Effects of single-dose ruminal infusions of high or low short-chain fatty acid concentrations and high or low pH on apparent total-tract digestibility and hindgut fermentation of preweaning dairy calves

Although the importance of pH and short-chain fatty acids (SCFA) on rumen development are well-known, their impact on the small and large intestine are unclear. This study investigated how single-dose ruminal infusions with high or low SCFA concentrations and high or low pH affect calves' productivity, as well as physiological parameters associated with hindgut acidosis at 3 time points in 49 d. Holstein bull calves (n = 32) were individually housed and fed milk replacer (900 g/d) twice daily and calf starter and water ad libitum. At d 10 ± 3 of life, the rumens were fistulated and cannulated. At d 14 of life, calves were grouped by BW and assigned in a 2 × 2 factorial arrangement of treatments: high or low SCFA concentration (285 vs. 10 mM) and high or low pH (6.2 vs. 5.2), creating 4 treatment groups: high SCFA concentration, high pH (HS-HP); high SCFA concentration, low pH (HS-LP); low SCFA concentration, high pH (LS-HP); and low SCFA concentration, low pH (LS-LP). On d 21, 35, 49, feces were sampled to calculate apparent total-tract digestibility, determinate organic acid concentrations (i.e., SCFA, branched-chain fatty acid [BCFA], and lactic acid), and pH. Afterward, the rumen was evacuated and underwent a single-dose infusion for 4 h with 1 of 4 treatment buffers. After completion of rumen infusion on d 49, calves were killed and the tissue weight and length were recorded, along with the digesta pH of the rumen, duodenum, jejunum, ileum, cecum, colon, and rectum. Data were analyzed with main factors as fixed effects and repeated measures for weekly measurements. Treatments did not affect performance parameters such as feed intake, ADG, apparent total-tract digestibility and gut measurements. In the duodenum, jejunum, and ileum, HS-HP had a greater digesta pH than LS-HP, whereas the hindgut digesta pH was only affected by the SCFA concentration. A high SCFA concentration increased the concentration of colonic isovaleric acid and fecal BCFA, whereas only colonic acetic acid and fecal lactic acid concentrations were affected by treatment. Fecal SCFA and BCFA concentrations increased mainly on d 35. In summary, 4 h of physiological buffer infusion in the rumen does not change apparent total-tract digestibility and gut measurements but does affect hindgut fermentation parameters (i.e., organic acid concentrations and digesta pH). In addition, calves can experience increased risks of hindgut acidosis around 35 d of life.