Brebis laitières : impact de la structure du troupeau et autres facteurs d’élevage sur la composition chimique du lait livré

Abstract

Flock level analysis of milk production aims to determine the effects of non genetic factors of variation for the fat and protein contents variations and their effects. Our purpose was to estimate the effects of the "flock structure" induced by the entrance and drying off of ewes in the milked ewes group. These effects result from elementary factors of variation such as age and days in milk of the ewes. A simulation study was carried out for two Pyrenean flocks where, either all the ewes would be milked during the entire milking period or, for the same flock, ewes slowly entered the milk group throughout the year. Under these conditions, the yearly fat and protein contents where 3.3 g/l and 1.6 g/l higher respectively for the first flock. Furthermore, our purpose was to simultaneously estimate the effects of other non genetic factors, like flock feeding, to compare their importance with "flock structure" effects. We applied a model to the bulk milk tank data, similar to the individual milk recording model used for the genetic evaluation of the breeding values. An analysis based on the milk tank composition of 253 flocks in the Roquefort area indicated that "flock structure" effects could explain around 1.9 g/l and 1.3 g/l, for yearly fat and protein contents respectively. It also showed that flocks fed a diet including grass silage had a higher fat content in their milk (+ 2 g/l).