Estimation of variance components for clinical mastitis and somatic cell scores for the Nordic dairy cattle populations

Abstract

Clinical mastitis (CM) is a disease which causes great losses to the dairy industry. Due to the low incidence of CM and its discrete nature, somatic cell scores (SCS), which are measured on a regular basis, are often included in genetic evaluation. As such, determining the genetic architecture of udder health traits at different risk stages is important. Thus, the objectives of this study were to estimate variance components for (i) CM events at two risk stages (early and late lactation) and at three lactations, and for (ii) SCS at three lactations. Data consisted of CM and SCS records for Holstein (HOL), Jersey (JER), and Red dairy cattle (RDC) cows. For CM, each risk period of each lactation was considered as a correlated trait, and for SCS each lactation was considered as a separate trait modelled using random regression. The genetic component was modelled using sire information (sire model). Variance components were estimated using Monte Carlo expectation-maximization residual maximum likelihood. Mean CM incidence ranged from 3.5% to 13.2% for HOL, from 5.6% to 13.6% for JER, and from 3.10% to 10.9% for the RDC breed. Combined heritability of CM was 5.4%, 6.2%, and 6.3% for HOL, JER, and the RDC breed, respectively. Heritability estimates for individual lactations and periods ranged from 0.73% to 3.48% for HOL, 1.11% to 2.82% for JER, and 1.62% to 3.39% for RDC. In addition, genetic correlations among CM traits ranged from 0.28 to 0.95, from 0.32 to 0.98, and from 0.44 to 0.93 for the HOL, JER, and RDC breeds, respectively. On the other hand, the combined heritability of SCS of 305 days in milk and for 1st, 2nd, and 3rd lactations was 0.14, 0.17, and 0.19 for the HOL breed, 0.17, 0.18, and 0.16 for the JER breed, and 0.18, 0.19, and 0.20 for the RDC breed. Genetic correlations for SCS among lactations were high (> 0.80) for all breeds. Furthermore, genetic correlations between CM and SCS traits ranged from 0.41 to 0.78 for HOL, from 0.29 to 0.69 for JER, and from 0.27 to 0.66 for RDC. Overall, the heritability estimates for traits related to udder health, including CM and SCS was low or moderate for all the breeds considered. On the other hand, genetic correlations among CM traits, and among SCS traits were moderately high to high.