The main purpose of this review is to discuss associations between within-litter variation in birth weight, and preweaning survival and postnatal growth in the pig, as the basis for suggesting that the developmental competence of pigs born, as well as the size of the litter, need critical consideration. Extremes of intrauterine growth retardation (IUGR) occur within a discrete subset of fetuses, substantially smaller than their littermates and commonly described as runt piglets. The lower preweaning growth of runt pigs cannot be entirely explained based on their lower birth weight, nor do they show full postnatal compensatory growth. Interestingly, this more complex reprogramming of development in runt pigs can already be identified by d 27 to 35 of gestation. Recently, we reported more universal IUGR effects in commercial dam-line sows, as an indirect response to selection for increased litter size. High ovulation rates (>30 ovulations) in a proportion of greater parity sows are associated with increased numbers of conceptuses surviving to d 30 of gestation, resulting in detrimental effects on placental development of uterine crowding in the early postimplantation period. In turn, this limits nutrient availability to the embryo during a critical period of myogenesis. Consequently, although a reduction in the number of conceptuses occurs by d 50, placental development in the surviving fetuses remains compromised, resulting in IUGR and reduced numbers of muscle fibers at d 90 and at birth, in all surviving littermates. These effects of uterine crowding on fetal and postnatal development are analogous to the detrimental effects of nutritional restriction in gestating sows on fetal myogenesis, birth weight, and postnatal growth. The incompatibility between increased numbers of conceptuses surviving to the postimplantation period, in the absence of increased uterine capacity, offers a biological explanation for increased variability in birth weight and postnatal growth performance reported in greater parity sows. We conclude that a strategy of introducing hyperprolific females into the breeding nucleus, as a means of increasing the numbers of pigs born, needs to be critically evaluated in the context of the overall efficiency of pork production.
Unmodified, third parity, control sows (CTR; n 5 30) or sows subjected to unilateral oviduct ligation before breeding (LIG; n 5 30), were slaughtered at either day 30 or day 90 of gestation and used to determine the effects of numbers of conceptuses in utero on prenatal, and particularly muscle fibre, development. Ovulation rate, number of conceptuses in utero, placental and fetal size, and (day 90 sows) fetal organ and semitendinosus muscle development were recorded. Tubal ligation reduced (P < 0.05) the number of viable embryos at day 30 and fetuses at day 90. Placental weight at day 30 and day 90, and fetal weight at day 90, were lower (P < 0.05) in CTR sows. All body organs except the brain were lighter, and the brain:liver weight ratio was higher in CTR fetuses (P < 0.05), indicative of brain sparing and intrauterine growth restriction in fetuses from CTR sows. Muscle weight, muscle cross-sectional area and the total number of secondary fibres were also lower (P < 0.05) in CTR fetuses. The number of primary fibres, the secondary:primary muscle fibre ratio, and the distribution of myosin heavy chainIb, -IIa, fetal and embryonic isoforms did not differ between groups. Thus, even the relatively modest uterine crowding occurring naturally in CTR sows negatively affected placental and fetal development and the number of secondary muscle fibres. Consequences of more extreme crowding in utero on fetal and postnatal development, resulting from changing patterns of early embryonic survival, merit further investigation.
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