The aim of the present study was to investigate, firstly, whether the substitution of dietary starch by rumen-protected fat results in visible changes in the protein metabolism by increasing the urea flux and decreasing of AA levels in blood plasma as indicators for a change of AA oxidation, secondly, whether the fat-fed cows use predominantly long chain FA in the processes of milk fat synthesis or as an energy source in oxidation processes, and thirdly, whether the ratio of glucogenic to lipogenic nutrients could affect the supply of glucose in the metabolism of dairy cows. Two experiments were conducted. In either experiments, about 1.8 kg of starch in the ration of the control group were substituted by about 0.7 kg fat as protected fat (Ca salts of palm, soybean and sunflower oils in addition to protected soybeans) in the ration of the fat group. The first experiment was carried out on 32 multiparous high yielding dairy cows (16 Holstein-Friesian cows in each group) during the first 100 d of lactation. The second experiment was carried out in four periods in a cross-over design. Two cows were used in each period during the 6th to the 10th lactation weeks. The cows were infused intravenously with D-[U-13C6]-Glucose. The substitution of starch by protected fat tended to increase the milk production and milk lactose output and to decrease the microbial protein synthesis in the rumen and plasma glucose level. Also, the levels of ß-HBA and NEFA in plasma, the milk urea content and the total urea-flux were increased (P<0.05). Milk protein content but not yield and plasma levels of insulin, Met, Ser and His decreased (P<0.05) but the branched chain amino acids in plasma increased (P<0.05). The oxidation rate of FA was lower in comparison to other sub`strates. In the second experiment, the enrichment of milk fat and blood CO2 by 13C decreased but the recovery of 13C in milk lactose increased (P<0.065) due to high fat intake. The results indicate that the substitution of starch by protected fat can save glucose in the intermediary metabolism for lactose synthesis in the mammary gland and the cows used fatty acids predominantly for milk fat synthesis and not for oxidation.

The aim of the present study was to investigate, firstly, whether the substitution of dietary starch by rumen-protected fat results in visible changes in the protein metabolism by increasing the urea flux and decreasing of AA levels in blood plasma as indicators for a change of AA oxidation, secondly, whether the fat-fed cows use predominantly long chain FA in the processes of milk fat synthesis or as an energy source in oxidation processes, and thirdly, whether the ratio of glucogenic to lipogenic nutrients could affect the supply of glucose in the metabolism of dairy cows. Two experiments were conducted. In either experiments, about 1.8 kg of starch in the ration of the control group were substituted by about 0.7 kg fat as protected fat (Ca salts of palm, soybean and sunflower oils in addition to protected soybeans) in the ration of the fat group. The first experiment was carried out on 32 multiparous high yielding dairy cows (16 Holstein-Friesian cows in each group) during the first 100 d of lactation. The second experiment was carried out in four periods in a cross-over design. Two cows were used in each period during the 6th to the 10th lactation weeks. The cows were infused intravenously with D-[U-13C6]-Glucose. The substitution of starch by protected fat tended to increase the milk production and milk lactose output and to decrease the microbial protein synthesis in the rumen and plasma glucose level. Also, the levels of ß-HBA and NEFA in plasma, the milk urea content and the total urea-flux were increased (P<0.05). Milk protein content but not yield and plasma levels of insulin, Met, Ser and His decreased (P<0.05) but the branched chain amino acids in plasma increased (P<0.05). The oxidation rate of FA was lower in comparison to other sub`strates. In the second experiment, the enrichment of milk fat and blood CO2 by 13C decreased but the recovery of 13C in milk lactose increased (P<0.065) due to high fat intake. The results indicate that the substitution of starch by protected fat can save glucose in the intermediary metabolism for lactose synthesis in the mammary gland and the cows used fatty acids predominantly for milk fat synthesis and not for oxidation.