Genetic evaluation of susceptibility to and recoverability from mastitis in dairy cows

In paper I, extensive simulation analyses were performed to develop a bivariate model for joint genetic evaluations of susceptibility to- and recoverability from mastitis. In paper II, the bivariate model with an added time function as well as several systematic effects was applied to real data to estimate genetic parameters in Danish Holstein cows. In paper III, genome-wide association studies were conducted to identify associated single-nucleotide polymorphisms and thereof candidate genes. In paper IV, a dynamic health classification, which takes severity of possible infection into account was introduced to further improve the genetic evaluation of mastitis.

Findings in paper I demonstrated that both traits can be modelled jointly and genetic parameters could be correctly reproduced. In paper II, we detected presence of genetic variation that resulted to heritability (ranging from 0.06 to 0.08) of similar size for both traits. The between trait genetic correlation was -0.83. Despite the strong negative genetic correlation, association signals in paper III did not overlap, suggesting that the traits are at least partially regulated by different genes. Complexity of the traits was manifested with the absence of strong association signals. In paper IV, considerable genetic variation was detected for cows’ presence in health classes defined for longer periods, whereas the variations in health classes defined for short-term and sudden changes (e.g., acute) were mostly attributed to environmental factors. Although susceptibility to- and recoverability from mastitis are strongly negatively correlated, recoverability which is as heritable as susceptibility could be considered a new trait for selection. Evaluating and modelling the ability of animals to overcome infection could be of specific benefit in situations of high disease incidence.