Chronic dexamethasone exposure retards growth without altering the digestive tract microbiota composition in goats

Dexamethasone (Dex), an artificially synthetic cortisol substitute, is commonly used as an anti-inflammatory drug, and is also employed to mimic the stress state experimentally. It is well known that chronic stress disturbs the gut microbiota community and digestive functions. However, no relevant studies have been conducted in ruminants. In this study, a low dosage of Dex (0.2 mg/kg body weight, Dex group) was consecutively injected intramuscularly for 21 days to simulate chronic stress in growing goats. Goats were injected with saline (0.2 mg/kg body weight) as the control group (Con, n = 5). Dex-treated goats showed a higher number of white blood cells and blood glucose levels (P &lt; 0.01), but lower dry matter intake (DMI) and body weight (P &lt; 0.01) than those of saline-injected goats. Plasma cortisol concentration decreased significantly in response to the Dex injection compared to the control (P &lt; 0.05). The Dex treatment did not change most ruminal volatile fatty acid (VFAs) concentrations before the morning feeding after 1–21 days of treatment (P &gt; 0.05); however, ruminal VFA concentrations decreased dramatically 2, 4, 6, and 8 h after the morning feeding on day 21 of the Dex injections. In this study, chronic Dex exposure did not alter the community structure of microbes or methanogenes in the rumen, caecum, or colonic digesta. Only <i>Prevotella</i> increased on days 7 and 14 of Dex treatment, but decreased on day 21, and <i>Methanosphaera</i> was the only genus of methanogene that decreased. Taken together, our results suggest that chronic Dex exposure retards growth by decreasing DMI, which may be mediated by higher levels of blood glucose and lower ruminal VFA production. Microbiota in the digestive tract was highly resistant to chronic Dex exposure.