Agriculture Reference
In-Depth Information
Whilst this strategy allows the mother to capitalise on resource availability during
lactation, which cannot be predicted in advance, and thus rear more offspring in good
seasons, it demands that the investment in any individual piglet prior to birth should be
low and that offspring that cannot be reared in that season should die as early as possible
and with least compromise to littermates. This is achieved by disproportionate allocation
of resources by the mother amongst her litter, both pre- and post-natally, resulting in
intense sibling rivalry to acquire limited nutrients and high mortality risk to weaker
offspring. In the prenatal environment the limited resource is uterine space for placental
development, and 30-50% of released ova do not survive through gestation (Geisert and
Schmitt, 2002; Geisert
et al.
, 1991; Pope, 1994). Asynchronous elongation allows some
conceptuses to be more advanced than others, releasing oestrogen (estradiol 17β) to create
a potentially hostile uterine environment and impede elongation of their less developed
littermates, resulting in degeneration. Subsequently, if the number of fetuses implanted is
too great for uterine capacity, restricted placental development results in reduced nutrient
transfer and fetal death or reduced viability at birth (Foxcroft
et al.
, 2009). In the postnatal
environment, the limited resource is milk and the piglet's advanced dentition at birth,
in the form of sharp needle teeth, arms it with weaponry that can be used to full effect
against littermates when defending a teat. When the number of offspring exceeds the
number of functional teats, the consequences are fatal for less capable piglets.
These challenges have been magnified under modern farming conditions because
the agricultural industry has capitalised on the heritability of economically important
production traits, implementing breeding programmes that have resulted in super-prolific
lines where over-production of offspring is the norm. In the course of domestication,
selective breeding has delivered more than 100% increase in litter size. Denmark has seen
the most dramatic progress in this trait; an increase in total born per litter of four pigs has
occurred over a 13 year period, resulting in an average of 16.6 total born per litter compared
with a general commercial benchmark of 12.7 in Europe (BPEX, 2013; Rutherford
et al.
,
2013). However, the associated mortality rates are considered by many to have become
unacceptably high, due to negative effects on other traits of survival and production (Baxter
et al.
, 2013; Rutherford
et al.
, 2013). In addition, a strong selection pressure for other
economically important traits, such as rapid lean tissue growth and reduced adiposity
in the carcass, has resulted in piglets with reduced physiological maturity at birth, and
thus poor vigour and thermoregulatory abilities (Herpin
et al.
, 1993). In consequence, the
reduction of piglet mortality under farm conditions is a continuing challenge.
In order to develop effective environmental and biological solutions to this challenge, it
is vital to understand the causes of the different types of piglet death and the risk factors
that interact to predispose these events.
11.2
Causes of mortality
Total mortality (live-born + stillborn deaths) currently equates to approximately 16-
20% of the litter (BPEX, 2013). There have been a number of comprehensive review
articles relating to piglet mortality that report the causes of pre-weaning death (Dyck
