Our objective was to examine the effect of trans-10, cis-12 conjugated linoleic acid (CLA) on production performance and energy partitioning. Holstein cows (n=32; 93±35 DIM) were randomly assigned to treatment sequence in a crossover design experiment and fed iso-energetic diets containing fat supplements differing in saturation. Treatment diets contained 2.5% palmitic acid-enriched triglyceride (BergaFat T-300, SAT) or 2.5% soybean oil (UNSAT), with 25% NDF, 32% starch, 18% CP, and 4.6% FA (DM basis). Treatment periods were 28 d in length with the final 5 d used for sample and data collection. The statistical model included the random effect of cow and fixed effects of treatment and period. Compared to the SAT treatment, UNSAT did not alter dry matter intake (DMI), energy intake, or milk yield but decreased milk fat concentration and yield, with reduced de novo fatty acid (FA) and 16-carbon FA yield. UNSAT also decreased fat-corrected milk (FCM) and energy-corrected milk (ECM). UNSAT increased body weight (BW) gain but did not alter body condition score (BCS) or fat thickness over the rump and rib. UNSAT tended to reduce NDF digestibility and increased FA digestibility in period 1 but not in period 2. UNSAT increased plasma insulin, NEFA, and triglyceride concentrations, with increased milk trans-10 C18:1 and trans-10, cis-12 C18:2. In conclusion, with similar NEL intake, the SAT diet containing the palmitic acid-enriched triglyceride partitioned more energy toward milk, while the UNSAT diet containing soybean oil partitioned more energy toward body gain.
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