Weed development in cereals under different growth conditions and control intensities

The development of weed communities in cereals was investigated over 5 years at four locations in Norway. Each year the same plots were treated with 0, 1/3, 2/3 or 3/3 of the ‘normal’ dose of either of two herbicides. The weed species that emerged varied between years and locations, but were unaffected by treatments. The total number of weed plants and the frequency of individual species varied in all fields, but the changes differed greatly between fields, dosages and years. The seed bank was increased by the lowest herbicide dose and unaffected by the higher doses. The percentage of emerged weeds decreased with time, but varied between the fields. The per cent emergence was lower on untreated than on treated plots. The weed abundance in previous years and the size of the seed bank affected the plant number, but the air temperature after sowing was just as important. Even soil temperature in spring prior to tillage influenced weed emergence.