The evolutionary strategy adopted by the pig is to produce a large number of relatively undeveloped offspring. Such a strategy demands that relatively little investment is made in each individual piglet pre-natally, and that piglets which are surplus to resources should die at an early stage with least prejudice to their littermates. Thus, inherent variation in neonatal competitiveness is an advantage and survival of the strongest is promoted. Genetic selection strategies resulting in increased litter size and reduced physiological maturity at birth have compounded these effects, making piglet mortality an intractable problem. Records of piglet mortality in commercial units indicate that most deaths of liveborn piglets are attributed to crushing and starvation, but these ultimate causes are often secondary to the effects of perinatal hypothermia. Piglet survival is the outcome of complex interactions between the sow, the piglet and the environment. Commercial strategy has focused on improving the farrowing environment to modify sow behaviour and increase human intervention. However, concerns about the welfare implications of confining sows in farrowing crates may limit the use of such approaches in the future and change the focus towards genetic selection for characteristics in both the dam and offspring which promote survival. Methods of improving the inherent viability of the piglet, for example by increasing birth weight, neonatal vigour and thermoregulatory ability, have been commercially less effective to date, with intra-partum hypoxia exerting an overwhelming influence. A better understanding of neonatal and maternal physiology in relation to placental transfer of nutrients, regulation of the parturition process, colostrum transfer and expression of appropriate maternal behaviour is required for the development of effective future solutions.

Sandra Edwards