Genetic parameters for clinical mastitis and somatic cell count for the Nordic dairy cattle populations

Clinical mastitis (CM) is a complex disease, causing substantial losses to the dairy industry. Observed CM is a binary trait and due to its low incidence, genetic evaluation is challenging. On the other hand, somatic cell counts (SCC) are measured on a regular basis and may aid in the genetic evaluation of udder health. Determining the genetic architecture of udder health traits at different risk stages during production is necessary for developing genetic evaluations. The objectives of the current study were to estimate genetic parameters for 9 udder health traits (i) CM at 2 risk stages (early and late lactation) and at 3 lactations (6 traits total), and for (ii) SCC at 3 lactations (3 traits total). CM and SCC records for Holstein (HOL), Nordic Red dairy cattle (RDC), and Jersey (JER) cows were available. Traits corresponding to SCC were modeled using random regression. Variance components for a sire model were estimated using Monte Carlo expectation-maximization residual maximum likelihood. Incidence of CM was, in general, low for all breeds but was lowest at the first lactation and highest at the third lactation. Similarly, heritability estimates for individual lactations and periods were very low, ranging from 0.73% to 3.48% for HOL, 1.62% to 3.39% for RDC, and 1.11% to 2.82% for JER. Estimates of heritability for SCC were moderately-low but increased toward the end of the lactation. These ranged from 7% to 14% for HOL, 9% to 14% for the RDC, and 7% to 12% for JER. Genetic correlations among CM traits were, in general, higher between successive CM events and ranged from 0.30 to 0.97 for all breeds. Similarly, genetic correlations for SCC among lactations were high (>0.80) for all breeds. Furthermore, genetic correlations between CM and SCC traits ranged from 0.41 to 0.78 for HOL, from 0.27 to 0.66 for RDC, and from 0.29 to 0.69 for JER. While the heritability estimates for CM traits were low for all the breeds considered, the moderate to high genetic correlations with SCC traits should enhance genetic evaluation of CM.