An experiment was conducted to compare body weight from birth to yearling age and pre-weaning and post-weaning growth rate of purebred Menz (MM), Corriedale x Menz (CM) and Awassi x Menz sheep (AM) under conditions where natural pastures were either supplemented or not supplemented (Control). A total of 237 (88 MM, 68 CM and 76 AM) lambs were involved in the study. Both AM and CM crossbreds exceeded purebred MM lambs in all weights recorded from birth to yearling age. Similarly, male lambs were significantly heavier than females at birth and yearling ages. Concentrate supplemented lambs were 10.7 kg heavier than non-supplemented ones at yearling age. However, the responses of the three genotypes to concentrate supplement was significantly different at this age; relative to the non-supplemented controls. The supplemented MM, CM and AM lambs weighed respectively 6.7, 11.7 and 13.8 kg more at yearling age. Likewise, supplemented male and female lambs weighed respectively 51% and 38% more than their non-supplemented counterparts. The results of this study indicate that CM and AM lambs in the highlands of Ethiopia responded better to improved feeding conditions than purebred MM sheep. Thus, to fully exploit the genetic potential of crossbred lambs, supplementary feeding in addition to grazing is recommended.