Genetic effects on stillbirth and calving difficulty in Swedish Red dairy cattle at first and second calving

In Swedish Red (SRB) dairy cattle, genetic effects on stillbirth and calving difficulty were studied in 804,268 first- and 673,150 second-calvers. Uni- and bivariate linear sire-maternal grandsire models were used to analyse calving data gathered between 1985 and 2000. Mean incidences of stillbirth were low and differed little between first and second parity, 3.6% vs. 2.5%. At first calving, the heritability of stillbirth on the visible scale was 0.7 - 1.3% for the direct effect and 0.5 - 0.9% for the maternal effect. For calving difficulty, the heritabilities were around 2.5% and 1.8 - 2.1% for direct and maternal effects, respectively. At second calving the corresponding heritabilities for the two traits were of the same order as at first parity, albeit somewhat lower for stillbirth. The genetic correlations between first and second calving results were around 0.8 for direct and maternal effects in stillbirth and around 0.7 for calving difficulty. Univariate models for first and second calvings, analysed separately and together in a repeatability model, were compared with a bivariate model which was assumed to be the most correct. The correlations between bulls' transmitting abilities for stillbirth were 0.94 - 0.96 between results for first calving analysed with a bivariate model and results from the repeatability model. It was concluded that calving traits at first and second parities could be treated approximately as the same trait. Bivariate analyses, or a repeatability model, including calving results for both heifers and cows should be preferred in genetic evaluations of SRB bulls as sires and maternal grandsires.