Description of herd level criteria for good and poor udder health in Norwegian dairy cows.

Data from the Norwegian dairy herd recording system were used to evaluate herd level criteria for good and poor udder health in dairy cows. All dairy herds with at least 10 cow-years were included (n = 12,780). The association between bulk milk somatic cell count (BMSCC) and clinical mastitis rate (CM) was weak (R2 = 0.002), indicating that studies based on BMSCC provide different information regarding udder health than studies based on CM. The strongest correlations were found between average cow milk somatic cell count (CMSCC) and BMSCC and between percentage of individual cow milk sample >200,000 cells/mL and the variables BMSCC and CMSCC. A new variable called new infection rate (NEWINF) based on individual cow somatic cell count measurements and CM was evaluated. The NEWINF is influenced by the frequency of individual cow milk samplings, and, to compare herds, it is necessary to adjust for the frequency of cow milk samplings on each farm. The NEWINF was found to be a useful tool for describing overall udder health status at the farm level, as it combines both CM and somatic cell count.