Responses of milk fat composition to dietary fat or nonstructural carbohydrates in Holstein and Jersey cows

Milk fat from Jersey cows contains less oleic acid (cis-C-18:1) and more sh ort- and medium-chain fatty acids than does milk fat from Holstein cows. Th e objective of this experiment was to determine responses in milk fat compo sition when Jersey and Holstein cows were fed diets either high (37% of dry matter) or low (27% of dry matter) in content of nonstructural carbohydrat es (NSC) and supplemented with either 0 or 2.5% (of dry matter) of a mostly saturated fat source. Four Holstein cows and four Jersey cows were used in a Latin square design with 28-d periods; diets were in a 2 x 2 factorial a rrangement. Fat supplementation decreased contents of fatty acids synthesiz ed de novo within the mammary gland and increased contents of C-18:0 and ci s-C-18:1. Low-NSC diets tended to increase C-16:0 and to decrease C-18:0, c is-C-18:1, and C-18:3 Despite the differences in fatty acid composition bet ween breeds, both breeds generally responded similarly to dietary treatment s. An interaction of breed and fat indicated that the content of CiS-C-18:1 in milk fat was increased more by supplemental fat in Holsteins than in Je rseys. Interactions of breed x fat and breed x carbohydrate type showed tha t the ratio of cis-C-18:1 to C-18:0 decreased when Jerseys were supplemente d with fat but increased for Holsteins, and decreased when Jerseys were fed the low-NSC diet but increased when Holsteins were fed low NSC. The data a re consistent with the hypothesis (Beaulieu and Palmquist, 1995, J. Dairy S ci. 78:1336-1344) that mammary activity of stearoyl-coenzyme A desaturase i s lower in Jerseys than in Holsteins.