EFFECT OF SELECTION FOR LEAN TISSUE-GROWTH ON BODY-COMPOSITION AND PHYSIOLOGICAL-STATE OF THE PIG AT BIRTH

The effects of selection for lean tissue growth on the metabolic and physiological state (i.e., level of maturity) of the pig at birth have been examined on newborns from three breeds that markedly differ with respect to birth weight and postnatal muscle growth potential: a primitive Chinese breed (Meishan, MS), a European breed (Large White, LW), and a composite line (CL) highly selected for high rate of gain. Within each breed, 40 pigs from eight litters were used for whole carcass and tissue sampling, blood sampling, and for a fat tolerance test at 2 h of age. The CL pigs were heavier (P < .001) than the LW and MS pigs at birth but exhibited lower percentages of carcass protein, fat, mobilizable fat, and ash than the MS pigs (P < .05). In addition, MS pigs had larger adipose tissue adipocytes than pigs from the two other breeds (P < .001). Despite their 31% higher RNA capacity in longissimus muscle (higher RNA:protein ratio, P < .05), CL pigs exhibited a lower percentage of muscle protein (P < .05) than did MS pigs. Relative liver weight was higher for LW than for CL pigs (P < .05), which had the lowest percentage of liver phospholipids (P < .01). The CL pigs exhibited lower hematocrit (P < .01), glucose (P < .01), albumin (P < .01), cortisol (P < .01), and thyroxine (P < .05) levels than the MS pigs. Finally, CL pigs had the lowest ability to clear and metabolize circulating triglycerides, as illustrated by their lower triglyceride clearance rate ( P < .0 7), lipoprotein lipase ( P < .0 7), and cytochrome oxidase ( P < . 0 1) activities in rhomboideus muscle. Our results suggest that selection for lean tissue growth may have effects on body and tissue composition, metabolic and hormonal state, and fat metabolism that lead to heavier but less mature pigs at birth. This may affect survival rate.