| Feed inputs account for the largest proportion of operating costs in dairy industry.With the rising price of corn and soybean meal,the feed costs have increasingly become a constraining factor of the development of China's dairy industry.Moreover,the increasing concern of environmental pollution leads to the pressure of energy saving and carbon emission reduction in the intensive large-scale dairy farms.Improving the feed efficiency of dairy cow have profound significance as it is expected to decrease feed costs,alleviate the contradiction between the supply and demand of feed,and improve the environmental benefits.Numerous studies have applied nutritional regulation to investigate the feed efficiency of dairy cow.However,the internal factors that affect the feed efficiency of dairy cows are still unclear.The objectives of this study,therefore,were to investigate the ruminal microbial function and mammary gland metabolism in dairy cows with different feed efficnencies.In part 1,we analyzed the individual differences in the feed efficiency of dairy cows.and then high efficient dairy cows and low efficient dairy cows were selected.In part 2,we analyzed rumen fermentation of the high and low efficient cows,and then in combination with metagenomics to identify the differences of rumen microbial composition and functional in dairy cows with different feed efficiencies.In part 3,we analyzed mammary amino acid(AA)metabolism along with metabolomics of mammary blood and milk,to reveal the differences in mammary AA metabolism in dairy cows with different feed efficiencies.PART 1The individual differences in the feed efficiency and lactation performance of dairy cows Fifty-three lactating Holstein cows with relatively higher milk protein yield(days-inmilk = 153 ± 20.3d,milk yield = 37.5 ± 5.26 kg/d,milk protein content = 3.28 ± 0.17%,BW= 634 ± 85.8 kg)were selected for the experiment.The trial lasted for 57 d,with the first 7 d used for adaptation.The data of feed intake and milk yield were recorded every day.On d 24 and 49 of the feeding period,blood samples were obtained from the coccygeal vein of each cow and then analyzed for blood paramaters.Fecal samples were collected and analyzed for apparent digestibility.Fecal and urine samples were analyzed for energy and nitrogen partitioning.Stepwise multiple linear regression analysis was used to calculate the expected feed intake.The residual feed intake(RFI)for each animal was calculated as the difference between the actual and expected average feed intakes during the trial.A low or negative RFI value represents high efficiency,while a high RFI value represents low efficiency.With a power of 99% for RFI,the 13 lowest RFI and 13 highest RFI cows were selected to form two RFI groups:low RFI(high efficiency,n = 13)and high RFI(low efficiency,n = 13).Obvious individual differences existed in feed efficiency among dairy cow herd.The high efficient cows consumed less dry matter than the low efficient animals(24.4vs.27.1 kg/d,P < 0.01),but cows in both groups produced similar milk yield(P = 0.66).The milk feed ratios in the high and low efficient cows were 1.42 and 1.25 kg/d,respectively(P <0.01),and the RFI in the high and low efficient cows was-1.37 and1.37 kg/d,respectively(P <0.01).High efficient cows tended to have higher concentrations of blood urea nitrogen than those in low efficient cows(P = 0.10).Furthermore,the high efficient cows had greater digestibilities of DM(P <0.01),CP(P= 0.03),NDF(P = 0.03),and ADF(P = 0.03)than low efficient cows.Digestible energy(P <0.01),metabolizable energy(P <0.01),and ratio of net energy for lactation to gross energy intake(P <0.01)were higher in high efficient cows.Nitrogen(N)intake in high efficient cows tended to be less than that in the low efficient cows(P <0.01).The high efficient cows had lower output of feces(P = 0.02)and its proportion of dietary N(P =0.03)than the low efficient cows,but the milk N was greater in high efficient cows(P= 0.02).Thus,high efficient cows partitioned a greater percentage of energy and N into milk protein,but the low efficient cow partitioned a greater percentage of energy and N into feces.These results suggested that the differences in feed efficiency of dairy cows is related to the changes in digestibility and energy and nitrogen metabolism.Therefore,the follow-up analysis will focus on the two important organs(rumen and mammary glands),which related to digestion and metabolism.PART 2The production of milk protein precursor and rumen microbiome of cows with different feed efficiencies The high and low efficient cows in part 1 were used in the current study.On d 49 of the feeding period,rumen samples were collected to analyzed fermentation variables.Urine samples were collected and then analyzed to calculat the production of microbial protein.Rumen metagenomic was employed to analysze microbial composition and function.The concentrations of acetate(P = 0.10),butyrate(P = 0.10)and valerate(P= 0.09)tended to be lower in high efficient cows.However,the concentrations of isovalerate(P = 0.09)and the molar proportion of propionate(P = 0.09)were tended to be higher in high