| Milk fat is one of important indicators to evaluate milk quality; however, low milk fat content has been a serious issue facing milk industry of China. Based on the realistic feed resource of China, three typical types of diets were formulated in this study to compare their effects on mammary uptake for fatty acids in dairy cows and investigate the uptake and utilization of fatty acids by mammary gland, to provide theoretical foundation and scientific basis for improving milk fat yield and quality by dietary means.Experiment 1 Effects of different diets on milk performance and apparent digenstibility of nutrients in dairy cowsThirty healthy Holstein cows with similar parity, body weight(554 ± 21 kg), lactation period(120 ± 24 d), and milk yield(24.30 ± 1.47 kg/d) were randomly assigned to one of 3 treatments with ten cows each in a single-factor block design. Three different diets were formulated: MF diet containing alfalfa, Chinese wild hey and corn silage; CS1 diet consisted of corn stalks only that contained almost equal nutrient level with MF diet; and CS2 diet with only corn stalks that contained the same roughage ratio with MF diet. The experiment lasted for 90 days with 30 days each period. TMR, milk, and blood samples were continuously collected in the last 3 days of each period and examined. The results showed that different treatments had no effect on dairy cow body weight, body condition and DMI(P > 0.05); milk yield changed signicantly MF > CS1 > CS2(P < 0.05); milk protein content of MF group was significantly higher than CS2(P < 0.05), and that of CS1 was between MF and CS2, but showed no difference with both of them(P > 0.05); the milk fat content of MF treatment was significantly higher than CS1(P < 0.05), that of CS2 was between MF and CS1 but showed no difference with MF and CS1(P < 0.05). Cows fed with MF and CS1 had a significantly higher level of crude protein digestibility compared with CS2 group, while CS1 and MF had no significant difference; Digestibility of dry-matter, crude fat, acid detergent fiber, and neutral detergent fiber showed no difference between 3 treatments.Experiment 2 Effects of different diets on arterial blood indexes of fat metabolism and milk fat profile in dairy cowsOn the basis of experiment 1 by feeding three different diets to dairy cows, arterial serum biochemical indexes GLU, CHO, TG, FFA, HDL, LDL, and INS that related to fat metabolism showed no significant differences(P > 0.05), but consentration of INS in CS2 treatment tended to decrease. Contents of C18:0(P = 0.08), trans-18:1(P = 0.09), C18:1, C18:3 and unsaturated fatty acid(P < 0.05), in milk fat were increased in CS1 and CS2 compared with MF, but C4:0-C16:0(P < 0.05) were reduced; whereas compared to cows fed CS2, cows receiving CS1 increased(P < 0.05) the content of C4:0 to C12:0 and C18:2. The results suggested that both feeding CS1 and CS2 diets increased contents of unsaturated fatty acids and long-chain fatty acids in milk, but blood biochemical indexes involved in fat metabolism and insulin level were not affected.Experiment 3 Effect of different diets on mammary uptake and utilization for fatty acids in dairy cowsOn the basis of experiment 1 by feeding cows different diets, Concentrations of C18:3 in artery was(P < 0.05) higher for cows with CS1 and CS2 than cows with MF, but no significant changes were observed with CS1 and CS2. Concentration of LCFA was trended affected by treatments: MF > CS1 > CS2(P = 0.07), but concentrations of C16:0, C18:0, C18:1 and C18:2 were not observed significant changes(P > 0.05). Blood flow of dairy cows in CS1 was significantly higher than MF, and MF was significantly higher than CS2(P < 0.05); Daily mammary supplies of LCFA were affected by treatments: MF and CS1 > CS2(5629.51, 6605.02 and 3878.91 g/d, respectively, P < 0.05). Mammary extractions of LCFA were influenced by diets: MF and CS2 > CS1(10.99, 10.84 and 7.39%, respectively, P < 0.05). Mammary uptakes of LCFA were affected by diets: MF > CS1 > CS2(618.69, 487.87 and 420.56 g/d, respectively, P < 0.05).Experiment 4 Effects of infusing milk fat precursor to external pudic artery on milk fat uptake in dairy cowsEight healthy Holstein cows with similar parity, body weight, lactation period(99 ± 11 d), and milk yield(20.17 ± 1.28 kg/d) were randomly assigned in 2 groups with 4 cows each. 2×2 crossover design was applied with 14 days of preliminary and 6 days of infusion. CS2 and MF diets were used in this experiment. The design principle was that take a certain ratio of difference between arterial supply of fatty acid in MF cows and that in CS2 as blood supplement. The total fat emulsion was about 280 g/d including 200g/d long-chain fatty acid and 80 g/d acetate, infusion began simutaniously with feeding and lasted for 6 h, and the volume of infusion emulsion was 2 L. The results showed that after infusing milk fat precursor to cows in CS2 group, milk yield(differed by 3.5 kg before infusion, and 1.3kg after infusion) and milk fat yield(differed by 0.19 kg before infusion, and 0.08 after infusion) were increased and were close to those in MF group; dry-matter intake of cows was decreased significantly(P < 0.05), but body weight and somatic cell count in milk were not affected(P > 0.05). The content of C15:0 in milk was reduced, whereas contents of linoleic acid and linolenic acid increased; contents of saturated fatty acid and mono-saturated fatty acid were reduced, but poly-saturated fatty acid content was increased(P < 0.05). Arterial infusion of milk fat precursor significantly increased concentration and supply of milk fat precursor in blood(P < 0.05); mammary uptake efficiency of milk fat precursor was decreased(P > 0.05); but the volumes of mammary uptake for milk fat precursor was increased significantly(P < 0.05). The results indicated that after infusing milk fat precursor in external pudic artery of dairy cows, although mammary uptake efficiency of milk fat precursor had reduced, the total concentration and supply of LCFA were increased, and the volumes and balance of mammary uptake for LCFA were increased, therefore increased milk yield and milk fat yield.The overall result indicated that compared to MF diet, feeding CS1 and CS2 diets significantly reduced milk yield and milk fat yield, but increased the content of unsaturated fatty acid in milk significantly. Cows with CS1 diet reduced the efficiencies of fatty acids by mammary uptake, therefore decreased milk yield and milk fat content; cows with CS2 diet reduced the volumes of fatty acid by mammary uptake, therefore decreased milk yield and milk fat yield. Blood flow and arterial fatty acid concentration codetermine fatty acid supply for mammary gland, and mammary uptake efficiency and mammary supply determine the volumes of mammary uptake. But compared to mammary uptake efficiency, mammary supply performanced more important for the volumes of mammary uptake, therefor affect milk fat yield in our three diets of study. |
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