Weed flora and the relative importance of site, crop, crop rotation, and nitrogen

Weed species composition and density were recorded in three identical field experiments established 26 to 30 yr ago in southern Sweden. Each experiment compared three 6-yr crop rotations and four rates of nitrogen application. The rotations differed by having (1) a 2-yr rotational grassland, (2) a 2-yr mixed rotational grassland (legume/grass), or (3) spring wheat followed by fallow. Other crops in the rotations were winter turnip rape, winter wheat, spring oats, and spring barley. Using multivariate analyses, the relative importance of site, crop, crop rotation, and nitrogen application rate on the weed flora was determined. The greatest difference was found between sites, and the second most important factor was crop species. Nitrogen application rate weakly influenced the weed flora, while differences between crop rotations were hardly detectable.