Contrasting Patterns in Rumen Fermentation, Digestibility, and Fatty Acid Biohydrogenation with Increasing Levels of Protected Coconut and Palm Fatty Acid Distillates

Tropical dairy systems face energy deficits from low-quality forages and high rumen fermentation heat, while interest grows in improving milk fatty acid composition, particularly CLA precursors; however, intensive biohydrogenation limits beneficial unsaturated fatty acids. This study evaluated the dose–response effects of coconut (CFAD) and palm (PFAD) fatty acid distillates on rumen fermentation, digestibility, and fatty acid biohydrogenation. An in vitro 2 × 5 factorial design was applied, comprising two FAD types and five inclusion levels (0%–4% DM) with 12 replicates. Fermentation characteristics, nutrient digestibility, microbial activity, and biohydrogenation indices were measured, and polynomial regression was used to determine optimal inclusion levels. CFAD exhibited stronger antimicrobial and defaunating effects, resulting in higher total VFA production, improved NH₃ utilization, enhanced microbial protein synthesis, and a more favorable unsaturation pattern at moderate inclusion levels, although it reduced trans-11 accumulation. In contrast, PFAD maintained more stable fermentation and promoted greater accumulation of trans-11 and CLA precursors, despite increasing methane-related pathways at higher inclusion levels. Overall, CFAD (2.5%–3% DM) is more suitable for improving fermentation efficiency, whereas PFAD (1.5%–2% DM) is more appropriate when the objective is to enhance CLA-related intermediates.