Stillbirth (co) variance components for a sire-maternal grandsire threshold model and development of a calving ability index for sire selection

Co) variance components for stillbirth in US Holsteins were estimated under a sire- maternal grandsire threshold model using subsets of data from the national calving ease database, which includes over 6 million calving records with associated stillbirth scores. Still-birth was coded as a binomial trait indicating whether the calf was alive 48 h postpartum. Records were selected for calves whose sire and maternal grandsire ( MGS) were among the 2,600 most frequently appearing bulls ( 2,578 sires and 2,586 MGS). Herd- years were required to contain at least 20 records and only single births were used. After editing, the data set included 2,083,979 calving records from 5,765 herds and 33,304 herd- years. Six sample datasets of approximately 250,000 records each were created by randomly selecting herd codes. Quasi- REML and Bayesian approaches were used to estimate ( co) variance components from each sample. The model included fixed yearseason, parity- sex, birth year group of sire, and birth year group of MGS effects and random herd- year, sire, MGS, and residual effects. Quasi- REML and Bayesian analyses produced similar results, although the Bayesian estimates were slightly larger. Marginal posterior means ( and standard deviations) from the Bayesian analysis averaged 0.0085 ( 0.0015), 0.0181 ( 0.0020), 0.0872 ( 0.0538), and 0.00410 ( 0.0001) for sire, MGS, and herd- year variances and the sire- MGS covariance, respectively. Mean direct and maternal heritabilities were 0.030 ( 0.003) and 0.058 ( 0.005), respectively, and the mean genetic correlation between the 2 effects was - 0.02 ( 0.16). A calving ability index combining stillbirth ( SB) and calving ease ( CE) was developed for inclusion in the Lifetime Net Merit index. The index was calculated as - 4( sire CE)- 3( daughter CE)- 4( sire SB) - 8( daughter SB).