The genetic evaluation of UK Holstein Friesian sires for calving ease and related traits

Records were analysed on more than 75 000 calvings by 685 Holstein-Friesian bulls tested in commercial dairy herds in England and Wales. The data were collected from 1988 to 1994. The traits analysed were the degree of diffic ulty at calving, assessed from no difficulty to serious difficulty, calf mo rtality to 48 h, gestation length and a subjective assessment of calf size. Information was available for the percentage Holstein genes in the sire, d am age (heifer or mature), sex of the calf and the year and month of the ca lving for all records, while lactation number of the dam and the regional l ocation of the herd were also recorded in 1993 and 1994 (14988 records). Es timates of heritability and genetic and phenotypic correlations were obtain ed for all traits by restricted maximum likelihood (REML) procedures using a sire model, while also fitting the effects of dam age, calf sex, year and season of calving and significant interations. Heifers had move difficult calvings, higher calf losses and shorter gestati ons than mature cows while seriously difficult calvings and mortality were higher for male calves (all P < 0.05). Summer months were associated with l ower incidences of difficult calvings, lower mortality and shorter gestatio ns. Heritability estimates were 0.05 (s.e. <0.01) for calving difficulty sc ore, 0.45 (s.e. 0.02) for gestation length, 0.02 (s.e. <0.01) far mortality and 0.08 (s.e. 0.01) for calf size score. Heritability estimates for the s cored categorical and binomial (mortality) traits were also obtained using threshold model analysis. As expected, these estimates were higher than the REML estimates; 0.12 (s.e. 0.01) for calving difficulty score, 0.08 (s.e. 0.01) for mortality and 0.14 (s.e. 0.01)for calf size score. Calving diffic ulty score was genetically correlated with calf size score (-0.81), mortali ty (0.40) and gestation length (0.34) and calf size score was moderately co rrelated with mortality (-0.40). In all cases, the genetic correlations wer e stronger than the corresponding phenotypic correlations. Sire genetic merit teas predicted for calving difficulty score and the dist ribution of these predictions is described. Sire predictions for calving di fficulty score on the underlying scale were backtransformed to predict the expected incidence of serious difficulties in future calvings.