Quarter milk samples were collected aseptically at monthly intervals (n = 13) from the cohort cows according to standard procedures (National Mastitis Council, 1999). Samples were frozen and transported to the laboratory of the Milk Control Centre Flanders (MCC, Lier, Belgium) for bacteriological examination. In addition to the monthly milk samples taken from the cohort cows, extra samples were taken when the farmer noticed clinical signs in a quarter (any visual abnormality of milk or the udder). Clinical milk samples were taken both from cows of the cohort and from cows outside the cohort. The average overall incidence rate of clinical mastitis (IRCM; all pathogens) was calculated as described by Barkema et al. (1998).