The Effects of a Diet Low in Hay and High in Flaked Maize on Milk-fat Secretion and on the Concentrations of Certain Constituents in the Blood Plasma of the Cow.

The original observations of Powell (1938, 1939, 1941) that, for lactating cows, diets containing only a small amount of roughage, or usual amounts of roughage offered in a finely ground form, are frequently associated with an unusually low content of fat in the milk have been fully confirmed (see Van Soest (1963) for a comprehensive review). Information on the associated changes in the composition of the milk fat is, however, incomplete. The few studies in which changes in the fatty acid composition have been measured by chromatographic techniques (Smith & Ronning, 1961 ; Brown, Stull & Stott, 1962; King & Hemken, 1962; Palmquist, Smith & Ronning, 1964) agree generally in showing an increase in the proportion of unsaturated acids when diets contain little roughage, but the reported effects on the other acids are inconsistent and in much of the work all the acids were not investigated. There is also still much uncertainty as to the cause of the fall in milk fat content (Van Soest, 1963; Rook & Storry, 1964). The experiments now described were therefore done to obtain additional information on the effect of a diet low in hay and high in flaked maize on the secretion in milk of the component fatty acids of milk fat and on the concentrations in blood plasma of several constituents that are possibly of importance in milk fat synthesis.