Weed suppression by cereals: Beyond 'competitive ability'

Much research on cereal-weed competition has been directed towards finding cultivars that have high 'competitive ability' with weeds. I argue that this objective is insufficient, because weed suppression is usually the result of interactions among genotype, environment and management rather than direct effects of genotype, and this limits the usefulness of competitive ability as a general concept. The clearest example of this is the effect of cereal crop density. Cultivars of wheat and maize that compete best with weeds are often different at high versus low crop density. This is especially relevant because effective weed suppression, a reduction of weed growth to very low levels by competition from the crop, is only possible at high crop density and spatial uniformity. The potential for improving weed suppression by crops under standard regional management practices, which are usually based on weed-free conditions, is limited, and there is likely to be a significant cost in yield. To make major improvements in weed suppression and thereby reduce greatly our dependence on chemical weed control, researchers should focus more on management strategies that benefit the crop in competition with weeds, and then on traits that improve weed suppression under the relevant management regime, rather than considering differences in management as a form of environmental variation and looking for traits promoting general competitive ability.