EFFECTS OF LEVEL AND TYPE OF ENERGY-SOURCE (VOLATILE FATTY-ACIDS OR GLUCOSE) ON MILK-YIELD, COMPOSITION AND COAGULATING PROPERTIES IN DAIRY-COWS

Four fistulated Holstein cows were arranged in a 4 x 4 Latin square de sign to study the effects of level and type of energy source on milk y ield and composition. Treatments consisted of a basal diet fed alone ( low energy treatment) or with 3.3 Meal of net energy for lactation fro m extra nutrients perfused either into the rumen (either propionic aci d or a mixture of volatile fatty acids) or into the duodenum (glucose) . Increasing the energy input without changing the vola tile fatty aci d profile improved milk yield and slightly increased milk protein and fat yields. Compared with the isoenergetic mixture of volatile fatty a cids, both propionic acid and glucose infusions significantly decrease d fat content (4.5 g/kg) and yields (respectively, -111 and -160 g/d), but affected fatty acid proportion and yield differently (more elonga tion process and less C18 with glucose infusion). Protein yield was sl ightly increased by propionic acid infusion but not by glucose because of the counterbalanced effects on milk yield (-1.3 kg/d) and protein content (1.5 g/kg). The coagulating properties of milk were directly l inked to variations in protein, casein and mineral contents. In conclu sion, propionic acid or glucose scarcely affected milk protein content , but induced a similar decrease in milk fat content probably through different metabolic pathways. (C) Inra/Elsevier, Paris.