Agriculture Reference
In-Depth Information
2.5
Hormonal interventions to support the maintenance of
pregnancy
Because progesterone is such an important driver of endometrial function and is recognized
as one of the key factors controlling embryo survival, supplementing progesterone could
overcome luteal insufficiency and thus improve litter size or salvage pregnancies that
would otherwise be unsuccessful. In gilts fed twice the maintenance requirement, which
would presumably cause a drop in systemic progesterone, supplementing progesterone
during the first three days after oestrus increased embryo survival at day 28 from 70
to 84% (Jindal
et al.
, 1997). However, there are risks associated with supplementing
progesterone or progesterone analogues. Indeed, supplementing altrenogest too soon
after ovulation seemed to compromise both transport of ova through the oviduct and
fertilisation rate, and supplementing altrenogest on days 4 to 6 after mating reduced litter
size in first-parity sows (Soede
et al.
, 2012).
Another interesting paradigm is the induction of accessory corpora lutea (Caldwell
et al.
,
1969; Guthrie and Polge, 1976) by treating pregnant sows with a follicle stimulating agent
such as eCG and subsequently with hCG or a GnRH analogue, to induce ovulation or
at least luteinisation of these follicles. This treatment was developed to extend the luteal
phase in cyclic gilts in order to synchronise subsequent oestrus by inducing luteolysis at a
desired time point. The accessory corpora lutea increase the total amount of progesterone
secreted and may therefore support the maintenance of pregnancy and increase litter
size (O'Leary
et al.
, 2011). A single treatment with a gonadotrophin may also increase
progesterone secretion by the existing corpora lutea without needing to induce accessory
corpora lutea (O'Leary
et al.
, 2011; Peltoniemi
et al.
, 1995). However, these models have
not been tested as remedies for pregnancy support. Induction of follicle growth on day
12 may have an additional effect on the maintenance of pregnancy since the secretion
of oestrogens by the follicles at this stage of pregnancy would mimic the endogenous
oestrogen signal from the embryos, which normally prevents luteolysis thus maintaining
pregnancy. Again, the model of induced oestrogen secretion has been used to induce
pseudo-pregnancy as a means to synchronise cyclic gilts (Noguchi
et al.
, 2010), but may
have applications for pregnancy support as long as the treatment with gonadotrophins is
performed on day 12. Salvaging small litters that would otherwise not be sustained may,
however, be a scenario that has to be considered.
2.6
Conclusions
During the first five days post-mating, limiting feed intake may increase embryo survival.
After this period there seems to be no risk associated with a higher feed allowance and
there are potentially benefits for maintaining pregnancy and embryo survival, because of
stimulating effects on secretion of progesterone which is transferred directly to the uterus
through local mechanisms despite lower systemic progesterone. Therefore, after day 5 gilts
and primiparous sows do not need to be restrict-fed and feed allowance can be increased
to match the desired growth curve and/or recovery from the previous lactation. In sows
that are at risk of pregnancy failure, such as in group-housing situations or periods of
