Use of a threshold animal model to estimate calving ease and stillbirth (Co)variance components for US Holsteins

Abstract Text:

(Co)variance components for calving ease and stillbirth in US Holsteins were estimated using a single-trait threshold animal model and two different sets of data edits. Six sets of approximately 250,000 records each were created by randomly selecting herd codes without replacement from the data used for the December 2015 national evaluations, and from a second extract using more stringent edits than the official run. The stricter edits required that records have a valid dam ID in addition to a known sire, cows have corresponding lactation records, and animals have a breed composition of at least 93.75% of the breed of evaluation. Calving ease was recorded on a five-point scale ranging from no assistance needed (most common) to extreme difficulty (least common). Stillbirth was coded as a binomial trait indicating whether or not the calf was alive 48 h postpartum. Gibbs sampling was used to estimate (co)variance components from each sample; 100,000 samples were drawn, the first 10,000 rounds were discarded as burn-in, and every fifth sample was retained. The model included fixed parity (1 through 5) and sex-of-calf effects, and random herd-year-season, animal (direct), maternal, maternal permanent environment, and residual error effects. (Co)variance components and heritabilities were averaged over the six replicates of each scenario for each trait and are shown (with standard errors) in Table 1. Direct animal effects in the animal model are comparable to sire calving ease and sire stillbirth in the sire-maternal grandsire (S-MGS) model, and heritabilities were similar for the S-MGS and animal models. Maternal heritabilties were slightly lower in the animal model. Heritability estimates were very similar between scenarios within traits, although maternal heritabilities were slightly higher using the new edits. These differences may be due in part to larger estimates of direct-maternal covariances than reported in previous studies, as well as stricter requirements for known parent IDs in the new edits. The implementation of an animal model for calving traits will provide direct estimates of genetic merit for all animals, not only males, and the adoption of stricter edits will improve data quality without having large effects on the (co)variances used in the evaluation. It also is anticipated that such a change will increase correlations of US evaluations with other Interbull participants for calving traits.

Keywords: animal model, calving traits, (co)variance components