Thirty-eight multiparous St. Croix ewes bred to Romanov or Suffolk rams were used in an 8- week experiment beginning 2.9 ±0.15 days after parturition to determine effects of sire breed (i.e., lamb growth potential), litter size (i.e., single vs twins), and different supplemental concentrate treatments on ewe and lamb performance. Wheat hay (10% crude protein and 71% NDF; DM basis) consumed ad libitum by ewes and lambs was supplemented for ewes (DM basis) with: 0.25% body weight (BW) of ground maize (control); 1.25% BW of maize, 0.3% BW of a mixture of fish (38.8%), blood (30.6%), and feather meals (30.6%; MR); or 1.25% BW of maize alone in weeks 1 to 3 and with high-protein feedstuffs in weeks 4 to 8 (M-MR). Control ewe BW decreased during the experiment at a rate of 67 g/day, whereas BW change for MR and M-MR treatments was 16 and -8 g/day, respectively (control vs MR and M-MR, P < 0.05; SE 7.8). Lamb BW gain also was lower for control than for MR (P < 0.05) and M-MR (P = 0.08), 144, 205, and 190 g/day for control, MR, and M-MR, respectively (SE 12.9). Lamb BW gain was greater (P < 0.05) for Romanov- vs Suffolk-sired lambs (196 vs 163 g/day; SE 8.7) and for single than for twin lambs (198 vs 162 g/day; SE 9.0).