Factors affecting the yield of milk and its constituents, particularly fatty acids, when dairy cows consume diets containing added fat

Abstract

Abstract Lactating cows and heifers were offered diets containing (a) no added oil, (b) added tallow and (c) an added mixture of palm oil and soya oil. Each of these diets was given at two levels of feeding. The diet containing no added oil at the high level of feeding was isoenergetic and isonitrogenous with diets (b) and (c) at the low level of feeding. This design allowed the effect of the decrease in fermentable substrate (consequent upon substituting fat for starch) on the output of short‐chain fatty acids (SCFA) in the milk fat to be quantified. Approximately 20% of the total decrease in the concentations of these acids in milk was found to be due to the loss of dietary starch when tallow was fed, the value being somewhat higher in the case of the more unsaturated acids of diet (c). These results are considered in the context of the established changes in citrate concentration of milk that occur on feeding fat. It is concluded that the major cause of decreased output of SCFA in the milk when dairy cows consume rations containing added fat is mediated within the mammary gland and is probably due to the dietary long‐chain fatty acids acting as partial inhibitors of the synthesis of the SCFA.