ESTIMATION OF GENETIC-PARAMETERS FOR LITTER SIZE IN DANISH LANDRACE AND YORKSHIRE PIGS

Variance components for litter size (total number of piglets born) wer e estimated from Danish purebred Landrace and Yorkshire litters by res tricting maximum likelihood. The data were collected from the national Danish breeding programme and consisted of 19 666 litters in Danish L andrace and 29 336 litters in Danish Yorkshire. Four different analyse s for litter size were conducted within breed. In the first two, genet ic groups were included in the model in order to account for the impor tation of animals from other countries; in the other two, genetic grou ps were removed from the model. Within each case, herd-year-type of in semination effects were fitted as fixed (H-fixed models), or herd-year -season-type of insemination effects were fitted as random (H-random m odels). Estimates of heritability ranged from about 0.11 to 0.14 in La ndrace and from 0.10 to 0.11 in Yorkshire. Variance due to herd-year-s eason-type of insemination ranged from 0.029 to 0.041 of total varianc e, values somewhat lower than those obtained for non-genetic permanent effects. In order to compare the four models, data were divided into different subsets, and records from the one subset were predicted usin g parameters estimated from the other subset. Both the correlation bet ween observed and predicted values, and the mean squared error of pred iction indicated that predictive ability was higher in the case of H-r andom models. There was no evidence that genetic groups improved the p redictive ability for litter size. However, group effects affected inf erences about genetic trend, particularly in Landrace, where genetic g roup composition changed consistently over the years.