Longitudinal characterization of plasma and fecal bile acids in dairy heifers from birth to first calving in response to transition milk feeding.

This study aimed to characterize plasma bile acid changes from birth to first calving and evaluate the effects of early transition milk (TM) feeding versus milk replacer (MR) during key stages. Fecal bile acids in TM-fed calves were also analyzed, offering insights into bile acid metabolism. Thirty female Holstein calves were fed TM or MR for the first 5 d, followed by 12 L/d MR. From d 14, calves were fed MR and starter with gradual weaning between wk 8 and 14. Blood samples were collected at 7 time points: 30 min and 12 h after birth, preweaning (wk 2, 6), weaning (wk 14), 8 mo, 13 mo, 3 wk before calving, at calving, and 3 wk after calving. Fecal samples were collected from a subset of TM-fed calves (n = 10) at birth, wk 6, wk 14, 8 mo, and calving. Samples were analyzed for bile acids using the Biocrates MxP Quant 500 kit. Cholic acid (CA) in plasma showed significant time-treatment interactions, with higher levels in TM-fed calves at weaning. Taurine- and glycine-conjugated bile acids had no treatment or time-treatment interactions, but all plasma bile acids showed significant time effects. Principal component analysis revealed that bile acid profiles at birth and after colostrum intake were tightly clustered. Plasma bile acid profiles showed greater dispersion during milk feeding and weaning, with tighter clustering observed postweaning, particularly at 13 mo, and in the transition period. Significant effects were observed for CA, deoxycholic acid (DCA) and chenodeoxycholic acid (CDCA), with CA showing a notable interaction and being higher in TM-fed calves at weaning than in MR-fed calves. Bile acid levels increased toward weaning, peaked at wk 14, and decreased after weaning. Glycine-conjugated bile acids changed over time, with glycocholic acid (GCA) and glycodeoxycholic acid (GDCA) peaking at weaning, and glycochenodeoxycholic acid (GCDCA) being elevated before weaning, decreasing thereafter, and increasing again at calving. Taurine-conjugated bile acids also showed temporal changes, peaking at wk 6. The shifts in bile acid composition from birth to postcalving, with taurolithocholic acid (TLCA), GDCA, and taurocholic acid (TCA) initially dominating, CA increasing at weaning, and GDCA and DCA dominating at calving, with CA increasing again postcalving. During the transition to calving, CA decreased whereas glycine-conjugated bile acids increased relative to taurine-conjugated bile acids in plasma, irrespective of treatment. Fecal bile acid profiles in TM-fed calves clustered distinctly at birth, evolving through pre- to postweaning and calving, with increasing profile overlap over time. Most fecal bile acids, except DCA and CA, were abundant at birth but declined over time. Both DCA and CA increased postweaning, mirroring plasma trends. From wk 6 to calving, DCA was the dominant bile acid, accounting for the highest percentage of total bile acids excreted in feces. Spearman's correlation analysis was performed to assess the relationship between plasma and fecal bile acids in TN-fed calves. A significant positive correlation was observed only for GCDCA (Spearman's rank correlation coefficient [rho] = 0.35), whereas all other bile acids were not correlated. These results illustrate the complex dynamics of bile acid profiles during calf development.