Plasma bile acid dynamics during 3- and 6-week weaning transitions in ad libitum milk replacer-fed calves

Elevated milk replacer (MR) feeding supports early-life growth in dairy calves but may increase the biological complexity of weaning by shifting calves from milk-derived intestinal nutrient absorption to solid-feed-dependent gastrointestinal maturation. Bile acids (BA) are involved in lipid digestion, enterohepatic cycling, gut-liver signaling, and host-microbial interactions; however, their trajectories during weaning under high-MR feeding systems remain poorly characterized. This companion study evaluated whether plasma BA profiling reveals metabolic remodeling during weaning in calves subjected to different weaning durations. Twenty Holstein calves, including 10 males and 10 females, received MR ad libitum from wk 1 to 7, were standardized to 12 L/d at wk 8, and were assigned to either a 3-wk or 6-wk step-down weaning protocol, with 10 calves per treatment. Both protocols included a final week at 2 L/d before complete MR withdrawal. Plasma samples were collected at calf age wk 10, 13, and 16, representing protocol-dependent stages of the weaning transition and early postweaning period. Bile acids were quantified using targeted liquid chromatography-tandem mass spectrometry. Individual BA were analyzed using repeated-measures mixed models, and global BA profile structure was evaluated using multivariate analyses. Plasma BA profiles were remodeled primarily across sampling week rather than by weaning duration as a broad treatment effect. The most consistent response was a decline in taurine-conjugated BA, including taurocholic acid, taurolithocholic acid, taurodeoxycholic acid, taurochenodeoxycholic acid, and tauromuricholic acid, from wk 10 to wk 16. In contrast, selected non-taurine BA contributed more strongly at later sampling points. Glycocholic acid increased from wk 10 to wk 13 and remained elevated at wk 16, whereas glycoursodeoxycholic acid declined after wk 10. Cholic acid, deoxycholic acid, and chenodeoxycholic acid showed weaning duration × sampling week interactions, indicating that the 3- and 6-wk protocols modified the temporal kinetics of selected BA species without producing a broad treatment-wide shift in the total measured plasma BA pool. Total measured plasma BA concentration did not differ by weaning duration or sampling week, suggesting that temporal remodeling was driven mainly by changes in individual BA species rather than by a broad shift in total circulating BA abundance. Multivariate analyses confirmed progressive separation of plasma BA profiles across wk 10, 13, and 16, with wk 13 occupying an intermediate position. Overall, these results show that circulating BA profiles capture a previously underexplored component of weaning physiology in calves fed elevated MR allowances, linking the transition from milk-derived lipid digestion to solid-feed-associated gastrointestinal and enterohepatic maturation.