Importance of average litter weight and individual birth weight for early postnatal performance and myofiber characteristics of progeny

Recent evidence suggests that different patterns of prenatal survival causing different levels of intrauterine crowding are occurring in prolific sows bearing litters with 10-15 piglets born, resulting in marked differences in average litter birth weight. Because early intrauterine crowding of embryos is known to have negative effects on prenatal development, the objective of the study was to compare litter characteristics, as well as histomorphometrical and molecular traits of myofibers of selected offspring born from litters of high (H: >1.7 kg) and low (L: <1.3 kg) average birth weight. The selected newborn littermates were either two females representing the intermediate (HI: 1.73 kg) and low (HL: 1.41 kg) birth weight from H-litters or two females representing the intermediate (LI: 1.26 kg) and high (LH: 1.55 kg) birth weight from L-litters. These piglets were sacrificed the day of birth and the longissimus (LM) and semitendinosus (ST) muscles were collected. The average birth weight of the selected HI and LI piglets differed (P<0.05) whereas that of the HL and LH piglets was similar. From birth to weaning, H-offspring grew faster (P<0.05), which was associated with greater (P<0.05) body weight loss of H-compared with L-sows during lactation, where their feed intake did not differ. In LI offspring, brain proportion of birth weight and brain to ST ratio was greater (P<0.05) than in HI but similar between LH and HL progenies. Expressed per mm2, the dark portion of the ST of HI piglets had fewer (P<0.05) primary but similar secondary myofiber numbers compared to LI progeny. Nevertheless, due to the larger ST, total myofiber number was greater in HI and LH than LI piglets. In the LM, myofiber characteristics were similar across birth weight classes, except for the greater (P<0.10) secondary to primary myofiber ratio in HL than LH offspring. In the ST, but not in the LM, the expression of the myoblast differentiation factor 1 was greater (P<0.05) in LH than HL offspring. No birth weight class differences occurred in myosin heavy chain isoform distribution in the LM. In conclusion, female progenies falling in the mid-weight range of the L-litters, but not their heavier littermates, carried some negative phenotypic traits normally associated with intrauterine growth restriction, such as greater brain to ST ratio and impaired myofiber hyperplasia.