Effects on weeds of management in newly converted organic crop rotations in Denmark

We investigated the effects on annual weeds of location, weed control, manure application and catch crops and their interactions in a crop rotation with cereals and pulses for grain during conversion to organic farming in order to better understand the combined effects of management. An experiment with a 4-year crop rotation (spring barley/undersown ley, grass-clover, winter wheat and pea/barley) was carried out from 1997 to 2000 under organic conditions at three locations in Denmark with four treatments: with and without catch crop and with and without manure. Mechanical weed control was reduced or absent in cereals or pulses with undersown catch crops or grass-clover. Manure application increased weed biomass, but not the proportion of total biomass that was weed biomass, indicating that crop as well as weeds benefited from manure. The effect of catch crop on weed biomass was linked to weed control, while direct effects of catch crops on weed biomass were of minor importance. At the location with most intensive weed control, weed biomass decreased in all crops over the years. At the two other locations, weed biomass was stable or increased slightly in winter wheat and pea/barley, in which some weed control was performed, but increased in spring barley where no weed control was performed. Catch crops reduced weed density. The entry point of the rotation, i.e. the crop at the start of the rotation in 1997, had a significant influence on mean weed biomass, but it differed between locations and could mostly be explained by differences in weed biomass between years and crops. This suggests that experiments that do not include all crops in the rotation every year may give biased results. Effect of management practices (manure, catch crop and weed control) was specific to sites, but with similar effects in the different crops at each location.