Effet du niveau d'apport azoté à des vaches au début de la lactation sur la production laitière et l'utilisation de l'azote

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Abstract
Using 2 groups of 8 cows, the influence of a severe protein underfeeding (association with an energy underfeeding) during early lactation on N metabolism and milk production was studied. All the animals received a restricted amount of a diet of constant composition (40% dry beet pulp, 30% concentrate, 20% straw and 10% hay). During the last month of pregnancy, the feeding schedule was the same for all animals. During the first 7 wk of lactation, 1 group (H) received a high protein level (by a partial substitution of soybean meal for barley) and the other group (B) a low level (protein underfeeding). From the 8th wk, all cows were again fed according to requirements and were kept in the cow-shed until the 10th wk and then on pasture. Milk production, composition, liveweight of the animals, N balance and the concentration of several constituents, especially urea, in the milk, the blood and the urine were examined.