Studies On Effects Of Milk Fat Substrates Infusion On Lactation Performance And Milk Fat Synthesis And Their Mechanisms In Lactating Dairy Cows

The objective of this study was to investigate the effects of infusion of milk fat substrates(MFS)on lactation performance,further study the mechanism of mammary uptake and utilization of MFS,as well as mammary metabolism of three main C18 unsaturated FA in dairy cows,to provide scientific basis for reasonable and effective regulation of milk FA composition.This study contained four experiments:Experiment 1 studied the effects of MFS infusion into external pudic artery on dairy cows under corn stover diet feeding condition and investigated whether the lactation performance and milk fat synthesis could reach similar levels with cows fed high quality roughage diet.Eight Holstein cows(DIM=99±11d)were averagely divided into two groups and assigned to two diets:corn stover(CS)or mixed forages(MF),respectively.The Experiment included two stages and each lasted for 27d with a 21d preliminary,a 3d carrier infusion and a 3d experimental treatment infusion.In the carrier infusion period,carrier emulsion was infused to cows in both groups(MFC1 and CSC).At the 3d experimental treatment infusion period,the MF cows were continued to infuse with carrier(MFC2),and the CS cows were infused with MFS(CSF).The MFS mainly consisted of acetate(72 g/d)and mixed long-chain FA(180 g/d).The results showed that DMI was not affected by treatments.The differences in milk yield,yields of milk fat,protein and lactose between CSC and MFC1 were compensated by MFS infusion in CSF as compared with MFC2.Infusion of MFS increased milk C18:2 n-6c,C18:3 n-3 and C12:0 yields significantly in CSF(P<0.05),but did not affect the yield of total short-and medium-chain FA in milk fat(P>0.05).Experiment 2 studied the effects of withdrawal infusion of C18 unsaturated FA intravenously on lactation performance,apparent nutrient digestibility and blood hormone in dairy cows.Eight Holstein cows(DIM=101±11d)were used in a 4 x 4 Latin square design.Treatments were jugular vein infusion of mixed oil consisting of:oleic,linoleic and linolenic acid(CTL);linoleic and linolenic acid(-C18:1);oleic and linolenic acid(-C18:2);and oleic and linoleic acid(-C18:3).Each period lasted for 12d,consisted of 5d of infusion and 7d of recovery.The results showed that DMI,contents of milk fat and lactose were not affected by withdrawal infusion(P>0.05).However,milk yield,yields of milk fat,protein and lactose were increased linearly by decreasing unsaturated degree of total FA in the infusate and were significant higher in treatment-C18:3 than in CTL(P<0.05).Apparent digestibility of nutrients was not affected by withdrawal infusion(P>0.05).Blood leptin concentration was lower in-C18:1 than in CTL(P<0.05).Experiment 3 studied the effects of withdrawal infusion of C18 unsaturated FA intravenously on milk FA contents and yields in dairy cows.The experimental design was the same as Experiment 2.The results showed that the milk content and yield of C18:2 n-6c and C18:3 n-3 were lower in treatment-C18:2 and-C18:3 respectively than in CTL.Milk fat content and yield of C18:1 cis-9 were not changed in treatment-C18:1(P>0.05).Content of C18:1 cis-9 was higher in treatment-C18:2 than in CTL(P<0.05)whereas its yield did not follow this response(P>0.05).Milk fat yields of C18:0(P = 0.06),C14:0(P?0.07),C16:0(P = 0.07)and total short-and medium-chain FA(P = 0.09)tended to increase linearly with decreasing unsaturated degree of total FA in the infusate.Experiment 4 studied the effects of withdrawal infusion of C18 unsaturated FA intravenously on mammary uptake and utilization of FA in dairy cows.The experimental design was the same as Experiment 2.The results showed that arterial and venous plasma C18:1 cis-9 concentrations were not changed in-C18:1(P>0.05).Arterial concentration of C18:2 n-6c was lower in-C18:2 than in CTL(P<0.05)and that in venous was not changed(P>0.05).However,treatment-C18:2 had higher arterial and venous concentrations of C18:1 cis-9 and C18:3 n-3 than CTL(P<0.05).Arterial and venous concentrations of C18:3 n-3 were lower in-C18:3 than in CTL(P<0.05).Mammary extraction rates of FA did not show difference between treatments.Withdrawal infusion of different C18 unsaturated FA decreased its mammary uptake respectively and had no interaction effects on others.Mammary balance of C8:0 and C16:0 were lower in-C18:2 than in CTL(P<0.05).Mammary FA balance did not show differences in treatment-C18:1 and-C18:3(P>0.05).In summary,supplement of MFS could enhance milk and milk fat yield in cows fed corn stover diet to a similar level with cows fed high quality roughage diet,and change milk FA profile.There might exist combined effect between C18 unsaturated FAs on lactation performance and milk fat synthesis in dairy cows.