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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The effect of serving sows at first or second estrus after weaning was determined using 114 Camborough sows (Parities 1 to 8). Eighty sows were paired according to weaning-to-estrus interval and parity. One sow of each pair was bred at first and the other at second estrus after weaning. A further 27 sows were catheterized on d 3 after weaning or d 20 of the first estrous cycle and bred at first or second estrus, respectively. Blood samples were taken every 30 min from 0800 to 1330 on the day after catheterization, every 6 h from catheterization until 1 d after standing estrus, and every 12 h for a further 3 d. Of the 80 sows, litter size increased (P < .002) by breeding Parity 1 and 2 sows at second rather than at first estrus (10.4 vs 12.8 total pigs born). These increases in litter size were achieved in the absence of any significant changes in sow weight or backfat during lactation and between weaning and breeding, and predicted changes in fat and protein during lactation. Similar but nonsignificant results (P = .15) were observed for the 27 catheterized sows. Plasma progesterone concentrations measured 50 h after the preovulatory LH surge were higher in Parity 1 and 2 sows bred at second rather than at first estrus (5.4 +/- .7 vs 3.7 +/- .3 ng/mL, P < .04). Sows of Parity 1 and 2 bred at first rather than at second estrus had higher plasma IGF-I (P < .05) and glycerol (P < .001) concentrations and tended to have lower preprandial insulin concentrations (P = .09).(ABSTRACT TRUNCATED AT 250 WORDS)

ContentsThe semen evaluation techniques used in most commercial artificial insemination centers, which includes sperm motility and morphology measurements, provides a very conservative estimate of the relative fertility of individual boars. As well, differences in relative boar fertility are masked by the widespread use of pooled semen for commercial artificial insemination (AI) in many countries. Furthermore, the relatively high sperm numbers used in commercial AI practice usually compensate for reduced fertility, as can be seen in some boars when lower numbers of sperm are used for AI. The increased efficiency of pork production should involve enhanced use of boars with strong reproductive efficiency and the highest genetic merit for important production traits. Given that the current measures of semen quality are not always indicative of fertility and reproductive performance in boars, accurate and predictive genetic and protein markers are still needed. Recently, significant efforts have been made to identify reliable markers that allow for the identification and exclusion of sires with reduced reproductive efficiency. This paper reviews the current status of proteomic and genomic markers of fertility in boars in relation to other livestock species.