Facteurs de variation de la numération cellulaire du lait chez des vaches laitières indemnes de mammites cliniques

Abstract

Changes in somatic cell count were analysed in 404 lactations of 208 dairy cows managed on an INRA experimental farm. Only lactations free of clinical mastitis were included in this study. On average, somatic cell count was minimal in the second month of lactation (50 000 cells/ml) and maximal in late lactation (200 000 cells/ml). Primiparous cows showed higher somatic cell count than multiparous cows in early lactation, and lower in late lactation. Somatic cell count was higher in Holstein cows than in Montbéliarde or Tarentaise cows. In late lactation, the difference between these two groups of breeds reached 120 000 cells/ml. This effect of breed is not linked to milk yield. Multiparous cows that were affected by more than one clinical mastitis during the preceeding lactations showed higher somatic cell count (+ 40 000 cells/ml over the whole lactation) than those that have had one or no clinical mastitis during the preceeding lactations. Somatic cell count increased slightly during summer, independently of lactation stage. However, all these physiological variation factors cannot explain bulk milk somatic cell count higher than 300 000 cells/ml.