Weed suppression by crops

Crop plants may suppress weed development physically through superior vigour, allelopathically by the release of phytotoxins, or a combination of both. In experiments to evaluate the effect of different crops on weed number and biomass, most crops suppressed weed growth and percentage ground cover rather than reduced seedling numbers. Allelopathy may have been a factor in the few crops that did reduce weed seedling emergence. In studies over a ten-year period, weed emergence in the presence or absence of a crop was not consistently less when the crop was present. Cover-crops sown in September reduced weed biomass over-winter but did not suppress weed emergence. Whatever feature of weed development is reduced, and whether a physical or an allelopathic effect is responsible, the suppression of weeds by crops should be exploited to improve weed control in organic systems.