Test day models and genetic parameters in Latxa and Manchega dairy ewes

Abstract

Test day (TD) milk yield and protein percentage models and genetic parameters estimation has been studied in Latxa and Manchega milk ewes. Genetic parameters for test day data were estimated under a multivariate animal model which considered production in successive test days in first lactation as different traits and under a univariate animal model considering milk and protein percentage test day as repeated measures of the same trait. Two contemporary groups (CG) were fitted, the classical herd-year-season of lambing (HYS) utilized with standardized lactation records and the herd-test day (HTD). In the multivariate approach, small differences in the variance component and genetic parameter estimates were obtained by fitting the HTD rather than the HYS effect. TD milk yield heritability estimates ranged from 0.20 to 0.33 and from 0.17 to 0.26 in Latxa and Manchega breeds, respectively. Residual variances and heritability estimates were also very similar for both models for protein percentage TD records (heritability ranged from 0.21 to 0.28). Genetic correlations were higher between adjacent TD records (from 0.87 to 0.97 for TD milk yields and from 0.61 to 0.99 for protein percentage TD records) than for records two or three TD apart (from 0.46 to 0.85 for TD milk yield and from 0.50 to 0.72 for protein percentage TD records), indicating that TDs could be affected by different genetic bases. In the univariate approach, the HTD model showed a lower residual variance and higher heritability estimates (0.20 and 0.21) than the HYS model (0.16 and 0.19) in both breeds and traits. TD heritability estimates under the univariate approach were lower than those of the multivariate approach for both CG and breeds. Permanent environmental variance was also affected by the CG included in the model. Higher repeatabilities were found in both breeds with the HTD model (0.35, 0.47 and 0.52) than with the HYS model (0.21, 0.37 and 0.40). How to consider TD measures within and among successive lactations and how to combine TD genetic values into a single animal breeding value are questions which need further investigation.