Field experiments, designed to study weed incidence in Poaceae and Fabaceae monocrops and their mixtures as influenced by an organic cropping system, were carried out during 2007-2009 at the Experimental station of the Lithuanian Research Centre for Agriculture and Forest in Perloja on a Hapli-Albic Luvisol. Short-lived weeds prevailed in the crops. They accounted for 93-95% of the total weeds. The highest counts of weeds were recorded at the beginning of the growing season when the conditions for their emergence were the most conducive ones. In 2007 and 2009 the weather conditions during cereal tillering stage were wet and warm, which created optimal conditions for weed emergence and the weed count per m -2 was 257-264. In the third ten-day period of April 2008, re-current night frosts inhibited cereal and weed emergence, which resulted in 43.6-51.7% lower emergence. With increasing crop competitive power (milk maturity stage), weed emergence tended to decline and was by 17.9-9.8% lower compared with the weed count at tillering stage. Weed air dry weight decreased proportionally, however, only at crop hard maturity stage, with the reduction in crop competitive power, weed air dry weight increased by 31.6%. Due to the weather conditions, weed air dry weight in the wet year 2007 was by 35-55% higher than that in 2008 and 2009. Lupines, oats and wheat and its mixture with peas performed best in suppressing weeds. A strong linear correlation was established between weed number and precipitation and weigh (r=0.901, r=0.758); between plant density and precipitation amount (r=0.745).
