Metabolic role of the amino acids in the adaptation of Holstein dairy cows to two contrasting challenging situations

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This thesis investigated the role of the amino acids (AA) during the transition period and lactation under grazing conditions, and heat stress (HS) as two contrasting challenging situations for the dairy cow. In the experiment 1, the branched-chain AA (BCAA) metabolism was studied in North American (NAH) vs. New Zealand (NZH) Holstein cows between -45 and 180 days in milk (DIM). Higher plasma concentrations of BCAA were confirmed in NAH vs. NZH in early lactation. The temporal profile of hepatic gene expression of specific BCAA transporters, especially LAT1 and Eeg1, differed between the genetic strains in association with the level of somatotropic axis uncoupling and insulin resistance in the liver. In the experiments 2 and 3, we studied the role of the metabolism of the AA against HS and its amelioration through nutritional strategies under controlled conditions. The plasma and milk metabolomics indicated that the HS triggered skeletal muscle mobilization and enhanced AA catabolism. Dietary supply of vitamin E and Se above requirements seemed to have modulated the skeletal muscle mobilization, while D3 and Ca did not show significant effects (experiment 2). The results of experiment 3 suggested that the partial amelioration of HS effects on productive and physiological parameters through high dietary supply of Met (+70 %), Lys (+65 %) and His (+21 %) were mediated by a recovery of phosphatidylcholines and decreased oxidation of Val, Trp and Leu due to enhanced one-carbon metabolism and Lys oxidation, respectively.