| Dairy industry in China enjoyed rapid development in recent years.How to improve the milk quality is a hot spot and difficulty of research in order to improve both international competitiveness and corporate competitiveness.Milk fat is affected by many factors,especially the diet.Increasing concentrate or oil in diet can easily cause a reduction of milk fat synthesis,and even milk fat depression(MFD)in serious situation.Most researches about the mechanism of MFD are focusing on the hydrogenation of rumen microorganisms and fatty acids in dairy cows,but changes of the transcriptome in liver and mammary gland under MFD are still unclear.In order to investigate the changes of the transcriptome in liver and mammary gland during MFD caused by diet,and explore the mechanism of MFD,eight primiparous Holstein cows(215±34d-in-milk;575±23kg of body weight)were randomly assigned to sequences in a 2×2 crossover treatment design.The test was divided into two periods,and each period was23 d including 16 d induction phase and 7d recovery phase.The cows were allocated to two kinds of dietary treatment as follows: the control group was fed a low-starch and low-oil diet with the ratio of concentrate to roughage is 50:50;the treatment group was fed a high-starch and high-oil diet(266 g/kg of fine ground corn and 46 g/kg of soybean oil were added in the control diet,DM basis)with the ratio of concentrate to roughage increased to 62:38.All cows were fed the control diet during the recovery phase.Milk samples were collected on the1,4,7,10,13,16,19,22 d for detecting milk yield and milk components;Milk samples were collected on the 15 d for detecting milk fatty acid composition;blood samples were collected on the 15 d for detecting energy metabolism,oxidative stress and liver function of plasma;liver and mammary gland tissues were collected on the 16 d for RNA-Sequencing.The results indicate that:(1)Compared with control group,DMI and milk yield in treatment group decreased markedly on the 13 and 16d(P<0.05);milk fat content and yield tend to decrease after diet treatment lasted for a week and significantly reduced’ on the 10,13,16 and 19d(P<0.05);the content of milk protein,lactose and nonfat solid(SNF)significantly increased on the13,16,19d(P<0.05)while the yield of them significantly decreased on the 13,16,19d(P<0.05).(2)Compared with control group,the concentrations of milk total saturated fatty acids(SFA),odd and medium-chain saturated fatty acids(C8:0,C10:0,C11:0,C12:0,C15:0,C17:0),long-chain saturated fatty acids(C14:0,C16:0,C18:0)and C16:0+C16:1 in treatment group significantly decreased(P<0.05)while total unsaturated fatty acid(UFA),monounsaturated fatty acids(MUFA)(C14:1,C16:1)and polyunsaturated fatty acid(PUFA)(C18:1n9t,C18:2n6t,C18:3n3)significantly increased(P<0.05).(3)Cows fed with high-starch and high-oil diet had a higher concentrations of plasma non-esterified fatty acid(NEFA)(P<0.01),malondialdehyde(MDA)(P=0.02)and glutamicoxalacetic transaminase(AST)(P<0.01).And the plasma blood urea nitrogen(BUN)(P<0.01),total antioxidative capacity(T-AOC)(P<0.01),super oxygen dehydrogenises(SOD)(P<0.01),glutathione peroxidase(GSH-Px)(P=0.03),glutamic-pyruvic transaminase(ALT)(P<0.01),albumin(ALB)(P=0.02),and the ratio of ALB/globulin(GLB)(P=0.02)are all significantly reduced.(4)419 differentially expressed genes were detected in liver tissues,250 genes of them were up-regulated and 169 were down-regulated.Functional enrichment analysis showed that results of GO analysis was mainly related to cell cycle process,cell division,protein complex biogenesis,α-amino acid catabolic process,organonitrogen compound catabolic process terms and so on;KEGG analysis showed that it was mainly related to cell cycle pathway,oocyte meiosis pathway,histidine metabolism pathway,alanine,aspartate and glutamate metabolism pathway and so on.235 differentially expressed genes were detected in mammary gland tissues,64 genes of them were up-regulated and 171 were down-regulated.Functional enrichment analysis showed that results of GO analysis was mainly response to inflammatory response,α-amino acid metabolic process,organonitrogen compound metabolic process,cell development,lipid biosynthetic process,fat cell differentiation terms and so on;KEGG analysis showed that it was mainly related to axon guidance pathway,NF-κB signaling pathway,calcium signaling pathway and so on.(5)Combined with the correlation analysis between the differentially expressed genes and lactation performance,some genes related to lipid regulation had been selected as candidate genes in this study,for researching lipid metabolism of dairy cows under MFD,including G0S2,FGF21 and NR0B2 which were up-regulated in liver,as well as FGFR4,VDR,HTR2 B,CCL21 and TYRP1 which were down-regulated in mammary gland.In summary,trial results indicated that dairy cows fed with high-starch and high-oil diet can induce MFD in dairy cows and changed the milk performance,fatty acid composition,blood metabolism and transcripts of liver and mammary gland.The down-regulation of m RNA for key genes of fat synthesis in mammary gland is one of the factors that cause the reduction of milk fat. |
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