Effects of heat stress on ruminal volatile fatty acid con- centrations and gastrointestinal microbiome in weaned Holstein calves supplemented with or without dietary organic acid and plant botanicals

Our objectives were to evaluate the effects of heat stress on ruminal volatile fatty acid (VFA) concentrations and gastrointestinal tract microbiome in post-weaned calves subjected to heat stress and dietary organic acid and plant botanicals (OA/PB) supplementation. In a completely randomized design, 62 bull and heifer calves were fed starter grain and assigned to one of 5 groups (n = 12–13/group) for 19 d: thermoneutrality without OA/PB (TN-Con), heat stress (temperature- humidity index: 75 to 83) without OA/PB (HS-Con), thermoneutrality but pair-fed to HS-Con (TN-PF), heat stress supplemented with low or high dose OA-PB (75 and 150 mg/kg of body weight; 25% citric acid, 16.7% sorbic acid, 1.7% thymol, 1.0% vanillin, and 55.6% triglyceride; AviPlusR, Vetagro, Italy). Rumen fluid VFAs (d 3 and 19) were measured using liquid chromatography and bacteria (d 19) were profiled using 16S rDNA sequencing. The statistical model included fixed effects of body weight at birth, treatment, time, and their interaction as well as the random effect of calf. Heat stress increased ruminal lactate concen- trations, relative to TN-Con (P < 0.05). TN-PF had greater lactate and isobutyrate concentrations than TN-Con or HS-Con (P < 0.01). Although the major phyla in ruminal content did not differ between HS-Con and TN-Con, relative abundance (RA) of Bacteroidetes was higher in calves fed high-dose OA/PB, relative to TN-Con (P < 0.05). Duodenal Fibrobacteres RA tended to be reduced in heat-stressed calves, relative to TN-Con (P = 0.06). Jejunal Euryaechaeota and Methanobacteria- ceae RA were greater in heat-stressed calves, relative to TN-Con (P < 0.05). The RA of ileal Bacteroidetes and Bacteroidaceae tended to be increased in heat-stressed calves (P ≤ 0.15). Fecal Proteobacteria was also increased in HS-Con compared with TN-Con calves, and high dose OA/PB reduced this phylum (P = 0.01), relative to HS-Con. We conclude that heat stress and dietary OA/PB supplementation modified the gastrointestinal microbiome. Such changes may explain the ability of HS to modify ruminal VFA concentrations.