Effect of CLA on milk fat synthesis in dairy cows: comparison of inhibition by methyl esters and free fatty acids, and relationships among studies.

CLA is a potent inhibitor of milk fat synthesis, as shown by investigations using mixtures of CLA isomers in FFA form. However, methyl esters of CLA can be initially formed in commercial synthesis, and their use in a supplement has certain manufacturing and cost advantages. Our objective was to compare abomasal infusion of methyl esters of CLA (ME-CLA) and FFA of CLA (FFA-CLA) on milk fat synthesis. Data were also combined with previous investigations to examine broader relationships between trans-10,cis-12 CLA and the reduction in milk fat. Three mid-lactation, rumen-fistulated Holstein cows were used in a 3 x 3 Latin square design. Treatments were (i) control, (ii) ME-CLA, and (iii) FFA-CLA. The ME-CLA and FFA-CLA treatments (4.2 g/d trans-10,cis-12 CLA) resulted in a comparable reduction in milk fat yield (38 and 39%, respectively) and pattern of reduction in individual FA. In contrast, milk yield, milk protein, and feed intake were unaltered by CLA treatment. Combining data across studies revealed strong correlations relating the reduction in milk fat yield to abomasal dose of trans-10,cis-12 CLA (R2 = 0.86), milk fat content of trans-10,cis-12 CLA (R2 = 0.93), and milk fat secretion of trans-10,cis-12 CLA (R2 = 0.82). Across studies, transfer efficiency of abomasally infused trans-10,cis-12 CLA into milk fat was relatively constant (22%; R2 = 0.94). Overall, ME-CLA and FFA-CLA were equally potent in reducing milk fat, and either form could be used to formulate a dietary supplement that would induce milk fat depression.