1 Department of Molecular Biology and Genetics - Center for Quantitative Genetics and Genomics, Department of Molecular Biology and Genetics, Science and Technology, Aarhus University2 Pig Research Centre, Breeding & Genetics3 Pig Research Centre, Breeding & Geneticsd4 Department of Molecular Biology and Genetics - Center for Quantitative Genetics and Genomics, Department of Molecular Biology and Genetics, Science and Technology, Aarhus University
Breeding for increased litter size in Danish Landrace and Yorkshire started in 1992. Since then, litter size has in-creased significantly. Commercial pig producers and nucleus breeders argue that increases in litter size increase the number of dead piglets. Pig breeders in Denmark responded by including the total number of live piglets at day five (TN5) in the breeding goal. The objective of this study was to investigate whether breeding for TN5 has affected total number of born (TNB) and the mortality rate in five-day-old piglets. Data included records of first litter from 43,432 Landrace sows and 34,446 Yorkshire sows in Danish nucleus herds from January 2004 to December 2010. At far-rowing, litter size was recorded as TNB including number of still-births. Litter size and mortality rate up to day five after birth were analyzed using a two-trait animal model assuming normality. The heritabilities of maternal effect on litter size were 0.079 and 0.095 in Landrace and Yorkshir e. The heritabilities of maternal effect on piglet-mortality rates were 0.069 and 0.082 in Landrace and Yorkshire. The genetic correlation between litter size and mortality rate were unfavourable; and the estimates were 0.36 and 0.25 for the Landrace and Yorkshire. These findings indicate that breeding for TNB has increased the mortality rate from 1991 to 2004. However the introduction of TN5 in the breeding goal reduced mortality. The annual means of BLUP values related to piglet mortality showed that the ge-netic gain has reduced the piglet mortality rate by 4 %-points in Landrace and Yorkshire from 2004 to 2010. The genetics gain was confirmed by decreased phenotypic annual mortality rates in the breeding and multiplier herds.