Seasonal disruption of pregnancy: the present view

5. Seasonal disruption of pregnancy: the present view

A definition of seasonal disruption of pregnancy, considered as the major economic Ž . loss related to seasonal infertility, has been given Love et al., 1993 . This definition contends that affected sows in summer–autumn first conceive, have embryos present for a short period of time, but that within a week after implantation the embryos die and the Ž . whole litter is lost Love et al., 1993 . Having undergone this wasteful process, sows typically exhibit signs of oestrus 25–30 days after mating. Furthermore, studies from Ž . Ž . Ž Australia Love et al., 1993 , Italy Enne et al., 1979 , and Finland Peltoniemi et al., . 1999a,b,c indicate the early embryonic deaths and loss of the whole litter as the cause of the reduced farrowing rate in summer–autumn. In addition, our recent work studying embryonic deaths as a cause for repeated breeding in different seasons in group housing system, indicates that loss of an established early pregnancy, determined by ultrasound Ž and progesterone profiles, was rare in spring but common in autumn Tast, Peltoniemi et . al., unpublished work . It seems more than a coincidence that such an irregular oestrus-to-oestrus interval occurs in sows in which embryos are removed by flushing on Days 11, 12 or 13 of Ž . pregnancy Meulen et al., 1988 . In these sows, the first phase of the embryonic signal has occurred, an effect that can be mimicked by two injections of oestradiol on Days 12 Ž . and 13 Pusateri et al., 1996a . A second signal produced by the embryos or further injections of oestradiol is required to extend the life of the corpora lutea beyond 30 days Ž . Pusateri et al., 1996b . An interesting question is whether the seasonal disruption of Fig. 2. Season–nutrition interaction of hormones involved in embryonic survival and establishment of pregnancy. Seasonally suppressed LH in feed-restricted sows may exert a downward effect on progesterone, Ž reducing secretory activity by the endometrium. This results in an inadequate second embryonic signal dotted . line within the second oestrogen surge and seasonal disruption of pregnancy with sows returning to oestrus before Day 30 after mating. pregnancy seen in seasonal infertility is due to inadequacy of the embryonic signals or a failure of the maternal response to these signals. The fact that both progesterone Ž . Ž secretion Wrathall et al., 1986 and LH pulsatility Peacock, 1991; Smith and Almond, . 1991; Peltoniemi et al., 1997a in the early pregnant pig are subject to seasonal suppression during late summer–early autumn would support the latter theory. However, as seasonal and nutritional effects on LH secretion were as such considered too weak to Ž cause regression of CL and seasonal disruption of pregnancy Peltoniemi et al., . 1997a,b , we have come to the conclusion that LH-mediated effects on progesterone may in fact lead to inadequacy of embryonic signals. This is why the sow fails to respond to the embryonic signalling, leading to a seasonal disruption of pregnancy. The present view therefore is that LH secretion is reduced in summer–autumn, and this effect is amplified by a restricted post-mating feeding strategy, which suppresses Ž . progesterone seretion Fig. 2 . This effect only becomes evident after the CLs become independent of pituitary support after Day 12. The reduced progesterone then has a negative impact on histotrophic secretions from the endometrium, exerting a negative effect on viability of embryos, and, more importantly, capability of the embryos to Ž . produce the second embryonic oestrogen signal on Day 18 required for maintenance of pregnancy beyond Day 30. The sow fails to respond to the inadequate embryonic signals, CLs regress and pregnancy is terminated.

6. Conclusion