Brassica cover cropping: II. Effects on growth and interference of green bean (Phaseolus vulgaris) and redroot pigweed (Amaranthus retroflexus).

Field experiments were conducted in Maine, USA, during the 2002 and 2003 cropping seasons, to determine the effects of Brassica cover cropping on the growth and interference of green bean (Phaseolus vulgaris) and redroot pigweed (Amaranthus retroflexus). Rape and yellow mustard were sown in early May, mowed in early July and the residues incorporated. Green bean and redroot pigweed were then sown at fixed densities. Plant height and biomass were measured weekly, while the leaf area and biomass of component plant parts were measured at 3 harvests. Based on analysis of variance (ANOY A) 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 to fallow in both years, but the magnitude of this effect was small. The rape cover crop did not affect growth. Brassica cover crops did not reduce redroot pigweed growth when it was grown in mixture with green bean. 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.