Microbiota-associated antioxidant responses to selenium source and probiotic supplementation in beef cattle

Selenium (Se) and micro-ecological preparations are widely recognized for their roles in enhancing animal health, antioxidant capacity, and production performance. However, their combined effects and underlying mechanisms, particularly involving rumen microbiota, remain poorly understood. This study aimed to investigate the effects of different selenium sources (inorganic vs. organic) combined with micro-ecological probiotic supplementation on growth performance, serum biochemical parameters, antioxidant status, and rumen microbial composition in beef cattle. A total of 24 crossbred Xinjiang Brown steers (average initial body weight: 407.48 ± 36.10 kg; age: 19 months) were assigned to a 2 × 2 factorial design following a 14-day adaptation period and a 37-day feeding trial, with selenium source (sodium selenite vs. yeast selenium) and micro-ecological probiotic supplementation (0 vs. 3%). Growth performance, serum biochemical indices, antioxidant parameters, and rumen microbiota (16S rRNA sequencing) were analyzed. Weighted gene co-expression network analysis (WGCNA) was further employed to explore the associations between microbial modules and host phenotypes. Results showed that selenium source significantly affected feed conversion ratio (FCR), with sodium selenite resulting in improved feed efficiency compared to yeast selenium (p = 0.027). Selenium source significantly affected total antioxidant capacity (T-AOC; p = 0.043), malondialdehyde (MDA; p = 0.005), and glutathione peroxidase activity (GSH; p < 0.001), whereas probiotic supplementation significantly affected superoxide dismutase activity (SOD; p = 0.032) and GSH activity (p < 0.001). A significant interaction between selenium source and probiotic supplementation was observed for GSH activity (p = 0.002). WGCNA identified key microbial modules associated with T-AOC and GSH, within which genera such as Romboutsia, Paeniclostridium, and Turicibacter were significantly correlated with antioxidant indices. Further analysis suggested that selenium source may modulate growth performance through a microbiota-antioxidant axis, particularly via Romboutsia-mediated regulation of oxidative status. In conclusion, selenium source and micro-ecological probiotic supplementation jointly influence antioxidant capacity and rumen microbial structure, and may regulate feed efficiency through microbiota-mediated mechanisms. These findings provide new insights into nutritional strategies for improving beef cattle production.