Nutritional manipulation of the rumen ecosystem provides a strategy to alter the content and composition of milk fat. Dietary fat supplements affect the content and composition of milk fat. The magnitude of changes is influenced by the degree of protection; as protection increases, the deleterious effects fatty acids on microbial activity decreases, and biohydrogenation of C18 unsaturated fatty acids is reduced. In addition, change is influenced by the transfer of dietary fat into milk, which is related to fatty acid composition, degree of ruminal metabolism, and efficiency of digestion. A cascade of metabolic events involving specific nutrients (e.g., trans fatty acids and cyclopropene acids) occurs that regulates the activity of key enzymes in pathways of endogenous fat synthesis within the mammary gland. When cattle are fed oilseeds (e.g., canola and cotton) with > 75% protection from biohydrogenation, the proportion of saturated to unsaturated fatty acids is changed, and the fat content of milk is increased. Human consumption of dairy products containing elevated proportions of C18 mono- and polyunsaturated fatty acids reduces the content of cholesterol in plasma low density lipoproteins. These fat-modified dairy products are more susceptible to autoxidation, which can be controlled by including vitamin E in the diet of lactating cow. These products also have much less solid fat, which improves spreadability of butter. By protecting different oilseeds from ruminal metabolism, demands for energy can be satisfied while producing milk fat that can be designed for consumer and manufacturing requirements.
