Phytogenic Modulation of Rumen Fermentation Reshapes Amino Acid Metabolism in Weaned Dairy Calves: A Mechanistic Insight

The impact of essential oils on amino acid metabolism in relation to the rumen fermentation outcomes and adaptation to a solid diet is not well documented. Our objective was to evaluate the impact of an essential oil blend (EOB; contains cinnamaldehyde, capsicum, curcumin, and saponin) on growth performance, rumen fermentation, and serum amino acids of weaned growing calves. Sixteen weaned Holstein calves were blocked by body weight (100 ± 6 kg), and randomly allotted into: 1—CON group (n = 8; received 0% EOB), and 2—EOB group (n = 8; received 0.5 g of the EOB/Kg dry matter for 60 d, sex was used to balance the groups, so each group has 4 male and 4 female calves, but it is not the main scope of this study. Growth performance, rumen fermentation, and serum amino acids were assessed. Growth performance was not affected by treatment. Rumen pH decreased, and valeric acid, acetate, and propionate levels were increased. Serum leucine was reduced, and aspartate increased. Metabolic pathway enrichment analysis identified alanine, aspartate, and glutamate metabolism as the most affected metabolic pathway. In conclusion, supplementation with this EOB did not impact growth performance, despite its impact on rumen fermentation and blood amino acids in weaned Holstein calves. This study indicates that phytogenic supplementation may contribute to metabolic modulation during the post-weaning period through modulation of amino acid-associated metabolic pathways.