Genetic selection for increased milk fat percentage leads to increased proportions of short-chain fatty acids in milk fat and decreased proportions of long-chain fatty acids (Karijord et al., 1982). Consistent with this, milk fat from Jersey cows has higher amounts of short and medium chain fatty acids (Beaulieu and Palmquist, 1995; DePeters et al., 1995) and lower ratios of cis 9 18:1/18:0 than milk fat from Holsteins. Milk fat composition is strongly influenced by stage of lactation; proportion of short chains is low initially and increases until at least 8 to 10 wk into lactation. Seasonal and regional differences in milk fat composition are measurable, most likely because of local differences in feed supplies (Palmquist et al., 1993).

     The fundamental processes of milk fat synthesis are well established and explain the occurrence of high amounts of saturated fatty acids in milk. Milk fat is synthesized from fatty acids (FA) which are obtained from blood or by de novo synthesis in the mammary gland. Fatty acids synthesized de novo contain 4 to 16 carbons and are saturated. Blood FA are derived from diet or from lipolysis in adipose tissue. Approximately 50 to 60% of milk FA are of dietary origin; therefore, FA composition of milk can be influenced by diet. Modifications of dietary FA prior to incorporation into milk fat include biohydrogenation of unsaturated FA in the rumen and desaturation of stearic acid by intestinal, adipose, or mammary tissue. Consequently, milk FA tend to be lower in polyunsaturated FA and higher in oleic acid than is dietary fat.

     The most thorough modern summaries of the distribution of FA in milk fat and dairy products are by Jensen et al. (1991) and Jensen (1992, 1995). A CRIS search revealed 50 projects related to milk production and/or feeding effects on milk composition, and/or milk fat quality. Of these, 23 were projects of members of the present W-181 Committee; 13 were from non-members in project states, and 14 were from non-project states. The Committee is making an effort to include more investigators, as well as the Cooperative Extension Service who have similar interests from both project and non-project states. Other dairy-related regional research projects include NC-185, "Metabolic Relationships in Supply of Nutrients for Lactating Cows", which is focused on rumen metabolism and supply of amino acids for milk protein synthesis. Two W-181 members (OH and SD) are also active members of NC-185, and both committees share the same CSREES representative, which should enhance coordination and reduce duplication between the committees. A second regional committee, NE-132 "Environmental and Economic Impacts of Nutrient Flows in Dairy Forage Systems" concerns forage use and the environment; therefore it does not address issues of product composition and quality.