Simple SummaryCalf stage is critical to the production performance and meat quality of growing beef cattle. Early weaning is gradually carried out in rearing calves because it can accelerate their gastrointestinal development. Nevertheless, calves experience a significant stress reaction due to the transformation of feed type in weaning. Therefore, mitigating the weaning stress and promoting the healthy growth of weaned beef calves are of great significance for modern large-scale beef cattle farming. Our study evaluated the effects of grape seed extract supplementation on growth performance, ruminal fermentation, nutrient digestibility and serum biochemical, antioxidative and immune parameters of weaned cross-breed beef calves. The findings obtained from the current study indicate that grape seed extract as a feed additive led to a significant improvement in growth performance, ruminal fermentation, antioxidative capacity and immunity of weaned cross-breed beef calves.AbstractThe purpose of this research was to evaluate effects of grape seed extract (Gse) supplementation on the growth performance; ruminal fermentation; nutrient digestibility; and serum biochemical, antioxidative, and immune parameters of weaned beef calves. A total of 30 Simmental crossbred male calves with similar age and body weight were randomly allocated to two groups: a control group with no Gse (CON) and a Gse supplementation group (GSE) (4 g/d Gse per animal). The results show that, compared with the CON group, the average daily gain significantly increased (p= 0.043) in the GSE group. The ruminal contents of microbial protein and butyrate in GSE group were higher (p< 0.05) than those in the CON group. Additionally, calves fed Gse displayed increased (p< 0.05) dry matter and neutral detergent fiber digestibility. Moreover, the serum concentrations of triglyceride, catalase, superoxide dismutase, immunoglobulin G and immunoglobulin M were higher (p< 0.05) in the GSE group than those in the CON group. However, opposite tendencies of non-esterified fatty acid, malondialdehyde, tumor necrosis factor-α and interleukin-6 were found between the two groups. Overall, the supplementation of Gse can improve ruminal fermentation, nutrient digestibility, antioxidant ability, and immunity, as well as promoting the healthy growth of weaned cross-breed beef calves.