Effects of dietary crude protein and/or crystalline amino acid concentration on serum IGF-I concentrations and IGF-I mRNA expression in growing pigs

Three experiments were conducted to investigate the effects of increasing dietary crude protein (CP) and/or crystalline amino acid (AA) intake on growth performance, serum insulin-like growth factor-I (IGF-I) concentration, and IGF-I mRNA expression in growing-finishing pigs. All three experiments utilized crossbred pigs with an initial body weight of 34 kg in 28-d (exp. 1) or 26-d (exp. 2 and 3) growth assays. Dietary treatments used in the experiments were standard corn-soybean meal diets (10, 14, 18, 22% CP) or low-protein, amino acid-supplemented diets (10% CP + AA, 14% CP + AA, and 18% +AA). Pig and feeder weights were recorded weekly during all three experiments for the determination of ADG, ADFI, and feed efficiency (ADG/ADFI). Blood samples were collected weekly and analyzed for plasma urea and serum IGF-I concentrations. On d 26 (exp. 2 and 3) and d 28 (exp. 1), real-time ultrasound backfat (BF) and longissimus muscle area (LMA) measurements were recorded and used for the calculation of fat-free lean gain (FFLG). There was no difference (P > 0.10) in ADFI among treatments throughout the growth period in all three experiments. Dietary protein concentration had linear and quadratic effects on ADG and ADG/ADFI (P < 0.01) in exp. 1 and 2. Results from experiment 3 showed that pigs fed the corn-soybean meal diets had greater ADG (P < 0.05) and ADG/ADFI (P < 0.05) than pigs fed the low-protein, amino acid-supplemented diets throughout the experiment. In exp. 2 and 3, IGF-I mRNA expression in the semitendinosus muscle increased as CP concentration increased in the diet. In all experiments, fat-free lean gain, plasma urea, and serum IGF-I concentration were increased (P < 0.01) by dietary protein concentration. Results from these experiments suggest that the consumption of a diet deficient in CP (10 and 14%) inhibits the expression in semitendinosus muscle and production of IGF-I which was associated with a reduction in growth rate and carcass protein accretion.