ESTIMATION OF GENETIC TRENDS FOR WEANING WEIGHT AND TEAT NUMBER IN IBERIAN PIGS USING MIXED MODEL METHODOLOGY

Mixed model methodology was used to estimate genetic and environmental trends over 23 years for the traits weaning weight and number of teats in three populations of Iberian pigs. Full genealogy was available. REML estimates of variance components were used to obtain the predicted breeding values. Annual rate of genetic trend was estimated as the regression of the yearly mean predicted breeding value on year. Environmental trend was directly obtained from the estimate of the effect of batch. Results obtained for weaning weight were consistent with positive selection differentials and the low selection intensity applied. Estimated environmental trends reflected a change to worse management conditions that occurred in the herd. However, the methodology seems to have failed to separate genetic from environmental effects correctly for teat number. The reason may be in the special characteristics of the trait, which is not normally distributed, and for which the theoretical genetic model may differ from that assumed in mixed model analysis.