乳脂是牛奶的重要成分，是衡量牛奶质量和价格的主要指标之一，同时也与奶牛健康密切相关。乳脂含量受多种因素影响，尤其是日粮组成。日粮中易发酵碳水化合物或多不饱和脂肪酸含量过高时可引起奶牛乳脂合成抑制，造成乳脂降低综合征。但是现有理论尚不能完全解释乳脂合成抑制的机制，且存在很大争议。因此，本研究拟选用8头装有瘤胃瘘管的泌乳高峰期荷斯坦奶牛，采用反转试验设计，利用微生物组学、代谢组学和转录组学技术，研究高精料和高多不饱和脂肪酸两种典型日粮诱导乳脂合成抑制及恢复过程中瘤胃发酵、微生物多样性和代谢产物的变化特征，挖掘肝脏和乳腺组织中的脂质代谢差异表达基因和通路，进一步整合多组学数据以揭示两种典型日粮诱导奶牛乳脂合成抑制时的脂质代谢规律和分子调节机制，为合理配制日粮和提高乳脂含量提供数据和理论支撑。

The fat content is very important in regard to milk quality and economic value， which is also closely associated with cow health; additionally, fat content could be affected by many factors, especially by the component of diet. Diet-induced milk fat depression (MFD) is a specific reduction in milk fat synthesis caused by feeding the diets with high contents of rapidly fermentable carbohydrates and polyunsaturated fatty acid (PUFA). However, the mechanism involved in MFD has not been fully elucidated, much controversy still remains. Eight Holstein cows at peak period of lactation with rumen fistula will be used in change-over experiment to evaluate the effect of MFD induced by high levels of concentrate diet or high levels PUFA diet on the variation of rumen fermention, microbial diversity and metabolites, and the difference in gene expression profiles and pathway related to lipid metabolism in liver and mammary gland by integrating the methods of microbiome, metabonomic and transcriptomics, further reveal the lipid metabolism and molecule mechanism of MFD, providing theoretical basis and practical parameters for the increasement of milk fat content and preparation of diets.