A Sire-Dam Threshold Model for U.S. Single-step Calving-Ease Evaluation

Abstract

The traditional U.S. calving ease (CE) evaluation currently uses a single-trait sire-maternal grandsire (sire-MGS) threshold model implemented in a stand-alone pipeline separate from other trait groups, while the genomic evaluation uses a multi-step approach with de-regressed proofs as input. As the Council on Dairy Cattle Breeding (CDCB) transitions national evaluations toward single-step genomic BLUP (ssGBLUP), a sire-dam threshold model using animal-model pedigree was investigated to better represent maternal information while accommodating incomplete calf identification in U.S. data. CE records from 2002 onward were analyzed using single- and multi-trait threshold models, with first and later calvings modeled either jointly or as correlated traits. Variance components were estimated from an informative subset using Gibbs sampler, and breeding values were subsequently obtained at national scale under both traditional pedigree-based BLUP (pBLUP) and ssGBLUP threshold models using a Newton-Raphson algorithm. Genetic correlations between first and later calvings were high for direct (0.95) and maternal (0.84) effects, supporting adoption of the single-trait model. In the final model, direct and maternal heritabilities were 0.092 and 0.115, respectively, with a genetic correlation of −0.40. Genetic trends from pBLUP and ssGBLUP were very similar, whereas comparisons with the current domestic sire-MGS pBLUP evaluation showed breed- and component-dependent re-ranking, particularly for the maternal effect. The sire-dam threshold model provides a practical framework for integrating CE into the CDCB single-step evaluation system while maintaining continuity with the current U.S. interpretation of the published Sire Calving Ease and Daughter Calving Ease traits.