tended to have a greater total VFA concentration (35.60 ± 11.4 vs. 11.90 ± 11.8 mM) but mean rumen pH was unaffected (6.25 ± 0.22 vs. 6.17 ± 0.21, respectively). Between PRE-S (wk 6) and POST-S (wk 9), calf starter intake increased (250 ± 219 vs. 2,239 ± 219 g/d), total VFA concentrations increased (35.6 ± 11.4 vs. 154.4 ± 11.8 mM), but mean rumen pH was unaffected (6.25 ± 0.22 vs. 6.40 ± 0.22, respectively). Compared with POST-S, POST-B calves had greater starter intake in wk 7, 8, and 9, but POST-B tended to have lower total VFA concentration (131.0 ± 11.8 vs. 154.4 ± 11.8 mM) and lower mean ruminal pH (5.83 ± 0.21 vs. 6.40 ± 0.22). In conclusion, the weaning transition does not appear to affect rumen pH and VFA profile, but supplementing rumen-protected butyrate during the weaning transition increased starter intake and average daily gain. Further, these data suggest that the ability of the rumen to manage rumen pH changes fundamentally postweaning. Why weaned calves with lower rumen pH can achieve higher calf starter intakes is unclear; these data suggest the effect of rumen pH on feed intake differs between calves and cows.

No abstract

In calves, changes in intracellular pH regulation dynamics and volatile fatty acid (VFA) transporter abundance through the weaning transition have not previously been described. The proton exporter NHE3 decreases in abundance between the preweaning and postweaning phase. Preweaning, the bicarbonate importer NBC1 increases in abundance in calves fed milk only versus those fed milk and solid feed. With solid feed consumption, no changes in abundance for the VFA transporter MCT1 were observed preweaning or postweaning. Some short-chain fatty acids (SCFA) are still imported through passive diffusion. Highlights• Calves may experience dysregulation of intracellular pH through weaning.• VFA concentration changes in the rumen do not equate to VFA transporter abundance.• The calf rumen can handle large amounts of fermentable feed without negative effects.• Cell size and relative cell circumference change as the rumen is developing.