Effects of supplementation with 2-hydroxy-4-(methylthio)-butanoic acid isopropyl ester on splanchnic amino acid metabolism and essential amino acid mobilization in postpartum transition Holstein cows.

The present study aimed to investigate the effects of 2-hydroxy-4-(methylthio)-butanoic acid isopropyl ester (HMBi) supplementation on splanchnic AA metabolism, essential AA (EAA) mobilization, and plasma AA status in postpartum transition dairy cows. The EAA mobilization was calculated by difference: EAA excretion in milk protein - net portal absorption of EAA or net splanchnic release of EAA. Eight Holstein cows fitted with permanent indwelling catheters in the hepatic portal vein, hepatic vein, mesenteric vein, and an artery in the dry period preceding second parturition were used in the study. Cows were randomly allocated to 1 of 4 treatments in a 2×2 factorial arrangement with factor 1: control (calcium carbonate) versus HMBi [1.5 g of HMBi/kg of dry matter (DM)] and factor 2: high dietary ethanol (19 g/kg of DM) versus high dietary propanol (16 g/kg of DM). Only factor 1 data are presented. Treatments were administered in 4 total mixed rations and initiated on the day of parturition. Cows were sampled 14 d before expected parturition and 4, 15, and 29 d after parturition. Supplementation with HMBi tended to increase milk fat content but not fat yield, tended to impose a slower rate of decrease in milk casein content with increasing days in milk (DIM), prevented the decrease in plasma Met associated with parturition for control, reduced plasma concentration of Ser, tended to reduce plasma concentrations of Gly and His, and tended to increase hepatic uptake of Met postpartum. Cows excreted 248±18 g more EAA in the milk at 4 DIM than was released from splanchnic tissues. The EAA deficiency decreased as lactation progressed and was not affected by HMBi supplementation. It was estimated that 4,700±600 g of EAA from extra-splanchnic tissues were secreted in milk protein during the first 29 DIM. Extra-splanchnic EAA mobilization can be crucial to sustain milk protein yield in the postpartum transition period and HMBi is a fast-working Met source that can improve Met status of postpartum transition cows.