POST-WEANING ANESTRUS IN PRIMIPAROUS SOWS - LH PATTERNS AND EFFECTS OF GONADOTROPIN INJECTION AND BOAR EXPOSURE

Post-weaning anoestrus was studied in eighteen primiparous sows, selec ted from a breed showing a high proportion of anoestrous sows. The sow s were studied from late lactation, through weaning at day 29 post-par tum (p.p.), until day 21 post-weaning (p.w.). Blood samples were taken once daily, and frequently (every ten minutes) on several days before and after weaning. Out of a total of ten anoestrous sows, three were exposed to a boar and seven were given gonadotropins (PG600) on day 21 p.w.. Serial blood samples were analysed for LH only and daily sample s were additionally analysed for oestradiol-17beta and progesterone, b y validated radioimmunoassay procedures. Analysis of variance of the b asal level, pulse frequency, pulse amplitude and mean level of LH show ed, retrospectively, that during lactation the basal and wean levels o f LH were significantly lower in anoestrous than in oestrous sows (P l ess-than-or-equal-to 0.05). Furthermore, the post-weaning basal and me an levels of LH were also significantly lower in anoestrous than in oe strous sows (P less-than-or-equal-to 0.05). However, because of the sm all number of oestrous animals (n = 3), these results should be interp reted with caution. Exposure of anoestrous sows to a boar did not resu lt in oestrus and/or ovulation within seven days, but did increase LH pulse frequency. Injection of gonadotropins resulted in an LH surge, o estrus and ovulation in only three sows, but oestradiol levels were in creased in six sows. From our experiments and from reports in the lite rature we conclude that a lowered secretion of LH may play a role in t he aetiology of post-weaning anoestrus in the sow.