Effects of protein nutrition in young pigs on developmental changes in the body and sekeletal muscles during growth.

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Two experiments were conducted with growing pigs killed at various predetermined weights. Effects of protein level and quality of chemical body composition and chemical and physical composition of selected muscles were studied. In Experiment 1, pigs were fed either 10 or 20% crude protein of good quality and identical amino acid ratios. The pigs were killed at weights that represented equal logrithmic spacing of their estimated lean mass (moisture plus protein). At similar lean masses, the empty bodies of pigs fed a diet of 10% protein contained considerably more ether extract than those of pigs fed a 20% protein diet, but, as predicted, the two groups had similar amounts of moisture and protein. Analyses of a rectus femoris muscle revealed that weight, average fiber diameter, DNA, RNA, protein, moisture, protein:DNA and RNA:DNA increased with increasing lean mass at slaughter. Only RNA, RNA:DNA and moisture were affected by protein level; RNA and RNA:DNA ratio being greatest and moisture least in pigs fed the 20% protein diet. In experiment 2, pigs were fed the same 10 and 20% protein diets of experiment 1, and a third group was fed a 20% protein diet with poor amino acid balance (poor protein quality). As in Experiment 1, empty bodies of pigs fed good quality protein at 10 and 20% levels had predictible contents of moisture and protein; these components, however, were overestimated for pigs fed the diet with 20% protein of poor quality. Analysis of a M. longissimus from each pig showed that weight, average fiber diameter, DNA, RNA, protein, moisture, protein:DNA and RNA:DNA increased as the pigs grew. RNA, protein:DNA and RNA:DNA were greater in the muscles of pigs fed 20% protein of good quality than in those of pigs fed 10% protein of good quality. RNA, protein, moisture and RNA:DNA ratio were greater in the M. longissimus of pigs fed 20% protein of good quality than in those of pigs fed 20% protein of poor quality. Estimates of numbers of nuclei (NN), muscle fiber cross-sectional area (CSA) and muscle weight (WT) were indicative of expected mathematical relationships. These expected relationships define an expanding cylinder, in which a constant ratio is maintained between length and diameter and there is a direct relationship between CSA and NN. Expected regression coefficients for log CSA on log WT, log NN on log WT and log NN on log CSA are .67, .67 and 1.0, respectively. Protein level and quality did not influence these relationships in either the rectus femoris or the M. longissimus