Effect of Dietary Crude Protein Versus Crystalline Amino Acids on Growth Performance, Serum Insulin-Like Growth Factor-I Concentration, and IGF-I mRNA Expression in Growing-Finishing Gilts

University of Nebraska 2006 Swine Day Report. Fifty-six crossbred gilts with an initial body weight of 73 lb were used in a 26-day growth study. The pigs were randomly allocated to one of seven dietary treatments and individually penned (8 replicates/treatment). The dietary treatments consisted of four standard corn-soybean meal diets, which were formulated by changing the corn and soybean meal (10, 14, 18, and 22% CP) ratio and three low-protein, amino acid-supplemented diets formulated to contain similar lysine, methionine, tryptophan, and threonine concentrations as the corn-soybean meal diets (10% CP + AA,14% CP + AA, and 18% CP + AA). Pig and feeder weights were recorded weekly for the determination of ADG, ADFI, and feed efficiency (ADG/ADFI). Blood samples were collected weekly and analyzed for plasma urea and Insulin-like Growth Factor I (IGF-I) concentrations. On day 26, real-time ultrasound backfat and longissimus muscle area measurements were recorded and used for the calculation of fat-free lean gain. There was no difference (P > 0.10) in ADFI among treatments throughout the 26-day period. Pigs fed the corn-soybean meal diets (14, 18, and 22% CP) had greater ADG (1.81 versus 1.68 lb; P < 0.05) and ADG/ADFI (0.44 versus 0.40 lb/lb; P < 0.05) than pigs fed the reduced CP amino acid-supplemented diets (10% CP + AA, 14% CP + AA, and 18% CP + AA) throughout the experiment. Fat-free lean gain increased as dietary CP or total amino acid concentration increased (P< 0.01); however, no differences (P > 0.40) were observed between gilts fed the corn-soybean meal (378 g/day) versus CP amino acid-supplemented diets (368 g/day). Increasing dietary CP or total amino acid concentration increased serum IGF-I concentrations on day 26 (P < 0.01). Serum concentration was different (P < 0.05) between gilts fed the corn-soybean meal versus low-CP, amino acid-supplemented diets (505 vs. 445 ng/mL, respectively). Real-time PCR results indicated an effect (P < 0.05) of dietary treatment on mRNA expression in the liver and semitendinosus muscle. Also, IGF-I mRNA expression was greater (P < 0.01) in the semitendinosus muscle and adipose tissue of gilts fed corn-soybean meal diets compared to gilts fed low-protein, amino acid-supplemented diets. These results suggest that the form of dietary amino acid supplementation affects serum IGF-I concentrations and mRNA expression in semitendinosus muscle and adipose tissue. The interaction between diet and the pigs growth potential are complex. The form and quantity of dietary amino acids impact this interaction. These results provide a basis to explore how diet affects the metabolic signals (e.g., IGF-I) regulating growth in the pig.