Genetic evaluation of mastitis liability and recovery through longitudinal analysis of transition probabilities

Genetic evaluation of mastitis was carried out by modeling the dynamic nature of somatic cell count (SCC) within the lactation. The SCC patterns were captured by modeling transition probabilities between assumed states of mastitis and non-mastitis. A widely dispersed SCC pattern generates high transition probabilities between states and vice versa. This method can model transitions to and from states of infection simultaneously, i.e. both the mastitis liability and the recovery process are considered. A multilevel discrete time survival model was applied to estimate breeding values on simulated data with different dataset sizes, mastitis frequencies, and genetic correlations.Correlations between estimated and simulated breeding values showed that the estimated accuracies for mastitis liability were similar to those from previously tested methods that used data of confirmed mastitis cases, while our results were based on SCC as an indicator of mastitis. In addition, unlike the other methods, our method also generates breeding values for the recovery process.The developed method provides an effective tool for the genetic evaluation of mastitis when considering the whole disease course and will contribute to improving the genetic evaluation of udder health.Mastitis is a common disease in dairy cattle with severe economic consequences [1]. It has been shown that susceptibility to the disease varies between breeds and individuals, with heritabilities ranging from 0.07 to 0.12 [2,3]. Genetic evaluation of the disease is an important issue and a wide range of methods is available. Methods can be divided into cross-sectional or longitudinal approaches. Cross-sectional methods consider each lactation as a static process, whereas longitudinal methods model changes in disease states during the lactation. The variables mostly used in mastitis analyses are recorded cases of clinical mastitis (CM) or somatic cell counts (SCC). Routine recording of CM is not performed in most coun