Effects Of Methionine And Lysine Supplementation On Lactation Performance And Nitrogen Utilization Efficiency Of Dairy Goats

Dietary protein levels are regarded as an important factor that restrict milk production and milk protein level of lactating ruminants.However,ruminants make low efficient use of nitrogen,excreting a large amount of nitrogen in urine and feces.The remaning nitrogen may cause serious resource waste and environmental problems.Reducing the dietary protein level is a direct way to promote nitrogen efficiency in ruminants,but it may decrease milk production,resulting in production losses.Therefore,there is an urgent need to find a way promoting the nitrogen utilization efficiency of ruminants while simultaneously maintaining their lactation performance.In present study,the effects of methionine and lysine on the lactation performance and nitrogen utilization efficiency of dairy goats fed with a metabolic protein(MP)-deficient diet were investigated.The experiments were conducted from the aspects of milk yield,milk composition,lactation related gene expression,rumen fluid index,apparent digestibility,amino acid utilization and plasma physiology and biochemistry.This study provides a theoretical reference for the practical application of rumen-protected methionine(RPMet)and rumen-protected lysine(RPLys)products in dairy goats.Expt.1 The effects of RPMet on production performance of dairy goats fed with low protein dietThirty multiparous Guanzhong dairy goats were blocked by parity,milk yield and body weight and randomly designated to receive three different treatments:An MP adequate diet(C);An MP-deficient diet(D;88%MP compared to C);Diet D supplemented with 1 g RPMet/goat daily(DM).The whole trial lasted for 7 weeks(2-week adaptation and 5-week experimental periods).During the experimental periods,fecal samples were collected in the first and third weeks,while rumen fluid and blood samples were obtained in the second and fourth weeks.Feed intake data was measured twice in the first and third weeks for two consecutive days.Milk production was measured on the last two days of each week in experimental periods,and the goat milk samples were collected for further analysis of milk composition.At the fifth week,4 out of 10 dairy goats from each group were randomly selected for mammary gland tissue biopsy sampling.The mammary gland tissue samples were collected for further analyses of the mRNA abundance of milk synthesis related genes and amino acid transporters genes.The results showed that there was no significant difference in dry matter intake(DMI)among the treatment groups,but the milk yield,energy corrected milk and 4%fat corrected milk yield in the DM group were significantly higher than that of the D group(P<0.05).The milk protein yield,lactose yield and lactose content in DM group were also significantly higher than that of the D group(P<0.05).Meanwhile,the mRNA abundance of milk protein synthesis related genes CSN1S1,CSN2,CSN3 and LTF in DM group were significantly up-regulated compared with the control group(P<0.05),and the mRNA abundance of milk fat secretion related genes BTNIA1,XDH and ADFR were also significantly up-regulated compared with the control group(P<0.05).Furthermore,compared to C group,the feed conversion rate was significantly higher in the DM group(P<0.05).The hydrolyzed methionine and proline concentrations in milk were obviously higher in the DM group than those in the C group(P=0.04,P=0.001),and the mRNA abundance of amino acid transporters SLC1A1,SLC1A4 and SLC43A1 were significantly higher in the DM group compared with C group(P<0.05).In conclusion,the low protein diet supplemented with RPMet not only increased the utilization of dietary nitrogen of diary goats,up-regulated the mRNA abundance of amino acid transporters in mammary gland,but also up-regulated the mRNA abundance of lactation related genes in mammary gland.These results suggest that methionine may promote the lactation performance of diary goats by increacing the efficiency of amino acid utilization and upregulation the mRNA abundance of lactation related genes.Expt.2 The effects of RPLys on production performance of dairy goats fed with low protein dietThirty multiparous Guanzhong dairy goats were used in experiment 2,which were also blocked by parity,milk yield and body weight and randomly designated to receive three different treatments:An MP adequate diet(C);An MP-deficient diet(D;88%MP compared to C);Diet D supplemented with 6 g RPLys/goat daily(DL).The specific test period,sampling and experiment method were the same as expt.1.The results showed that there was no significant difference in dry matter intake among the treatment groups,but the milk protein content in the DL group was significantly higher than that in the other two groups(P<0.05).Compared with the C group,the urea nitrogen level in the milk(MUN)was tend to decrease in DL group(P=0.11)with the urea nitrogen level in the blood(BUN)significantly decreased(P<0.05).Meanwhile,the mRNA abundance of milk protein synthesis related genes CSN1S1 and CSN2 in DL group were significantly up-regulated compared with the C group(P<0.05).In addition,the concentration of hydrolyzed branched amino acids(BCAA)and cysteine in milk was significantly higher in the DL group than that of the C group(P<0.05),and the mRNA abundance of amino acid transporters SLC1A1,SLC1A4 and SLC43A1 in the DL group were significantly up-regulated compared with C group(P<0.05).The results suggested that supplementing RPLys in the low protein diet decreased the level of BUN and increased the mRNA abundance of the amino acid transporters in mammary gland.Finally,the concentration of hydrolyzed BCAA in milk and milk protein content were increased.In conclusion,both RPMet and RPLys supplementation in low protein diet had an effect on the production performance of dairy goats,but they may act in different ways.Methionine may promote the lactation performance of dairy goats through enhancing nitrogen metabolism and up-regulating the mRNA abundance of amino acid transporters and lactation related genes in mammary gland.Lysine may increase the milk protein content of dairy goats by enhancing the amino acid balance of the body and up-regulating the mRNA abundance of amino acid transporters in mammary gland.Overall,this study systematically evaluated the role of RPMet and RPLys in low protein diets,which not only provided a theoretical basis for the application of RPMet and RPLys in diary goat production,but also supplied an explanation for the differential mechanism of methionine and lysine influencing milk synthesis.