Mammalian stress hormones in saliva and rumen fluid affect rumen microbial fermentation, nutrient digestibility, and gas production in vitro.

Two experiments were conducted to evaluate the impacts of stress hormones on ruminal fermentation, nutrient digestibility, and gas production in vitro. For experiment 1, artificial saliva containing 0, 3, 6, or 9 ng cortisol/mL was infused into four continuous culture fermentors in a 4 × 4 Latin square. Each period included 7 d of adaptation, a 3-d cortisol treatment period (CORT period), and 3-d post-treatment (POST period) where no cortisol was supplied. During the CORT period, pH, total volatile fatty acids, and isobutyrate tended to increase quadratically (P ≤ 0.15) as greater concentrations of cortisol were included in the artificial saliva. Butyrate (P = 0.07) and isovalerate (P = 0.06) quadratically decreased with increasing cortisol concentrations. Digestibility of NDF was greater (quadratic; P = 0.09) for 0, 3, and 6 vs. 9 ng/mL cortisol. During the POST period, NH3-N linearly decreased (P = 0.04) with increasing concentrations of salivary cortisol, and a quadratic effect (P = 0.10) was observed for the molar proportion of isovalerate. For experiment 2, no stress hormone (CON), epinephrine (EPI), norepinephrine (NOR), cortisol (CORT), and a combination of EPI, NOR, and CORT (ALL) were added to flasks containing ruminal fluid and McDougal's artificial saliva. Ruminal fluid was collected for analysis of pH, NH3, and VFA concentrations, and gas production. Neither pH, NH3-N, nor total VFA were different (P ≥ 0.40) among treatments. Acetate and isovalerate were less (P ˂ 0.01) for EPI and NOR than CON, CORT, and ALL. Butyrate was greater (P = 0.03) for EPI and NOR than CON, and intermediate for CORT and ALL. From 8 to 12 h incubation, gas production was greatest for CON and CORT, intermediate for NOR, and least for EPI and ALL (treatment × h; P ˂ 0.01). Therefore, mammalian stress hormones impact rumen microbial fermentation, digestion, and gas production.