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A study was conducted to investigate the effects of increasing dietary protein intake on growth performance, carcass composition and serum insulin-like growth factor-I (IGF-I) concentration in growing-finishing barrows and gilts. Seventy crossbred pigs (35 barrows and 35 gilts) with an initial body weight of 75.1 lb were used in a 26-day growth study. The pigs were allocated randomly to one of four dietary treatments. The diets were standard corn soybean meal diets, which were formulated to contain 10, 14, 18, or 22% crude protein by changing the ratio of corn to soybean meal in the diet. At the termination of the experiment, pigs were slaughtered to determine carcass accretion rates of protein, water, fat and ash. Pig and feeder weights were recorded weekly for the determination of average daily gain (ADG), average daily feed intake (ADFI), and calculation of feed efficiency (ADG/ADFI). Weekly blood samples were collected to evaluate dietary effects on plasma urea and IGF-I concentrations. There was no difference (P > 0.10) in ADFI among treatments; however barrows consumed more feed than gilts (3.94 versus 3.70 lb/d; P = 0.01) throughout the 26-day period. Dietary protein concentration had linear and quadratic effects on ADG and ADG/ ADFI (P < 0.01). Also, barrows gained weight faster (ADG: 1.57 versus 1.41 lb; P < 0.01) and were more efficient (ADG/ADFI: 0.40 versus 0.38 lb/lb; P = 0.02) than gilts throughout the experiment. Increased dietary protein concentration resulted in increased fat-free lean gain, cold carcass weight (linear, P < 0.01; quadratic, P < 0.01) and dressing percentage (quadratic effect, P < 0.01). Protein concentration had a linear effect (P < 0.01) on plasma urea during weeks 1 through 4 and had a quadratic effect (P < 0.01) during weeks 1 and 4 of the experiment. Also, dietary crude protein concentration had linear and quadratic effects (P < 0.01) on serum IGF-I concentrations during weeks 2 and 4 of the experiment. In summary, dietary protein concentration had linear and quadratic effects on final body weight, ADG, feed efficiency, fat-free lean gain, cold carcass weight, plasma urea and serum IGF-I concentration. Thus, the interesting finding in this experiment was that the decrease in fat-free lean gain and protein accretion rate in pigs fed the 18% CP diet were not associated with a decrease in serum IGF-I concentration. This finding suggests that nutritional and (or) physiological factors are inhibiting the actions of IGF-I by causing a decrease in protein accretion rate. Thus, the future focus of this research is to determine the effects of dietary crude protein and crystalline amino acids on serum IGF-I concentration and metabolic actions.