Agriculture Reference
In-Depth Information
Size and vigour
Birth weight is widely believed to be the most important determinant of live-born
survival (Baxter
et al.
, 2008, 2009; Herpin
et al.
, 2002; Kerr and Cameron, 1995; Knol
et al.
, 2002b; Roehe and Kalm, 2000; Tuchscherer
et al.
, 2000). Roehe and Kalm (2000)
reported 40% pre-weaning mortality in pigs with a birth weight of lower than 1 kg, 15%
for birth weights between 1-1.2 kg and only 7% when birth weight was above 1.6 kg.
However, as discussed previously, it is still important to consider degree of development
and not just size, and to make the distinction between a small for gestational age (SGA)
low birth weight piglet and one having undergone IUGR. Although definitions vary,
piglets weighing less than the tenth percentile of the litter at birth yet displaying normal
allometry, are often classified as SGA. On the other hand, piglets that are disproportional
(suggesting that they have not reached their intrauterine growth potential) are classified
as IUGR (Bauer
et al.
,
1998; Hales
et al.
, 2013). The distinction matters because SGA
piglets may have more potential to recover given proper management than IUGR piglets
that have other abnormalities predisposing them to low viability. Small piglets, provided
they are physiologically mature and vigorous, can survive the vulnerable perinatal period,
as evidenced by Baxter
et al.
(2008). Using a sophisticated measure of vigour based on
the strength and persistence with which the newborn piglet manipulated an artificial teat
linked to a computer registration system, these authors demonstrated that a small yet
vigourous piglet could survive equally well as its larger conspecifics. Vitality or vigour
describes the piglet's active survival behaviour and this can be highly variable at birth
(Herpin
et al.
, 1996; Zaleski and Hacker, 1993). Piglets that are quicker to reach the udder,
find a functional teat and suck colostrum are those which survive (Baxter
et al.
, 2008;
Herpin
et al.
, 2002; Tuchscherer
et al.
, 2000). Subsequently, piglets will fight to gain and
maintain possession of a preferred teat and, if they fail to perform optimal massaging
and suckling behaviours at this teat, colostrum and milk yield and piglet survival may
be impaired. Piglets failing to establish teat fidelity grow more slowly (De Passillé
et al.,
1988), with inability to gain access to the most productive teats resulting in starvation or
a lower milk intake when they do manage to gain access to a teat. De Passillé and Rushen
(1989) found that heavier piglets, born earlier in the birth order, won more teat disputes,
established teat fidelity quicker, suckled more frequently and ultimately were at a distinct
advantage over less behaviourally and physically vigorous littermates. The competitive
nature of the neonatal environment is discussed in greater detail in Chapter 13 (Špinka
and Illmann, 2015).
The degree of physiological maturity at birth not only affects the piglet's behavioural
ability to achieve early ingestion of colostrum, but immature organ development may
impact upon the piglet's ability to process any colostrum it obtains. There is a finite
amount of time after birth when the gut is permeable to macromolecules, such as the
immunoglobulins conferring passive immunity, before gut closure commences (at
approximately 48 h) (Cranwell, 1995). During this time it is important for the piglet
to obtain and process the maximum amount of colostrum, as the process of closure is
stimulated by colostrum ingestion. There are two major windows of opportunity for
pathogens to establish in the piglet's systemic circulation. The first is within the initial
24 h of postnatal life and is influenced by delayed colostrum intake which can delay gut
