Milk fat synthesis in dairy cows is progressively reduced by increasing supplemental amounts of trans-10, cis-12 conjugated linoleic acid (CLA)

Conjugated linoleic acid (CLA) supplements containing a variety of isomers reduce milk fat yield. We have recently identified trans-10, cis-12 CLA as the isomer responsible for inhibiting milk fat synthesis in dairy cows. Our objectives were to determine milk fat yield and fatty acid composition res ponses to different doses of trans-in, cia-12 CLA. Multiparous Holstein cow s (n = 4) were used in a 4 x 4 Latin square design. Treatments consisted of a 5-d abomasal infusion of four doses of trans-10, cis-12 CLA, i.e., 0.0, 3.5, 7.0 and 14.0 g/d. Milk fat yield was decreased 25, 33, and 50%, and mi lk fat concentration was reduced 24, 37 and 46% when cows received 3.5, 7.0 and 14.0 g/d of trans-10, cia-12 CLA, respectively. Feed intake, milk yiel d, and milk protein content and yield were unaffected by treatment. Milk fa tty acid composition revealed that de novo synthesized fatty acids (short a nd medium chain) were extensively reduced when cows received the two highes t doses, but at the low dose (3.5 g/d), decreases in de novo synthesized fa tty acids and preformed fatty acids were similar. Changes in milk fatty aci d composition also demonstrated that ag-desaturase activity was inhibited a t the two high doses of trans-10, cis-12 CLA, but was unaffected by the low dose. Results indicate minimal quantities of trans-10, cis-12 CLA (0.016% of dietary dry matter) markedly inhibited milk fat synthesis (25% reduction ) and that a curvilinear reduction in milk fat yield occurred with increasi ng quantities of trans-in, cis-12 CLA.