Genetic aspects of maternal ability in sows

The purpose of this thesis was to gain knowledge about the genetic background of different aspects of maternal ability in sows. Today’s selection for increased litter size at birth will put higher demands on the sow’s ability to take care of her litter, and may cause problems with increasing piglet mortality and decreasing piglet growth. This motivates inclusion of a measure of maternal ability into the breeding goal. The studied traits are different measures of piglet mortality, piglet birth weight, piglet growth, sow behaviour and sow body condition. Both direct genetic effects and maternal genetic effects influenced traits measured on the piglet. The maternal effect was more important than the direct effect for all traits measured prior to weaning. Piglet birth weight was favourably genetically correlated to survival of live born piglets. However, we found unfavourable genetic correlations between stillbirth and birth weight of the piglet. This indicates that selection for increased birth weight is not a recommendable strategy to improve piglet survival at birth. We also conclude that a breeding programme aiming at improving both pre-weaning survival and pre-weaning growth should include both these traits, as we found indications of unfavourable genetic correlations between them. The sow’s capacity for a high pre-weaning growth rate and a high survival rate in her litters was genetically associated with a greater loss of weight and backfat in the sow during lactation. A large loss of body reserves may increase the risk of reproductive problems. Low heritabilities, ranging from 0.01 to 0.08, were estimated for the sow behaviour traits: sow’s response to a screaming piglet, sow’s response to piglet handling, fear of and aggression towards the stockperson. A strong response to a piglet scream was genetically associated with a lower mortality rate, and a high fear response was genetically associated with a higher mortality rate. In conclusion, it seems possible to improve piglet survival rate through selection. However, selection for piglet birth weight is not a good strategy. Sow behaviour, for example fear of the stockperson, could be a possible selection criterion to improve piglet survival. Some attention needs to be paid to the sow’s body condition during lactation when selecting for improved piglet survival and growth.