Effects of increasing dietary levels of a palmitic acid–enriched supplement on fiber digestibility, rumen fermentation, and microbial composition in high-fiber diets

Recent research has highlighted that palmitic acid (PA)-enriched supplements increase NDF digestibility, likely due to rumen microbial composition and changes in metabolism. However, the effect of different PA doses on fiber digestibility and rumen metabolism remains unclear. Thus, this study aimed to investigate the effects of increasing dietary levels of a PA-enriched supplement on fiber digestibility, rumen fermentation, and microbial composition using continuous culture fermenters. Diets were assigned to 8 dual-flow continuous culture fermenters arranged in a replicated 4 × 4 Latin square with four 11-d experimental periods. The treatments included a basal control diet (50:50 forage:concentrate) without a PA-enriched supplement and the basal diet supplemented with incremental levels of supplemental PA at 0.85%, 1.70%, or 2.25% of DM. The supplement contained 85.0% C16:0, 3.10% C18:0, 9.70% cis-9 C18:1, and 97.4% total fatty acids. Buffer dilution and solids passage rate were maintained at 7.0%/h and 5.0%/h, respectively. Samples were collected in the last 4 d of each period. Data were analyzed using a mixed model considering treatments as fixed effects and period and fermenter as random effects. Digestibility of NDF responded quadratically to PA inclusion, with 0.85% PA resulting in the highest digestibility, followed by 1.70% PA and 2.55% PA, and ADF digestibility showed a similar quadratic trend as the PA dose increased. No treatment effects were observed for OM, starch, or hemicellulose digestibility. Propionate flow also tended to respond quadratically, peaking at 0.85% PA, whereas isobutyrate and valerate flow increased linearly with PA inclusion. Microbial growth per kilogram of degraded ADF and NDF showed a quadratic response increasing at 2.55% PA, indicating improved microbial efficiency. Although we did not observe treatment effects on the α- and β-diversity of the microbial population, the relative abundances of several bacterial phyla, families, and genera responded quadratically to PA, increasing Prevotellaceae and decreasing Lachnospiraceae relative abundance. These findings underscore the potential of PA to enhance fiber digestibility through shifts in bacterial metabolism and the abundance of fiber-degrading bacteria. However, further research is necessary to understand broader implications for microbial energy use, growth, and efficiency.