Three experiments were conducted to investigate the effects of low-crude protein (CP) amino acid (AA)-supplemented diets formulated on Lysine (Lys):Net energy (NE) basis and ractopamine (RAC; Paylean) on growth performance, carcass characteristics, and IGF-I and IGFBP-3 mRNA expression in late finishing pigs. In all experiments, pigs were individually penned and had ad libitum access to feed and water. In Exp. 1, 36 pigs (initial BW = 69.6 kg) were randomly allotted to 1 of 4 dietary treatments with 10 or 16% CP and 0 or 20 ppm RAC for 42 d. In Exp. 2, 36 pigs (initial BW = 70.1 kg) were randomly allotted 1 of 6 dietary treatments consisting of 1 corn-soybean meal diet and 2 low-CP AA-supplemented diets (5.23, 5.23 and 6.31 g of Lys:Mcal of NE, respectively), and their RAC-supplemented counterparts (20 ppm RAC) for 28 d. In Exp. 3, 24 pigs (initial BW = 83.44 kg) were randomly allotted to 1 of 6 dietary treatments consisting in 5 low-CP AA-supplemented diets with addition of RAC (16% CP; 5 ppm RAC; 3.35, 3.95, 4.57, 5.2, and 5.83 g of Lys:Mcal of NE) and a corn-soybean meal diet (20% CP; 5.24 g of Lys:Mcal of NE; 0 ppm RAC) for 28 d. In Exp. 2, RAC increased ADG and G:F ( P = 0.019 and 0.021, respectively). In Exp. 3 increased dietary Lys:NE linearly decreased backfat (P = 0.008). Hot carcass weight increased with RAC supplementation (Exp. 1 and 2; P = 0.028 and 0.001, respectively). In adipose tissue (Exp. 3), the IGF-I/IGFBP-3 mRNA expression ratio linearly increased with increased Lys:NE (P = 0.019). Low-CP AA-supplemented diets provided energy and AA in adequate amounts to allow RAC to improve growth performance. The improvement in growth performance due to dietary RAC inclusion was not associated with changes in IGF-I or IGFBP-3 mRNA expression.

Key words: pigs, ractopamine, growth performance, IGF-I, IGFBP-3, carcass characteristics