Agriculture Reference
In-Depth Information
Most other studies, however, did not find a negative effect of a high feed level beyond day 3
on embryo survival, despite a consistent reduction in systemic progesterone, and a number
of studies even reported a positive effect of a high feed level on embryo survival (Quesnel
et
al.
, 2010; Athorn
et al.
, 2011a). Some of the reported benefits of a higher feed level during
established luteal function not only apply to embryo survival, but also to pregnancy rate,
as in Dyck and Strain (1983) and Virolainen
et al.
(2004) cited above. Similarly, Athorn
et
al.
(2011b) and Langendijk
et al.
(2011) showed that when first parity sows or gilts were
classified according to their growth rate during the first 25 days of gestation, as a measure
of feed intake, those with the highest 25% growth rate had pregnancy rates of 100 and 92%,
and those with the lowest 25% growth rates had pregnancy rates of 92 and 85%, respectively
for these two studies. These effects of a high feed level on maintenance of pregnancy may
be a direct effect of nutrition on luteal function reducing the percentage of sows with luteal
insufficiency, but may also be the reflection of a greater embryonic survival, leading to less
litters that are numerically too small to sustain sufficient oestrogenic signalling to rescue
the corpora lutea and maintain pregnancy.
These observations suggest that the first few days post-mating, a high feed level may
reduce systemic progesterone and negatively impact the uterine conditions for later
embryo survival. Beyond the first few days, a high feed level may have a different effect
on luteal function, uterine conditions and possibly directly on embryo development, and
may actually support embryo survival and maintenance of pregnancy. This is reasonable
if one considers that the dynamics of progesterone production during the first few weeks
of gestation change from a small quantity of luteal tissue secreting a limited amount of
progesterone into the systemic circulation, to fully functional luteal tissue by ten days
after gestation. What should also be considered is the direct transfer of progesterone from
the ovaries to the uterine horns, which bypasses the systemic circulation and depends on
the gradient of progesterone in the utero-ovarian circulation. The effects of feed levels
on the dynamics of progesterone secretion and on the local transfer of progesterone
have previously been overlooked and only recently been recognised (Langendijk and
Peltoniemi, 2013; Virolainen
et al.
, 2005a).
2.4.3
Local versus systemic progesterone
The effects of nutrition on uterine progesterone supply are a result of effects on systemic
progesterone, which include secretion and clearance, and of local transfer of progesterone
directly from the ovary to the uterine horns. The latter occurs through countercurrent
transfer and anastomoses between ovarian veins and uterine arteries (Langendijk and
Peltoniemi, 2013). The direct supply of progesterone forms a significant contribution
to the uterine supply, since unilateral removal of one ovary reduces embryo survival in
the ipsilateral horn even though initial embryo distribution over the two horns remains
unaltered (Athorn
et al.
, 2011a). A direct supply of progesterone also explains why
progesterone in uterine arteries is considerably greater than in the systemic circulation,
and is also greater in uterine arteries proximal to the ovary compared with those distal
to the ovary (Stefanczyk-Krzymowska
et al.
, 1998). As the local supply of progesterone is
direct, it is not subject to hepatic metabolism such as is the case for systemic progesterone.
Therefore, an increase in feeding hypothetically increases progesterone secretion by the
