Physiological responses and gut microbiota dynamics in beef cattle following transport stress syndrome with melatonin or dexmedetomidine administration

Beef cattle transport commonly induces stress, which may disrupt physiological homeostasis and alter the gut microbiota, thereby affecting animal welfare and production performance. The present study aimed to evaluate the effects of melatonin and dexmedetomidine on transport-induced stress. A total of 15 healthy Simmental bulls were enrolled and administered melatonin, dexmedetomidine, or physiological saline prior to a 6 h transport period (n = 5 per group). Blood and fecal samples were collected before transport and on days 1, 4, and 7 post-transport. Hematological parameters, biochemical indices, stress-related hormones, and oxidative stress markers were measured, and fecal samples were subjected to 16S rRNA gene sequencing analysis. The results demonstrated significant treatment × time interaction effects for red blood cell (RBC), hemoglobin concentration (HGB), adrenocorticotropic hormone (ACTH), and superoxide dismutase (SOD) (P < 0.05). In the melatonin (MT) group, RBC and HGB levels were significantly increased on day 7, with HGB levels exceeding those in the control (CON) group (P < 0.05). ACTH levels were significantly elevated in both the MT and dexmedetomidine (DEX) groups on day 7 (P < 0.05). In addition, SOD levels in the MT and DEX groups were higher than those in the CON group on day 7 (P < 0.05). Regarding the gut microbiota, the Shannon diversity index in the CON group decreased significantly on day 7 (P < 0.05), whereas no significant changes were observed in the MT and DEX groups. Beta diversity analysis revealed a significant separation in microbial community structure among groups (P < 0.05). At the genus level, an increase in Ruminococcus was observed in the DEX group on day 7, while Bacteroides abundance decreased in the MT group (P < 0.05). These findings suggest that melatonin and dexmedetomidine may exert regulatory effects on transport-induced stress and gut microbiota; however, further studies are required to validate these effects.