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1 Département des Sciences Animales, Université Laval, Ste-Foy, QC, Canada, G1K 7P4
2 Dairy & Swine Research and Development Centre, Agriculture and Agri-Food Canada, Lennoxville, QC, Canada, J1M 1Z3
3 Rowett Research Institute, Aberdeen, UK, AB21 9SB
4 Faculté de Médecine Vétérinaire, Université de Montréal, St-Hyacinthe, QC, Canada, J2S 7C6
Corresponding author: H. Lapierre; e-mail: lapierreh{at}agr.gc.ca.
The response of splanchnic tissue metabolism to different levels of metabolizable protein (MP) was measured in 6 catheterized multiparous lactating Holstein cows. Three diets, balanced to provide similar energy intakes and increasing amounts of MP (g/d)—1922 (low), 2264 (medium), and 2517 (high)—were fed during 21-d experimental periods according to a replicated Latin square. On d 18, 19, or 20, six hourly blood samples were collected simultaneously from the portal and hepatic veins plus an artery to determine net fluxes of nutrients across the portal-drained viscera and the liver. Yields of milk and protein increased, as did urinary N excretion with increasing MP. Portal absorption of essential amino acids (EAA) increased linearly with increasing MP supply, as did liver removal of His, Met, and Phe. In contrast, liver removal of the branched-chain AA (BCAA) and lysine was unaffected by diets. With increasing MP, the ratio of milk output to postliver supply of BCAA, Thr, and Lys decreased linearly, indicating oxidation of these AA in the peripheral tissues. Concomitant to a decreased catabolism of EAA in the liver (His, Met, Phe, and Thr) and/or in peripheral tissues (BCAA, Lys, and Thr), the efficiency of transfer of absorbed EAA into milk protein decreases markedly as protein supply increases. The efficiency of transfer of absorbed AA into milk also varies greatly between AA. These 2 important factors should be taken into account when building predictive schemes for milk protein output.
Key Words: amino acid • splanchnic tissue • mammary gland • cow
Abbreviation key: BCAA = branched-chain AA, EAA = essential AA, MP = metabolizable protein, pAH = para-aminohippurate, PDV = portal-drained viscera.
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