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* Unité Mixte de Recherches Production du Lait, Institut National de Recherche Agronomique, 35590 Saint-Gilles, France
Agribrands Europe San Juan 189 Barcelona, Spain
Division of Animal Nutrition and Physiology, Institute of Animal Genetics, Nutrition and Housing, University of Berne, CH-3012 Berne, Switzerland
Corresponding author:
S. Rigout; e-mail:
Sophie.Rigout{at}rennes.inra.fr.
Four lactating dairy cows were arranged in a 4 x 4 Latin square design to study the effect of intestinal glucose supply on milk fat synthesis. Glucose (0, 443, 963, and 2398 g/d) was continuously infused in the duodenum over 14-d periods. Grass silage-based diets were formulated to be isoenergetic and isonitrogenous and met 100 and 110% of energy and protein requirements according to INRA (1989). Mammary uptake of nutrients was estimated through assay of arteriovenous differences and blood flow measurements. Glucose infusions decreased arterial concentrations of acetate, ß-hydroxybutyrate, and nonesterified fatty acids linearly and total glycerides curvilinearly. Milk fat yield was slightly decreased (– 52 g/d) between 0 and 963 g/d of glucose and milk fatty acid composition was modified by a marked decrease in long-chain fatty acids and an increase in de novo synthesis. The decrease in long-chain fatty acids, related to the decreased mammary uptake of plasma total glycerides, was likely due to a decrease in lipoprotein lipase and esterification activities. In regards to the evolution of metabolite concentrations in milk, the enhanced de novo synthesis and chain elongation was probably allowed by a greater availability of NADPH synthesized through pentose phosphate pathway. The greatest dose of glucose clearly decreased milk fat yield (–234 g/d). A mammary cell mediated intracellular reaction likely caused a homothetic decrease in milk fatty acids. However, reduced synthesis was not due to a shortage of glycerol-3-phosphate because its milk concentration remained unchanged. In conclusion, changes in exogenous glucose supply, in cows fed a grass silage-based diet, decreased milk fat production and modified milk fatty acid composition.
Key Words: glucose infusion • mammary metabolism • milk fat • dairy cow
Abbreviation key: AV = arteriovenous, G0 = 0 g/d of glucose infused in the duodenum, , G1 = 443 g/d of glucose infused in the duodenum, , G2 = 963 g/d of glucose infused in the duodenum, , G3 = 2398 g/d of glucose infused in the duodenum, , MBF = mammary blood flow, PDI = protein truly digested in the small intestine
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