efficient cows,and the molar proportions of isobutyrate(P = 0.05)and isovalerate(P <0.01)were significantly greater in high efficient cows.These results indicated that high efficient cows had greater potential in generating energy.The production of microbial protein and metabolizable protein was not different between the two groups.However,the high efficient cows had higher proportion of metabolizable protein converted into milk protein(45.4% vs.40.4%,P = 0.02),indicating more efficient utilization of AA in mammary gland in these cows.Based on metagenomics and analyses of rumen microbial function and correlation between microbial species and function,we found stronger abilities of carbohydrate degradation in low efficient cows,and more intermediate metabolites such as pyruvate might be produced by the low efficient microbiome.Futhermore,the rumen microbiome of low efficient cows had higher abundances of butyrate-producing species and functions in butyrate metabolism.The rumen microbiome of low efficient cows also had higher abundances of organisms with methanogen and methanogenesis functions,suggesting that these cows may use more pyruvate to produce butyrate and methane.Compared with cows in low efficient group,the high efficient cows had higher abundance of genes encoding enzymes related to propionate production in the rumen,indicating that high efficient cows tend to produce propionate from pyruvate,thereby improving the efficiency of production.Additionally,several Lachnospiraceae species had positive relationship with carbohydrate metabolism and quorum sensing,implying that these species may be the key microorganisms that affect the difference in feed efficiency.PART 3Mammary gland metabolism in dairy cows with different feed efficiencies The high and the low efficient cows in part 1 were used in the current study.On d 49-50 of the feeding period,milk samples,blood samples from coccygeal artery and the mammary vein were collected and then analyzed for AA concentration and metabolomics.The arterial concentrations of essential AA(EAA,P = 0.06),branchedchain AA(BCAA,P = 0.10)and total AA(P = 0.07)tended to be lower in the high efficient cows than in low efficient cows.The mammary plasma flow did not differ between the two groups(P > 0.10).The high efficienct cows tended to be lower in arteriovenous difference of EAA(P =0.07),BCAA(P =0.10)and total AA(P = 0.08).Mammary uptake of EAA(P =0.09),BCAA(P =0.09)and total AA(P = 0.09)were tended to be lower in the high efficient cows than in low efficient cows.The high efficient cows tended to be lower in U:O ratio for EAA(P =0.08),BCAA(P =0.07)and total AA(P =0.09).These results suggested the higher efficiency of mammary AA utilization in high efficient cows.Metabolomics analysis revealed that 14 different metabolites were identified in arterial serum,of which the relative concentration of 4 substances was higher in high efficient dairy cows.A total of 19 significantly different metabolites were identified in the mammary vein serum,of which the relative concentration of 13 different metabolites was significantly higher in high efficient group.Futhermore,16 different metabolites were identified in milk,of which the relative concentration of two substances was higher in high efficient cows.We found that dairy cows affect the energy and nitrogen metabolism of the mammary glands through the urea cycle,tricarboxylic acid cycle related pathways and substances,thereby causing differences in feed efficiency.The metabolism of glycine,serine and threonine is the main differential metabolic pathways in the mammary glands of dairy cows with different feed efficiencies.Low efficient cows had higher concentrations of AA related metabolites in arteries and veins,and higher concentrations of urea in milk,suggesting the lower AA utilization in mammary gland of low efficient cow.In summary,obvious individual differences existed in feed efficiency among dairy cow herd.High efficient cows tended to partition a greater percentage of energy and N into milk protein,while the low efficient cow tended to partition a greater percentage of energy and N into feces.The high efficient cows had greater digestibilities than low efficient cows,but low efficient cows had stronger abilities of carbohydrate degradation,and more intermediate metabolites such as pyruvate tended to be used to produce butyrate and methane.The high efficient cows tended to use pyruvate to produce propionate,isobutyrate and isovalerate.Additionally,the high efficient cows had higher proportion of MP converted into milk protein.The lower AA U:O ratio in high efficient cows indicated higher mammary protein synthesis efficiency in this group.Cows with different feed efficiencies may affect mammary metabolism through the urea cycle,tricarboxylic acid cycle-related pathways and substances,leading to differences in feed efficiency.This study reveals some mechanisms that affect the feed efficiency of dairy cows from the perspective of rumen microbiome and mammary gland metabolism,and provides new insights for improving the efficiency of dairy cows. |
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