WS Salivary cortisol concentrations affect rumen microbial fermentation and nutrient digestibility in vitro

This study investigated effects of cortisol on fermentation and digestibility of nutrients by rumen microorganisms. Four dual-flow continuous culture fermentor (2700 mL) systems were used in a 4 x 4 Latin square design with 4 periods and 4 treatments. Each experimental period consisted of 13 d, which allowed 7 d for microbial adaptation, 3 d for cortisol treatment application, and a 3 d post-treatment period. Treatments consisted of 0, 3, 6, or 9 ng/mL of cortisol mixed into artificial saliva and continuously infused at a rate of 1.55 ± 0.05 mL/min. For the 3 d treatment period and the 3 d post-treatment period samples were collected at 0, 3, 6, 9, 12, 24, 36, 48, 60, and 72 h and were analyzed VFA, and NH3. During the 3-d sampling period effluent was composited for analysis of OM, NDF, and CP digestibility. During cortisol treatment, cortisol x h (P ≤ 0.01) was observed for acetate and valerate, total VFA and isobutyrate tended to increase from 0 to 3 ng/mL, but were not different among 3, 6, and 9 ng/mL (quadratic, P ≥ 0.12), butyrate tended to decrease from 0 to 3 ng/mL cortisol, and was not different among 3, 6, and 9 ng/mL (quadratic, P = 0.07), and isovalerate was not different among 0, 3, and 6 ng/mL, but decreased from 6 to 9 ng/mL of cortisol (quadratic, P = 0.06). Digestibility of OM (g/d and % of intake) tended to be lower (quadratic, P = 0.12), and NDF digestion (g/d and % of intake) was lower (quadratic, P = 0.09) for 9 ng/mL cortisol compared to 0, 3, or 6 ng/mL. Digestibility of CP (g/d and % of intake) was not different (P ≥ 0.23) among treatments. In the period after cortisol treatment, a cortisol x h (P = 0.03) was observed for isobutyrate, NH3 decreased linearly (P = 0.04) with increasing cortisol, valerate tended to increase linearly (P = 0.14) with increasing concentrations of cortisol, and isovalerate was lower for 9 ng/mL than 0, 3, and 6 ng/mL cortisol (quadratic, P = 0.10). Digestibility (g/d and % of intake) of OM, NDF, and CP were not different (P ≥ 0.51) among treatments. Results indicate that cortisol may influence rumen microbial fermentation and digestion when present in saliva at 9 ng/mL.