The effect of change from low-roughage to high-roughage diets on rumen fermentation, blood composition and milk fat secretion in the cow.

I. Changing cows from a diet with low hay and high flaked maize content, which depressed milk fat secretion, to a high-roughage diet resulted in increased milk fat secretion and in changes in the pattern of rumen fermentation and in the composition of the blood plasma. 2. During the 1st week following the change in diet the proportions of propionic and valeric acids in the rumen decreased and the proportion of acetic acid increased. These changes in rumen volatile fatty acids were associated with decreases in the concentration of lactic acid, increases in pH and increases in the rate of breakdown of cotton threads in the ventral sac of the rumen. After I week, only minor fluctuations in the proportions of volatile fatty acids occurred from day to day. 3. The percentage of fat in the milk increased over a period of 3 weeks following the change in diet and, with the exception of linoleic acid which decreased, the yields of all fatty acids in milk progressively increased over this period. The increase in yield of oleic acid was relatively much smaller than the increases in yield of saturated acids. 4. The concentrations in blood plasma of acetic acid and 1-hydroxybutyric acid increased in two cows and, in all cows, saturated triglyceride fatty acids increased, whereas the concentrations of the unsaturated triglyceride fatty acids decreased, during the 1st week after change to the high-roughage diet. The recovery in milk fat secretion is discussed in relation to the observed changes in rumen fermentation and blood composition.