Effects Of Rumen-Protected Methionine On Performance,Plasma And Amino Acid Metabolism In Cows At Peak Lactation

Dairy cows mobilize body fat to meet the energy required for lactation during the peak lactation period,which is more likely to occur nutritional metabolic diseases and induce oxidative stress than other lactation periods,making it difficult for dairy cows to fully exert their lactation ability.Methionine is the first limiting amino acid in dairy cows fed a diet consisting mainly of corn and soybean meal.When methionine is deficient,it limits the utilization of other amino acids.Rumen protection technology can prevent methionine from being decomposed by rumen microorganisms to reach the small intestine and improve nitrogen utilization rate.Numerous studies have shown that feeding cows ruminal methionine can improve performance and health status of cows.In addition,RPM is of great significance for alleviating the shortage of protein feed resources and protecting the environment.However,studies on the improvement of performance of dairy cows by rumen methionine mainly focus on the perinatal period,and there are few studies on the performance,apparent digestibility and amino acid metabolism of dairy cows at peak lactation.Therefore,this study explored the effects of methionine on apparent digestibility and ruminal fermentation of dairy cows at peak lactation,combined with blood biochemical indexes and amino acid metabolism,to provide new ideas and theoretical basis for explaining the regulation of methionine on performance and health of dairy cows at peak lactation.EXP.1 The effect of rumen methionine on the production performance of dairy cows during peak lactationSixteen healthy Holstein dairy cows at peak lactation with similar body condition,parity and milk yield were randomly divided into 2 groups.The control group was fed basal TMR daily,and each cow in the RPM group was fed 15 g RPM alone daily.The experimental cattle were kept in the same barn and fed daily after milking,with the amount of residue controlled to less than 10%and free access to water throughout the day.The experiment lasted for 8 weeks,including 14 days of pre-test and 42 days of test.At the end of the two sessions,body weight was measured before morning feeding and body condition was scored within one hour after morning feeding.Milk yield was recorded from 30 to 35days in the experimental period,and dry matter intake was recorded from 33 to 35 days in the experimental period.Milk samples were collected for 2 consecutive days from 35 to 36days for the determination of milk composition.The results showed that RPM supplementation had no significant effects on body weight,body condition and dry matter intake of dairy cows at peak lactation period（P>0.05）;Milk yield and milk composition were not significantly different between RPM cows and control cows（P>0.05）.EXP.2 Effects of ruminal methionine on ruminal fermentation and apparent digestibility in cows at peak lactationThe experimental design and dairy cattle feeding and management were the same as experiment 1.Dry matter intake and fecal weight were recorded on days 33-35 of the test period,and fecal samples were collected for subsequent determination of nutrient content and apparent digestibility.Rumen fluid was collected within 1 hour before morning feeding on the 40th day of the trial period to measure rumen fermentation parameters.The results showed that there were no significant differences in most rumen fermentation indexes and nutrient digestibility of cows at peak lactation when RPM was supplemented at 15 g per day（P>0.05）;However,compared with the control group,the RPM group had numerically higher digestibility of dry matter,crude protein,neutral detergent fiber,acid detergent fiber,starch,organic matter and methionine（P>0.05）;In addition,RPM supplementation tended to reduce ruminal butyrate content（P=0.069）.EXP.3 Effects of ruminal methionine on plasma biochemistry and amino acid metabolism in cows at peak lactationThe purpose of this study was to investigate the effects of RPM on plasma biochemistry and amino acid metabolism in cows at peak lactation,and the design and management of the cows were the same as experiment I.Blood samples were collected from the jugular vein at the end of the pretest period and before morning feeding at the end of the experimental period,and blood samples were collected at 3 h and 6 h after morning feeding on day 42.The results showed that RPM significantly increased blood urea nitrogen（BUN）and total carbon dioxide（t CO₂）levels（P<0.05）;The contents of albumin,globulin,total protein,triglyceride and cholesterol in plasma were increased,while the contents of glucose and insulin were not significantly different.There was no significant difference in alanine aminotransferase and aspartate aminotransferase between the two groups（P>0.05）.RPM had no effect on blood amino acid levels before and 6 hours after morning feeding（P>0.05）,but there was a tendency to decrease the arginine（P=0.072）and aspartic acid（P=0.091）contents in blood three hours after morning feeding,and the proline content was significantly lower than that in the control group（P<0.05）.Adding RPM tended to reduce the concentration of aspartate in plasma(P_treatment=0.070).Plasma concentrations of methionine,cysteine,arginine,serine,alanine,aspartic acid,glutamate,taurine,and glutamine were significantly time-specific(P_time<0.05),and glycine had a tendency to be affected（P=0.085）.There was no interaction between treatment and time on amino acid levels(P_interaction>0.05).This study suggests that RPM supplementation may improve the health status and performance of dairy cows at peak lactation period by improving methionine digestibility,amino acid metabolism and body protein synthesis,but the specific dosage and duration of supplementation need to be further studied.