Factors influencing variation in birth weight and its impact on later performance in Swedish Yorkshire pigs

Sammanfattning: The phenotypic variation in piglet birth weight caused by gender, birth parity number and litter size, and later how this variation in birth weight influenced growth rate until 100 kg and backfat thickness at 100 kg live weight were analysed in this study. Data on purebred Swedish Yorkshire pigs from 13 nucleus herds was the base for the analyses. Records on 32,531 animals born by first and second parity sows were included in the analysis of factors influencing variation in birth weight. Analysis of associations between birth weight and field performance test included 8,827 animals, which were divided into birth weight groups; low birth weight (<1.0 kg), normal birth weight which were divided into nine groups of 0.1 kg (1.1-1.9 kg), and high birth weight (2.0-2.8 kg). Analysis of variance and multiple comparisons of least square means was applied. Piglets born from first parity sows weighed on average 120 grams less, when corrected to the same litter size, and the litters were one average one piglet smaller compared to litters born from second parity sows. Also, liveborn piglets were on average heavier than stillborn piglets by 210 grams (P<0.001). Litter size influenced litter mean birth weight: a decrease with 33 grams per each total born piglet in the litter (P<0.001). Pigs born from first parity sows took on average 4.4 more days to reach 100 kg live weight compared to pigs born from second parity sows (P<0.001), when corrected to the same litter size. Also, males were 2.2 days younger at 100 kg live weight compared to gilts (P<0.001). The heavier the piglet was at birth, the faster it reached 100 kg live weight and the thinner was the backfat thickness at 100 kg live weight. Backfat thickness in pigs born by first parity sows were thinner by 0.2 mm compared to pigs born from second parity sows (P<0.001) and males had thinner backfat thickness than gilts by 0.8 mm (P<0.001). The results indicate, regarding growth performance, that it is better to breed for smaller litters with higher mean birth weight. However, for a farrow-to-finish herd, also litter size is of economic importance.