herbicide’s efficacy. The objective of this study was to examine the interaction between increasing seeding rate and herbicide dose to control weeds. To meet this objective, the herbicide fluthiacet-methyl was applied to field-grown lentil, with Indian mustard, a proxy for wild mustard, used as a model weed. The experiment was a factorial design with four lentil seeding rates and seven herbicide rates. Overall the herbicide dose response was altered by changing lentil seeding rate. Increasing lentil seeding rate decreased the weed biomass production when herbicides were not applied. In two of the four site-years, increasing lentil seeding rate lowered the herbicide ED50, the dose required to result in a 50% reduction in weed biomass. Increasing the crop seeding rate altered the dose response to provide greater weed control at lower herbicide rates compared with normal crop seeding rates. Increased seeding rates also resulted in higher and more stable crop seed yields across a wider range of herbicide dosages. These results suggest that dose–response models can be used to evaluate the efficacy of other weed management practices that can interact with herbicide performance.

The escalating evolution of weed species resistant to acetolactase synthase (ALS)-inhibitor herbicides makes alternative weed control strategies necessary for field crops that are dependent on this herbicide group. A fully integrated strategy that combined increased crop seeding rates (2X or 4X recommended), mechanical weed control with a minimum-tillage rotary hoe, and reduced-rate non–ALS inhibitor herbicides was compared with herbicides, rotary hoe, and seeding rates alone as a method of controlling ALS inhibitor–tolerant Indian mustard as a model weed. The full-rate herbicide treatment had the lowest weed biomass (98% reduction) and the highest yield of all treatments in 3 of 4 site-years, regardless of seeding rate. The fully integrated treatment at the 4X seeding rate had weed suppression rates equal to the full herbicide treatment at the recommended seeding rate. The fully integrated and reduced-rate herbicide treatments at the 4X seeding rate reduced weed biomass by 89% and 83%, respectively, compared with the control at the recommended seeding rate. The rotary hoe treatment alone resulted in poor weed control (≤38%), even at the highest seeding rate. Fully integrated and reduced-rate herbicide treatments at 2X and 4X seeding rates had yields equal to those of the full herbicide treatment at the recommended seeding rate. Partially or fully integrated weed control strategies that combine increased crop seeding rates and reduced-rate non–ALS inhibitor herbicides, with or without the use of a rotary hoe, can control weeds resistant to ALS-inhibitor herbicides, while maintaining crop yields similar to those achieved with full-rate herbicides. However, combining increased seeding rate, reduced-rate herbicides, and mechanical rotary hoe treatment into a fully integrated strategy maximized weed control, while reducing reliance on and selection pressure against any single weed control tactic.