Agriculture Reference
In-Depth Information
in the first three weeks of gestation (Geisert and Schmitt, 2002) and, more specifically,
between days 12 and 21. Less developed embryos may simply acquire insufficient space
at implantation to survive, and advanced embryos may also start to secrete oestrogens at
a stage when retarded embryos are compromised by the same oestrogens (Geisert
et al.
,
2006). This could be either through the direct effects of these oestrogens on less developed
litter mates or through indirect effects on the uterine environment. Currently, there have
been no indications of nutritional effects on intrinsic embryo quality, or of nutritional
manipulation on the variation between embryos. For example, nutritional manipulations
that slow down development of the more advanced embryos in a litter, or at least reduce
or temporarily inhibit their secretion of oestrogens, would have considerable implications
for the dynamics of embryo survival. This is a research area with potential.
Even though embryo development during the first three weeks of gestation may not
be directly affected by plane of nutrition, there clearly are indirect effects of nutrition
during these early stages on luteal function and uterine environment. These functions are
important because they are instrumental in creating the conditions for implantation that
determine the magnitude of embryo losses when placental area becomes more limiting
towards the latter part of the embryonic phase. There is no doubt that progesterone is
important for remodelling the endometrium to allow successful implantation of, and
supply of, nutrients to the concepti, and hence, systemic progesterone during early
gestation (72 h post ovulation) correlates with embryo survival to day 35 (Foxcroft, 1997,
r=0.48; Zak
et al.
, 1998, r=0.72). Beyond this window, embryo survival is not always
reported to be correlated to progesterone (Athorn
et al.
, 2012a; Gerritsen
et al.
, 2008).
Effects of nutrition on progesterone concentrations in the early- and mid-embryonic
phases are discussed further in this chapter, as are effects of nutrition on the functional
placenta area.
2.2.2
Failure to establish successful pregnancy
Pregnancy failure through resorption of the conceptus or abortion later in the embryonic
phase is hard to quantify, but ranges between 5 and 40% under commercial conditions,
although some of these failures may not be true failures if pregnancy was never established
per se. Pregnancy failure may already be initiated around day 15 when the luteolytic
mechanism is triggered in the absence of sufficient embryos. This occurs mainly in the
case of unsuccessful or poor fertilisation, and may not be related to nutrition, except in
cases where pre-mating nutritional status, such as in first-litter sows or cases of severe
malnutrition, limits the number of oocytes and/or peri-ovulatory events (discussed in
Chapter 17; Soede and Kemp, 2015). Post-implantation, when functional placental area
becomes limiting, nutrition may directly affect available nutrients and previous nutrition
during luteal formation may have a carry-over effect on uterine environment and embryo
survival, thereby resulting in an insufficient number of embryos to maintain the estrogen
signal and luteal function.
Embryo losses between approximately days 15 and 25 may result in a delayed luteolytic
signal, causing loss of pregnancy and delayed return to oestrus, due to insufficient signalling
from the conceptus and a failed rescue of the corpora lutea as a consequence (Tast
et al.
,
