Weed-crop competition studies in swedes.

Hand-weeding experiments were conducted over a three-year period in field-grown crops of swedes in north-east Scotland. When weeds were left to grow all season they reduced the dry matter yield of swede roots by 62% in 1980 and 79% in 1981 but by only 42% in the dry year 1982, in comparison with a weed-free crop. In all three years there was at least one time when a single removal of all weeds was sufficient to prevent yield loss by weed competition. In 1980 this was 6 wk after crop sowing, but the intervals between weed removal times were too great to reveal how critical this timing was. With shorter intervals in 1981 the timing of a single removal of weeds to achieve maximum crop yield was found to be highly critical at 6 wk after sowing, but in 1982 when weed biomass was much lower a single weeding any time from 4–7 wk after sowing gave as high a yield as a crop kept weed-free all season. Earlier weeding allowed subsequently emerging weeds to become competitive and reduce crop yield, while delaying weeding until after the optimum time allowed early competition from weeds emerging with the crop to depress final yield. In all years if weed removal was delayed until 12 wk after sowing, the crop yield was only slightly or no higher than if weeds were left to grow all season. It is concluded that swedes, unlike red beet or sugar beet, are incapable of recovery from the severe restriction to growth caused by weeds competing with the crop from 6–12 wk after sowing.