Field studies have shown that weed density and biomass were lower in crops following incorporation of brassica cover crops compared with fallow but have not determined whether weed-suppressive effects are solely a consequence of reduced establishment, as evidenced in our companion paper, reduced growth of established plants, or both. In 2002 and 2003, canola and yellow mustard were seeded in early May, mowed in early July, and the residues incorporated. Green bean and redroot pigweed were then planted at fixed densities. Plant height and biomass were measured weekly; leaf area and biomass of component plant parts were measured at three harvests. Based on analysis of variance (ANOVA) at discreet sampling points, growth of redroot pigweed and green bean in monoculture or mixture were similar following fallow and incorporated brassica cover crops. However, based on aboveground biomass fitted to a Richards function, redroot pigweed growth in monoculture was reduced by the yellow mustard cover crop compared with fallow in both years (P = 0.007), but the magnitude of this effect was small; the canola cover crop did not affect growth (P = 0.179). Brassica cover crops did not reduce redroot pigweed growth when it was grown in mixture with green bean (P ≥ 0.382). Redroot pigweed competition reduced green bean yield, but incorporated brassica cover crops did not affect green bean growth and yield, nor did they confer a competitive advantage to the crop. Thus, brassica cover crops may suppress the growth of established weed and crop plants, but the magnitude of suppression was less than previously documented for effects on weed establishment.

Nomenclature: Canola, ‘Hyola’, Brassica napus L.; redroot pigweed, Amaranthus retroflexus L. AMARE; yellow mustard, ‘Idagold’, Sinapis alba L.; green bean, ‘Provider’, Phaseolus vulgaris L.