The Endocrine Control of Parturition

The pig evolved as a litter bearing species with the maintenance of pregnancy dependent on the continued presence of the litter in utero and on continued production of ovarian progesterone. The length of gestation is reasonably precise, being approximately 112-116 days depending on the breed, size of litter and season (Cox, 1964; Bichard et al. , 1976; Aumaitre, Deglaire and LeBost, 1979). Parturition occurs slightly more frequently in the late afternoon and at night (Bichard et at , 1976; Boning, 1979) but no differences are seen in the frequency of day and night delivery when parturition is Artificially induced (Hammond and Matty, 1980). For an individual herd a knowledge of the average length of gestation and exact breeding dates are essential for optimal piglet survival and growth when parturition is to be induced. The entire parturition process requires 2-5 hours with piglets being delivered at approximately 12-16 minute intervals (Sprecher et aL, 1974; see Table 16.1). Piglets are delivered randomly from the two uterine horns (Dziuk and Harmon, 1969; Taverne et al., 1977). They sometimes pass each other in birth order (Taverne a aL , 1977) and the placentas are delivered either in part after the emptying of one uterine horn or within approximately four hours after the last piglet is delivered (Jones, 1966). This is at a time when plasma levels of oxytocin are elevated (Taverne, 1979). Parturition is normally preceded by udder oedema, attempted maternal nest building and a milk ejection response (First and Bosc, 1979). Parturition is not without complications. Body temperature increases 13±4.1 hours before delivery of the first piglet, reaches a peak of 0.6 °C-1.2 °C above the normal of 38.3 ±0.3 °C and in healthy sows returns to near normal within 24 hours (Elmore et al. , 1979). Parturition is often complicated by a disease called Mastitis—Metritis—Agalactia in which the sow's temperature remains greatly elevated for a prolonged period and she refuses to provide milk for the piglets. Not all piglets survive farrowing; at least 6% are born dead (Randall, 1972; Sprecher et , 1974; Leman, Hurtgen and Hilley, 1979) and the last piglet in each uterine horn has less than a 50% chance of survival (Sevier and Dziuk, 1976). The factors influencing piglet survival were reviewed recently by Leman, Hurtgen and Hilley (1979). It is apparent from their