Genetic analysis of somatic cell score and female fertility of Israeli Holsteins with an individual animal model.

The individual animal model was used to analyze the conception index, defined as the inverse of the number of inseminations to conception, and mean lactation somatic cell scores (SCS) of Israeli Holsteins. In addition, multitrait variance components were estimated for these traits and for milk production traits. The animal model analyses included 440,558 lactation records for conception index and 224,869 SCS records collected over 10 yr. First parity heritabilities were 0.035 for conception index and 0.158 for SCS. Genetic correlations were negative between conception index and milk production traits and positive between SCS and milk production traits. Genetic correlations between first and second lactations were 0.9 for conception index and 0.7 for SCS. Phenotypic correlations were 0.08 and 0.3, respectively. Despite the low heritability for conception index, the standard deviation of the sire evaluations was 2%, and the range of evaluations was 10%. Predicted genetic trends for conception index and SCS were computed based on the current selection index of Israel. The realized genetic trend for the conception index was slightly positive, despite the predicted negative trend, and the realized genetic trend for SCS, although positive as predicted, was fourfold the predicted value. The reasons for these discrepancies are not known.