Abstract：The mechanism of high concentrate diet which induces milk fat depression in lactating dairy cows (Bos taurus) has not been well explained. The purpose of this study is to investigate the changes of metabolism of milk fat precursors in liver by high concentrate diet fed, and to reveal the mechanism of milk fat depression (MFD) in lactating dairy cows. Ten Holstein cows (25±5 days in milk) were divided into two groups and assigned to 2 dietary treatments: Control group (forage-to-concentrate ratio: 4∶6) or a high concentrate group (forage-to-concentrate ratio: 6∶4). The liver vascular cannula was performed after 20 weeks feeding followed by convalescence for 4 weeks. The following experiments were carried out: 1) Milk production was recorded, and milk fat content was tested once a week; 2) Fasting blood from portal and hepatic vein was collected to test the concentration of triglyceride (TAG) and free fatty acid (FFA); 3) Live biopsy samples were collected to detect mRNA expression of fatty acid metabolism related genes. Results showed that milk yield of high concentrate group was increased during 6~18 week, while milk fat content was significantly decreased after 18 weeks (P＜0.05) compared with control group. Net content outputs of TAG and FFA in high concentrate group were significantly (P＜0.05) or extremely significantly (P＜0.01) decreased compared with control group. The mRNA expression of sterol regulatingelement bindingprotein-1c (SREBP-1c), acetyl-coacarboxylase (ACC), fattyacidsynthetase (FAS) in liver of high concentrate group were significantly (P＜0.05) or extremely significantly (P＜0.01) decreased, while the mRNA expressions of peroxisome proliferators-activated receptor α (PPARα) and carnitinepalmitoyl transterase-1 (CPT-1) were significantly increased (P＜0.05) compared with control group. High concentrate diets decreased fatty acid synthesis but enhanced fatty acid catabolism, which led to the redistribution of milk fat precursors in liver, and decreased its output, and affected milk fat synthesis. This study revealed the mechanism of high concentrate diet fed inducing milk fat depression in lactating dairy cows, and provides a theoretical basis for improving milk fat content.
