A field experiment was established in 1997–2000 at Uhrusk Experimental Station of Lublin Agricultural University. The experiment was conducted in a randomized block design with 10 m2 plots in 4 replications. The soil used was rendzina of light loam texture. The subjects of the experiment were: undersown green-manure Italian ryegrass (Lolium multiflorum v. westervoldicum Wittm.) plughed under every year; undersown green-manure seradella (Ornithopus sativus L.) ploughed under yearly; farmyard manure (FYM) – 30 t ha-1 plughed under every 4 years; FYM 15 t ha-1 ploughed under every year. The control object was spring wheat cultivated in crop rotation. It was shown that cultivated spring wheat in monoculture caused essential reduction of the yield of grain in comparison with control object. However, the size of this reduction was different, because after undersown green-manure Italian ryegrass and seradella it was on average about 61.2–62.8%, while after FYM from 29.2 to 33.8%. It was found out that the number of weeds and air dry mass produced by them were indeed greater after undersown green-manure Italian ryegrass and seradella under FYM.