Litter size in swine, although lowly heritable, has been receiving increasing interest especially with the availability of computer programs to allow estimation of genetic merit for this trait using family information. Genetic changes in a trait over time can be monitored by estimating best linear unbiased prediction (BLUP) of breeding value for individuals exhibiting the trait. Data from a recording programme within a Canadian province allowed estimation of genetic and phenotypic trends for three measures of litter size.

Data from purebred Yorkshire and Landrace litters were obtained from the Quebec Record of Performance Sow Productivity Program. First parity litters born between 1977 and 1987 were analysed for total numbers born, numbers born alive and numbers weaned. Data were edited to include only herds with ten or more litters in Yorkshire and 40 or more litters in Landrace. Also, in order to reduce the number of records further, due to computing limitations, a sow was only allowed one daughter per full-sib family. A total of 2024 Yorkshire gilts (from 467 sires and 1539 dams) and 1920 Landrace gilts (from 421 sires and 1436 dams) provided records.