Within-litter variation in muscle fiber characteristics, pig performance, and meat quality traits.

The objective of this study was to examine the intralitter variation in postnatal growth performance, meat quality, and muscle fiber characteristics when littermates were categorized by carcass weight. Thirty-nine litters were weaned at 4 wk of age and had free access to feed from 2 wk of age until slaughter. They were slaughtered by litter at an average BW of 104 +/- 14 kg, and six pigs per litter were selected for analysis: the heaviest- (HW), middle- (MW), and lightest-weight (LW) pig of each sex. Categorizing littermates in LW, MW, and HW pigs at the same age reflected the differences in postnatal growth rate within a litter; thus ADG, muscle mass, and muscle deposition rate differed across pig weight groups (P < 0.001). Also, the total DNA content was different among pig weight groups (P < 0.001) and reflected differences in muscle growth rate. The difference in muscle growth rate between LW and MW pigs could be explained by a larger (P < 0.05) mean fiber area (MFA) in MW pigs, whereas the number of muscle fibers was similar. Growth rate differences between MW and HW pigs could in part be explained by a higher number (P < 0.01) of equal-sized muscle fibers in HW pigs. The difference in MFA was due to a higher estimated DNA and RNA content per muscle fiber in MW and HW compared with LW pigs (P < 0.05). Pigment content was higher in MW and HW compared with LW pigs (P < 0.01), but no other measured meat quality traits were significantly different across pig weight groups. These results indicate that both the number and the growth rate of muscle fibers contribute to intralitter variation in postnatal growth performance.