La sous-alimentation énergétique et/ou azotée chez la vache tarie. Effets sur les acides gras volatils du rumen, quelques métabolites et hormones du plasma et l'excrétion urinaire de la 3-méthylhistidine

Abstract

In order to study the effects of a nitrogenous and/or energetic underfeeding on some metabolites and hormones, 4 dry cows were fed in 2 equal daily meals with hay-based rations. Four rations were distributed. They supplied too much (HN) or too little (LN) digestible protein in the small intestine and too much (HE) or too little (LE) net energy, ie the diets were HN-HE, HN-LE, LN-HE, and LN-LE. Continuous collection of rumen liquor samples (volatile fatty acids or VFA determination), venous blood samples (amino acids (alanine Ala, glutamine Gln, glycine Gly and serine Ser), urea, glucose, insulin and growth hormone (GH) measurements) and urine samples (determination of the amount of 3-methylhistidine 3-MeHi excreted) was carried out for 24 consecutive hours. By comparison with the reference diet (HN-HE), underfeeding was accompanied by a reduction in ruminal VFA concentrations, an increase in those of plasma Gly and GH, and a maintenance of glycemia levels. Furthermore, nitrogenous and energetic underfeeding (LN-LE) was accompanied by a decrease in plasma Ala, Gln and insulin concentrations. Nitrogenous underfeeding (LN-HE) caused a decrease in uremia levels, an increment in those of serinemia and no variation in circulating insulin concentrations. The energetic underfeeding (HN-LE) was accompanied by a reduction in insulinemia levels and an increase in those of alaninemia and serinemia. Such changes suggest that dry cattle adapt to underfeeding by mobilizing endogenous proteins. However, only the 2 diets with reduced energetic supplies were accompanied by an increase (P < 0.05) in the amount of the urinary 3-MeHi excreted per 24 h, suggesting that the level of energy in the diet is the main trigger for protein mobilization. Finally, the study of the 24-h evolution of the parameters revealed the influence of the feeding time (morning or evening) and the level of the dietary nitrogen and energy supplied on the variation in their concentration.