The Processes of Nutrition and Metabolism Affecting the Biosynthesis of Milk Components and Vitality of Cows with High- and Low-Fat Milk.

In order to clarify the mechanism of the depression of milk fat formation and preserve the health of animals, the aim of the research was to study the characteristics of rumen digestion, energy metabolism, and milk composition in high-producing dairy cows with high and low levels of milk fat that are fed the same diet. Two groups of cows with normal milk fat content (3.94 ± 0.12; n = 10) and low milk fat content (2.95 ± 0.14, n = 10) contained in the same diet were identified. Gas exchange (O2 uptake and CO2 output) was studied in cows and blood samples, rumen contents (pH, NH3-N), and VFA and milk (fat, protein, and fatty acid composition) were collected and analyzed. It was determined that cows with low fat milk are more efficient at using the metabolized energy of their diets due to the tendency to have a decrease in the proportion of heat production (by 6.2 MJ; p = 0.055) and an earlier start of a positive energy balance. At the same time, the fat content in milk did not depend on the level of hormones in the blood or on the formation of acetate in the rumen. An analysis of the duration of the productive use of cows on this farm (n = 650) showed that the number of lactations was inversely correlated with the level of fat in milk (r = −0.68; p < 0.05, n = 1300). These results indicate the advantages of cows that can reduce the fat content of their milk in the first months of lactation.