A series of experiments was carried out during the period 1997-2004 to study the effects of crop rotations with a different proportion of winter and spring crops, various legume preceding crops and cover crops and their biomass use for green manure on weed infestation in cereals on a clay loam Endocalcari-Endohypogleyic Cambisol ( CMg-p-w-can) under North Lithuania's conditions. Crop rotations with prevailing winter crops predetermined the reduction in perennial weeds in cereals. The occurrence of weeds in cereals was determined by the weed incidence in the preceding crops tested: red clover ( Trifolium pratense L.), lucerne ( Medicago sativa L.) and vetch-oats ( Vicia sativa L., Avena sativa L.) mixture and by competitive ability of cereals. The biomass of clover and vetch-oat mixture incorporated as green manure tended to increase weed incidence in cereals. Undersowing of cover crops ( Trifolium pratense L., Lolium multiflorum Lam., Dactylis glomerata L.) tended to reduce the weed emergence in cereals, compared to the crops without undersowing. Undersown red clover performed best at suppressing weeds during the cereal post-harvest period. Its positive effect persisted in the following year after incorporation of red clover biomass for green manure. The effect of post-harvest sown cover crops ( Raphanus sativus L., Sinapis alba L.) on weed incidence in cereals was lower compared to undersown crops.
