Dietary Level Of Glycine And Cystine And Their Combinative Effects On Stimulation Of Growth Of Broiler Chickens Fed Low Protein Diet

It is a common sense that glycine(Gly)stimulates the growth of broiler chicks fed low protein(LP)diet,but the specific mechanism is still unclear.The level of cystine(Cys)in the diet may be closely related to the growth promoting effect of Gly,and their interative effects were pending to uncover.In this study,the optimal combination of Gly and Cys in the LP diet was determined under the condition that the total sulfur containing amino acids meet the requirement.Then,the mechanism of the combinative effect of Gly and Cys in the LP diet to promote the growth of broilers was discussed by using metabonomics technology,which provided a scientific basis for the rational application of Gly and sulfur containing amino acids(SAA)in production and provided technical support for nitrogen emission reduction in poultry industry.Experiment 1: Study on the appropriate dose of Cys in LP diet of broilers containing Gly.Experimental treatment: the positive control(PC)group was fed a normal protein diet;the LP diet in NC group was decreased by 4.5 percentage points and supplemented with 1.06% Gly;the Cys groups were supplemented with 0.05%,0.10%,0.15% and 0.20% Cys on the basis of NC group,respectively.The results showed as follows: dietary Cys linearly and quadratic increased the average body weight(ABW),average daily feed intake(ADFI)and average daily gain(ADG)of broilers during starter period(P <0.05);adding 0.05% and 0.10% Cys to LP diets significantly decreased ABW of broilers during grower period and whole period(P < 0.05).Dietary Cys quadratically improved the breast muscle percentage(BMP)and leg muscle percentage(LMP)of broilers at 42 days of age(P < 0.05).The quadratic curve fitting results showed that the optimal growth performance and carcass composition of broilers were obtained when the Cys addition amount in LP diet was 0.08% to 0.14%.Compared with PC group,the nitrogen retention rate(NRR)of broilers in LP diet groups was extremely significantly increased(P <0.01).Adding 0.20% Cys to LP diets significantly decreased serum uric acid(UA)content of broilers at21 and 42 days of age(P < 0.05).Based on the results of growth performance,the PC group,NC group and NCC group(0.10% Cys group)were selected for subsequent analysis.Compared with NC group,the serum contents of threonine,phosphoserine,glutamate and carnosine in NCC group were significantly increased(P < 0.05),and methionine(Met)content was significantly decreased(P < 0.05).Compared with NC group,the m RNA expression of glycine-N-methyltransferase(GNMT)in NCC group was significantly down-regulated(P < 0.05),and the expression of glutathione synthase(GSS)was significantly up-regulated(P < 0.05).In conclusion,under the condition that total sulfur amino acids and Gly meet the requirements,LP diet supplemented with 0.10% Cys could improve the growth of broilers by changing the serum free amino acid metabolism and the expression of SAA metabolism-related genes in liver.Based on growth performance and carcass composition,the optimal addition dosage of Cys in LP diet for broilers is 0.08% to 0.14%(Cys accounts for 35% to 41% of the total sulfur containing amino acids).Experiment 2: Study on the appropriate dose of Gly in LP diet of broilers containing Cys.Experimental treatment: the PC group was the same as experiment 1;the LP diet in NC group was decreased by 4.5 percentage points and supplemented with 0.10% Cys;the Gly groups were supplemented with 0.35%,0.70%,1.05% and 1.40% Gly on the basis of NC group,respectively.The results showed as follows: dietary Gly linearly and quadratically improved ABW of broilers during every period(P < 0.05),linearly and quadratically increased ADG during starter and whole period(P < 0.05),and linearly and quadratically decreased F/G during starter and whole period(P < 0.05);compared with NC group,ADG of broilers in 0.70% and 1.05% Gly groups was significantly increased(P < 0.05),and F/G was significantly decreased(P < 0.05)during grower period.Gly addition linearly and quadratically elevated BMP of broilers at 42 days of age(P < 0.05).The quadratic curve fitting results showed that the optimal growth performance and carcass composition of broilers were obtained when the Gly addition amount in LP diet was 0.70% to 1.30%.Compared with the PC group,the NRR of broilers in LP diet groups were extremely significantly increased(P <0.01),and the UA level was significantly reduced(P <0.05).Based on the results of growth performance,the PC group,NC group(0.10% Cys)and NCG group(0.10% Cys+ 0.70% Gly)were selected for subsequent analysis.Compared with NC group,the serum levels of Gly,serine(Ser)and phenylalanine in NCG group increased significantly(P < 0.05),and the contents of arginine(P = 0.071)and taurine(P = 0.096)showed an increasing trend.Compared with NC group,the m RNA expression of cysteine sulfinic acid decarboxylase(CSAD)in NCG group was significantly upregulated(P < 0.05).In conclusion,under the condition that total sulfur-containing amino acids meet the requirements and Cys is suitable,LP diet supplemented with 0.70% or 1.05% Gly could improve the growth performance of broilers by changing body metabolism,up-regulating CSAD m RNA expression level and increasing the content of Gly,Ser,phenylalanine,arginine and taurine in serum.Based on growth performance and carcass composition,it is recommended that the optimal addition dosage of Gly in LP diet is 0.70% to 1.30%(total Gly + Ser content in LP diet is 1.62% to 2.57%).Experiment 3: Study on the mechanism of the combinative effects of Gly and Cys in LP diets on the growth of broilers based on metabonomics.Based on the growth performance results of experiment 2,we selected PC group,NC group(0.10% Cys)and NCG group(0.10% Cys + 0.70% Gly)for metabonomics analysis.Student’s test,principal component analysis(PCA)and partial least squares discriminant analysis(PLS-DA)were used to screen out the metabolites(P < 0.05,VIP > 1)between groups,and the pathway enrichment analysis was performed.The results showed as follows: compared with PC group,the metabolite levels of amino acids(such as glutamic acid and glutamine,etc.)and lipids(such as eicosapentaenoic acid and lysophosphatidylcholine,etc.)in NC group were significantly increased(P <0.05),while the metabolite levels of spermine,linoleic acid and leukotriene C4 were significantly decreased(P < 0.05);compared with the NC group,the metabolite levels of lumichrome,linoleic acid,spermine and leukotriene C4 in NCG group were significantly increased(P < 0.05),while the metabolite levels of 5’-methylthioadenosine were significantly decreased(P < 0.05).The results of pathway enrichment analysis showed that compared with PC group,amino acid metabolism(such as D-glutamine and D-glutamic acid metabolism,etc.),lipid metabolism(such as sphingolipid metabolism,etc.)and Fox O signaling pathway were significantly up-regulated in NC group(P < 0.05);compared with NC group,the metabolism of riboflavin and linoleic acid in NCG group were significantly up-regulated(P <0.05),and the metabolism of β-alanine and glutathione tended to be up-regulated(0.05 ≤ P < 0.10).In conclusion,the appropriate combination of Gly and Cys(0.10% Cys + 0.70% Gly)in LP diet could change liver metabolites(such as linoleic acid,lumichrome and spermine,etc.),and improve growth performance of broilers by regulating linoleic acid,riboflavin,glutathione and β-alanine metabolism pathways.Taken together,the appropriate combination of Gly and Cys(0.10% Cys + 0.70% Gly)in LP diet(reduced by 4.5 percentage points of protein compared to the the normal protein diet)could up-regulate the m RNA expression of CSAD in liver,and increase the levels of Gly,Ser,phenylalanine,arginine and taurine in serum,and promote the growth of broilers by regulating liver linoleic acid,riboflavin,glutathione and β-alanine metabolic pathways in the liver.