The influence of divergent selection for body weight at 63 days of age in rabbits on muscle characteristics at a same age or at similar weight

Two lines of rabbits divergently selected for body weight at 63 days during 6 generations (H: high line; L: low line) and a cryo-preserved control group (C) were used to assess the effects of growth rate on muscle characteristics, either at a fixed age (63 days) or at the same weight (2297 + 10 g). In the two series of slaughter, body weight at weaning (28 days) was decreased in the rank order L < C < H. At 63 days of age (n = 20 per line), the decrease in slaughtering body weight between the three lines (L: 2.32 kg < C: 2.62 kg < H: 2.87 kg) induced a proportional decrease in the weight and total cross-sectional area of the semitendinosus (ST) muscle. Mean cross-sectional area of ST myofibers was the lowest in the L line (-15%, P < 0.001), but it did not differ among H and C lines. Total number and type frequency of the myofibers were similar in the three groups. At a same weight (n = 20 per line), the decrease in slaughter age in the order H < C < L (51 d, 57 d, and 63 d, respectively) did not influence ST myofiber histological characteristics, however ultimate pH was slightly lower in the L line compared with the two other groups. In conclusion, divergent growth rate selection had asymetrical effects on myofiber size in rabbits at a same age, whereas it did not influence myofiber traits at a same weight.