GENETIC-FACTORS CONTRIBUTING TO VARIATION IN LITTER SIZE IN BRITISH LARGE WHITE GILTS

The number of ova released (ovulation rate) by 516 Large White gilts born between 1986 and 1989 was recorded. The weight of the gilt at birth, weaning and time of ovulation rate measurement and her number of teats were also recorded. Farrowing data (number born alive and litter weight at birth) corresponding to the ovulation rate were recorded from 382 of the gilts, enabling calculation of prenatal survival (number born alive/ovulation rate). The data were analysed using univariate and multivariate restricted maximum likelihood (REML) techniques with an individual animal model. The additive genetic direct and maternal components of variance and the common family and residual environmental components of variance and the additive genetic and residual environmental covariances between traits were estimated. The univariate REML analyses showed that the additive genetic direct component was a significant source of variation for gilt weight at birth and weaning, teat number, ovulation rate on the left hand side, total ovulation rate and litter weight at birth. Common family environmental effects were significant sources of variation for gilt weights and teat number. The multivariate REML analyses indicated that the genetic correlations between total ovulation rate and ovulation rate from the left and right ovaries were close to unity, with an estimate of the heritability of total ovulation rate of 0.37 +/- 0.09. In the data from gilts that farrowed, the heritabilities of ovulation rate, number born alive and prenatal survival were 0.30 +/- 0.10, 0.09 +/- 0.06 and 0.00 +/- 0.00, respectively. The genetic correlation between ovulation rate and litter size was close to unity, suggesting that genetic variation in ovulation rate explains virtually all of the genetic variation in number born alive in the population of Large White gilts under study.