MUSCLE CELLULARITY AND POSTNATAL-GROWTH IN THE PIG

Fast-growing strains of pigs and of other animals tend to have more mu scle fibers than do slow-growing strains. The relationship between fib er number and growth rate was extended by examining pigs within the sa me strain. Seven litters of pigs (n = 66) were weighed at monthly inte rvals from birth to approximately 80 kg. The semitendinosus muscle was removed and an estimation of total fiber number, total primary fiber number, and mean secondary:primary fiber number (S:P) ratio was made f or each animal. Pig growth was divided into three approximately linear phases: birth to 6 kg, 6 to 25 kg, and 25 kg to slaughter weight. Ave rage daily gain in the first two phases was correlated with birth weig ht (r = .3614, P < .05 and r = .5873, P <.001, respectively) but was n ot correlated with total muscle fiber number. In the third growth phas e ADG was correlated with muscle fiber number (r = .4149, P < .001) an d was not correlated with birth weight. The gain:feed ratio was also c alculated for each pig during the third growth phase and was found to be correlated with fiber number (r = .4191, P < .001). These results s uggest that muscle fiber number is an important determinant of postnat al growth such that pig littermates with a high fiber number tend to g row faster and more efficiently than littermates with a lower fiber nu mber.