Effect And Mechanism Of Partially Substituting Dietary Corn Grain With Sucrose And Lactose On Milk Fat Synthesis Of Dairy Cows

Soluble sugar is considered to be an alternative energy substance,which can improve the milk fat rate and milk fat yield of dairy cows by partially substituting high-starch grain.This effect may be related to no reduction of ruminal pH,the increase of butyrate production and the reduction of trans-10 hydrogenation intermediates which can inhibit the synthesis of milk lipids in the rumen.However,the effect of the soluble sugar on milk fat synthesis is still unclear,and the rumen fermentation of soluble sugar in various studies varied considerably.Besides,direct study that about the effect of soluble sugar on ruminal fatty acids metabolism is limited,which related evidence also need to be added.In order to further study the effect of soluble sugar on milk fat synthesis in dairy cows and the possible related mechanism,this study used sucrose and lactose selected from the previous study,in which sucrose had a more effective inhibition on trans-10 fatty acid production and lactose had a better promoting effect on butyrate production.Twelve lactating Chinese Holstein dairy cows were used in a 4×4 Latin square design to study the effect of substituting 5% corn in dairy cows' diets with 5%sucrose,5%lactose,and an mixtion of 2.5% sucrose and 2.5%lactose(mixed group)on lactation performance,rumen fermentation characteristics,milk fatty acids composition,and fatty acid metabolism in the rumen.The results were summarized as follows.1.Using sucrose,lactose or the mixture had no significant effect on dry matter intake and milk production of dairy cows(P > 0.10),but it tended to improve energy corrected milk(P = 0.084)and fat corrected milk(P = 0.076).And lactose group had a better effect than sucrose group and mixed group.Soluble sugar partially substituted corn had no significant effect on milk composition(P > 0.10),but tended to increase milk fat production(P = 0.052),which mainly due to the higher data of milk yield and milk fat yield in sugar group.2.Using lactose and sucrose as a partial substitution of dietary corn did not affect the rumen pH(P > 0.10),but tended to increase the total volatile fatty acids concentration(P = 0.085).Substituting corn with sugar significantly affected the molar proportion of individual volatile fatty acids.Compared with the basic diet group,all sugar substituting groups were significantly improved the molar proportion of butyrate(P < 0.05),decreased the molar proportion of isobutyrate(P < 0.05).Besides,lactose group and sucrose group also significantly increased the molar proportion of pentanoic acid(P < 0.05).Compared with the basic diet group,sugar had no significant effect on lactic acid concentration(P > 0.10),but had a tendency to reduce the concentration of lipopolysaccharide(P = 0.088),and ammoniacal nitrogen in the ruminal liquid(P = 0.086).3.The composition of milk fatty acids in dairy cows were affected by the substitution of sucrose,lactose and the mixture.Substitutng dietary corn with sugar significantly decreased the proportions of C18:1trans-10 and C18:1cis-9(P < 0.05),but significantly increased the proportions of short-chain fatty acid and C18:3n-3(P < 0.05).Besides,sugar tended to increase the proportions of C18:1trans-11(P = 0.058),but tended to decreased the proportions of CLAtrans-10,cis-12(P = 0.094).4.Substitutng dietary corn with sugar also had effects on the fatty acid composition in rumeminal liquid,which including the significant increasing of C13:0,C13:0anteiso,C17:0 and OBCFA proportions(P < 0.05),the significant reducing of C18:1trans-10,C18:2n-6 and CLAtrans-10,cis-12(P < 0.05).Besides,the ratio of trans-11 fatty acids to trans-10 fatty acids in sucrose group and lacrose group significantly increased(P < 0.05).5.Using sucrose and lactose partially substituting corn had no effects on glucose,insulin and urea nitrogen concentration in plasma(P > 0.10),but tended to increase the concentration of ?-hydroxybutyric acid(P = 0.068)and non-esterified fatty acid(P = 0.096).6.Partially substituting dietary corn with sugar affected the abundance of bacteria related to the rumen fatty acid biohydrogenation(P < 0.05).Compared with the control group,sugar groups had no effects on the abundance of B.hungate and B.fibrisolvens(P > 0.05),but significantly decreased the abundance of S.bovis,M.elsdenii and P.acnes(P < 0.05).The above results show that substituting corn in dairy cows dietary with sugar had no significant effects on production,milk fat rate and milk fat yield of dairy cows,however,it had effects on the composition of milk fatty acids.And the mechanism may be related to the increase of rumen butyrate and valerate molar production,no further reduce of rumen pH,the decrease of trans-10 fatty acids proportion and the increase of trans-11 fatty acids proportion in ruminal liquid.Moreover,the change of trans-10 fatty acids proportion and trans-11 fatty acids proportion in milk and ruminal liquid was considered to be caused by the abundance of relative bacterias in the rumen.