Factors affecting days of discarded milk due to clinical mastitis and subsequent cost of discarded milk.

Records of clinical mastitis for 6.5 yr from one Florida dairy with 1050 to 1350 cows milking per month were used to predict duration of occurrence of clinical mastitis and to estimate dollar value of discarded milk per lactation with occurrences of clinical mastitis. Only 6.1% of lactations accrued more than 28 d of discarded milk and were responsible for 52.7% of days of discarded milk. Accurate maintenance of herd mastitis records enables calculation of actual days of milk available for sale and provides a useful tool for culling and treating decisions. Duration of each occurrence of clinical mastitis was modeled by fixed effects of breed, season of occurrence, current lactation history of mastitis, number of quarters with clinical signs, and average maximum temperature-humidity index for the 2 d prior to the occurrence (to second order). These effects explained 1.8% of observed variation in duration of occurrence. Random effects of cow explained 5 to 6% of variation. Prediction of duration of occurrence had a mean value of 6.6 d. When economic status of individual animals is evaluated, comparison of days of milk production available for sale is preferable to total days of milk production. Estimated average cost of discarded milk for parities 1, 2, 3, 4, and 5 ranged from $29.73 to 223.98 per lactation.