Selective weed suppression by cover crop residues: effects of seed mass and timing of species' sensitivity

Laboratory bioassays have shown that large-seeded species better tolerate cover crop residue-mediated stress than small-seeded species. This provides the potential for selective suppression of small-seeded weeds in large-seeded crops. We conducted two field experiments in which seedling emergence of a range of crop and weed species was monitored in soil without cover crop residues and soil with recently incorporated lucerne, winter oilseed rape or winter rye residues. A positive relationship was found between seed mass and tolerance to winter oilseed rape and lucerne residues in both experiments, whereas winter rye residues did not inhibit seedling emergence. Both the gradual reduction in residue-mediated inhibitory potential with time in the first experiment, as well as the sudden increase in lucerne residue-mediated inhibitory potential following a severe rainfall event in the second experiment, illustrated the importance of temporal dynamics. Therefore, we hypothesised that time between residue incorporation and receptor plant emergence could serve as an additional variable for explaining the variation in inhibition of seedling emergence among receptor plant species. Re-analysis of data from two previously published field experiments revealed that seed mass and time of emergence only contributed significantly to explaining variance in receptor plant emergence when both these factors were included in the analysis. Our findings suggest that considering temporal synchrony of receptor species' sensitivity and potential residue effects in field studies provides a valuable framework for analysing crop residue-mediated effects.