An Approximate Bayesian Analysis of Somatic Cell Score Curves in Holsteins

Somatic cell score (SCS) decreases to a minimum at around 60 days postpartum and then increases until the end of lactation. This pattern was described with three non-linear mixed effects models. Parameters of the models were linear in fixed and random (cow) effects. Data were 1705 SCS test-day records (155 primiparous Holsteins) taken from 14 to 300 days in milk. Point estimates were the mode of the joint posterior distribution of fixed and random effects (given dispersion parameters) and approximate marginal maximum likelihood estimates of covariance components. Intramammary infection during lactation, season and period of calving affected SCS patterns. Estimates were similar with or without a relationship matrix. There was between-cow variation of trajectories, suggesting that SCS curve parameters can be modified by selection in some advantageous manner.