Weed competition in drilled summer cabbage

SUMMARY In four field experiments in different years with summer cabbage drilled in May, naturally‐occurring weed populations of 50–540 plants per m2 reduced the yield of marketable cabbage by 47–100% from that of weed‐free crops. Species composition of the weed population and the rate of emergence and early growth relative to that of the crop were important factors in determining the extent of yield loss. If weeds were removed no later than about 3 wk after 50 % crop emergence and the crop was subsequently kept clean there was no yield loss. If the crop was kept clean without soil disturbance for the first 2 wk or longer, weeds that established after this did not affect yield. There was thus no ‘critical period of weed competition’ and a single weeding 3 wk after 50% crop emergence gave yields similar to those of crops kept weed‐free throughout.