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Physiological and molecular mechanisms associated with performance, immunometabolic status, and liver function in transition dairy cows fed rumen protected methionine or choline
The mechanisms underlying benefits of rumen-protected (RP) methyl donor supplementation during the transition period are largely unknown. Objectives were to evaluate performance, immunometabolic status, liver function, and the underlying mechanisms in response to RP methionine (MET) or choline (CHO) supplementation. Eighty-one multiparous Holstein cows were used in a randomized complete block design with 2×2 factorial arrangement of MET (Smartamine M, Adisseo NA) and CHO (ReaShure, Balchem Inc.) level (with or without). Treatments were control (CON), no MET or CHO; CON+MET (SMA); CON+CHO (REA); and CON+MET+CHO (MIX). From −50 d to −21 d before expected calving all cows received the same diet (1.24 Mcal/kg DM). From −21 d to calving, cows received the same basal close up diet (1.54 Mcal/kg DM) and were assigned randomly to treatments (CON, SMA, REA, or MIX). From calving to 30 d in milk (DIM), cows were on the same postpartal diet (1.69 Mcal/kg DM) and continued to receive the same treatments through 30 DIM. MET supplementation was adjusted daily at 0.08% of diet DM and CHO was supplemented at 60 g/cow/d. Blood (-10, 4, 8, 20, and 30 d) and liver (-10, 7, 21, and 30 d) samples were harvested for biomarker and molecular analyses. MET supplementation led to greater DMI during close-up (P = 0.01) and first 30 d postpartum (P = 0.02). Milk yield (P = 0.03) and milk protein % (P < 0.01) also were greater in MET- compared with CHO-supplemented cows. The greater overall plasma albumin concentration (P = 0.04), blood neutrophil phagocytosis capacity (P = 0.01), and neutrophil oxidative burst (P = 0.03) in MET-supplemented cows underscored a better liver function and immune status. In addition, the greater concentrations of reduced and total (P = 0.01) glutathione in liver tissue indicated a lower degree of oxidative stress in cows fed MET compared with CHO. The similar (P > 0.05) activity and mRNA expression of hepatic betaine homocysteine S-methyltransferase, and the lower (P = 0.04) MET synthase activity in CHO-supplemented cows indicated little re-generation of MET from supplemental CHO. In contrast, the greater (P < 0.01) phosphatidylethanolamine N-methyltransferase mRNA expression in response to MET indicate increased CHO synthesis from supplemental MET. Overall, results indicate that MET supplementation improves performance of transition cows through a combination of better immunometabolic status and a reduction in oxidative stress; insufficient regeneration of MET may be one reason for the lack of effect with supplemental CHO.
Keywords: methyl donor, enzyme activity, oxidative stress