Agriculture Reference
In-Depth Information
of events that take place during early gestation and to explain how nutritional strategies
must take into account the dynamics of luteal function and embryo development.
2.2
Early gestation issues: establishment of pregnancy and
embryo mortality
Paramount to establishing a successful pregnancy and maximising embryo survival is the
formation of sufficient luteal tissue once the luteinising hormone (LH) surge has triggered
luteinisation of the pre-ovulatory follicles at oestrus and during the ensuing two weeks.
As in other mammals, porcine luteal tissue develops at an impressive speed and reaches
its full size between day 10 and 12 after ovulation, at which time total luteal mass ranges
from 6 to 8 g in gilts and 10 to 15 g in multiparous sows (Langendijk and Peltoniemi,
2013). A major factor determining total luteal mass is ovulation rate, with the correlation
between ovulation rate and luteal mass ranging from 0.45 to 0.62 (Almeida
et al.
, 2001;
Athorn
et al.
, 2012a; Willis
et al.
, 2003). This explains why multiparous sows have more
luteal tissue than gilts and first-parity sows. Systemic concentrations of progesterone are
related to total luteal mass (r=0.26-0.45; Athorn
et al.
, 2012a), although this relationship
is probably underestimated because it is generally based on blood samples obtained at
a different time point than the assessment of luteal mass. Reflecting this relationship,
progesterone in the circulation roughly follows the development in luteal tissue mass.
2.2.1
Embryo mortality
Prenatal losses in the pig range between 30 and 50% in commercial genetic lines (Geisert
and Schmitt, 2002). The majority of these losses occur during the embryonic phase
(before day 35), with 20 to 30% of embryos lost by the third week and another 10 to
15% being lost by the end of the embryonic phase (Ford
et al.
, 2002). Experimental
alterations of the available uterine space per embryo, by using superovulation (Dziuk,
1968), superinduction (Rampacek
et al.
, 1975), ligation of uterine horns (Chen and Dziuk,
1993), hemi hystero-ovariectomy (Père
et al.
, 1997), and unilateral oviduct ligation (Town
et al.
, 2004) have shown that by day 30 to 35 of gestation more embryos are lost when
uterine space is limiting, hence the term uterine capacity. Most embryos survive until
day 12 (93-96%; Anderson
et al.
, 1993). After that, elongation (days 11-13), spacing, and
implantation (days 15-17) occur; however, there are few reports of losses during these
early periods. Studies in gilts indicate that by day 25 losses range between 18 and 35%,
and are mostly independent of space (Dziuk, 1968; Pope
et al.
, 1972; Webel and Dziuk,
1974), with space becoming limiting for survival only after day 25.
In terms of nutrition, it is reasonable to expect that competition for available nutrients
starts only after 3 weeks of gestation, at which time limitation of space begins to affect the
size of implantation areas and the size of embryos (Père
et al.
, 1997; Town
et al.
, 2004).
Hence, the plane of nutrition per se is not likely to influence embryonic growth prior
to 3 weeks, however, specific nutrients may influence the development of embryos or
variation in development between embryos. Although its origin is poorly understood, the
variation in embryonic development is thought to be the major source of embryo losses
