TO EVALUATE THE EFFECT OF COVER CROPS AND NITROGEN (N) RATES ON WEED TRAITS AND YIELDS OF FORAGE SORGHUM (CV. KFS3) UNDER TILLAGE SYSTEM REDUCED A SPILT PLOT ARRANGEMENT BASED ON RANDOMIZED COMPLETE BLOCKS DESIGN WITH THREE REPLICATIONS WAS CARRIED OUT, AT RESEARCH FARMS OF COLLAGE OF AGRICULTURE AND NATURAL RESOURCES, UNIVERSITY OF TEHRAN, KARAJ, DURING THE GROWING SEASONS OF 2005-2006. N RATES (0, 75 AND 150 KG.HA-1) WERE ASSIGNED TO MAIN PLOTS; WITH WINTER COVER CROPS (RYE AND HAIRY VETCH AND FALLOW OR NO COVER CROP) CONSTITUTED THE SUBPLOTS. THE RESULTS SHOWED THAT N RATES HAD SIGNIFICANT EFFECT ONLY ON SORGHUM CROP WEED NUMBER SPECIES. COVER CROPS HAD SIGNIFICANT EFFECTS ON WEED TRAITS. RYE COVER CROP RESULTED A BETTER SUPPRESSION OF WEEDS. COVER CROPS AND NITROGEN RATES TREATMENTS SIGNIFICANTLY AFFECTED FORAGE SORGHUM. THE HIGHEST FORAGE YIELDS WERE OBTAINED FROM VETCH COVER CROP AT N RATE OF 150 KG HA-1.