Distribution of clinical mastitis among quarters of the bovine udder.

Clinical mastitis episodes occurring from 1962 through 1991 in the Louisiana State University Dairy Research Herd were studied to determine the distribution of clinical mastitis among quarters of the udder. Data were detailed records of all mastitis episodes that occurred during 1630 Holstein lactations. Incidence of episodes, categorized according to which quarters within a cow were clinical for a given case, were compared with mathematical expectations based upon the assumption that quarters were independent. Results indicated that quarters within a cow were more alike with respect to clinical mastitis than would be expected if quarters were independent. More episodes occurred in which either no quarters or all four quarters were clinical. Deviation of observed frequencies from expectation could have resulted from generalized cow differences, such as cow milk yield, immune competency, mammary type characteristics, and general health. Front quarters had less clinical mastitis than rear quarters when all episodes were considered. No difference was observed in incidence between front and rear quarters when only first episodes in first lactations were considered. No difference was observed between incidence in left and right quarters. Diagonal pairs occurred less often than expected and at about half the frequency of other pairs. Dependence between quarters should be taken into consideration when experiments are designed and analyzed.