The effects of intraruminal infusions of acetic, propionic and butyric acids on the yield and composition of the milk of the cow.

I n an earlier publication (Rook & Line, 1961) it was suggested that the increases in the yield and solids-not-fat (s.N.F.) content of milk that occur in response to an improvement in the plane of energy nutrition of the cow are due to increases in the ruminal production of acetic and propionic acids. Evidence was given supporting the view that the acids differ in their effects on the synthesis of milk constituents by the mammary gland, acetic acid affecting synthesis of both lactose and proteins, propionic acid affecting specifically the synthesis of proteins. The effect of the ruminal production of the volatile fatty acids on the fat content of milk is already well established (Balch, Balch, Bartlett, Bartrum, Johnson, Rowland & Turner, 1955): unusually low fat contents occur when cows receive diets lacking in physical fibrousness, and these diets characteristically lead to a low ratio of acetic acid to propionic acid in the rumen. Balch & Rowland (1959) suggested that the low fat content is due to a low ruminal production of acetic acid on these diets but, as discussed elsewhere (Rook, 1959), the amounts of propionic and butyric acids produced in the rumen also may be important in determining the extent of fat synthesis in the mammary gland. This paper describes three experiments to determine, with cows, the effects of continuous intraruminal infusions of acetic, propionic and butyric acids on the yield and composition of milk. Success was dependent on the development of a technique for the daily addition to the rumen of amounts of acids with a calorific value of not less than 4000 kcal, which is roughly equivalent to that of the smallest quantity of supplementary concentrates known from previous work (Rook & Line, 1961) to produce significant changes in the yield and composition of milk. Previously volatile fatty acids have been added to the diets of dairy cattle as sodium salts of the acids, either mixed with the concentrate part of the diet or as a solution, sometimes in admixture with small quantities of acetic acid, given directly through a fistula into the rumen (Balch & Rowland, 1959; Tyznik & Allen, 1951). I n this way daily additions of up to I kg sodium acetate (the equivalent as acetic acid would provide 1600 kcal) have been achieved, but larger quantities have produced harmful physiological effects (unpublished observation) probably resulting from the uptake of large amounts of sodium. A technique of infusing the acids in dilute aqueous solution directly into the rumen through a fistula suitable for continuous use over periods of several months was therefore developed in collaboration with M r R. C. Campling of this Institute.