The aim of this study was to investigate the effect of dietary supplementation with zinc (Zn) amino acids at different concentrations on immunity, antioxidant capacity, and gut microbiota composition in calves. Twenty-four one-month-old healthy Angus calves of comparable body weight were randomly divided into three groups (four males and four females in each group) based on the amount of Zn supplementation added to the feed the animals received: group A, 40 mg/kg DM; group B, 80 mg/kg DM; and group C, 120 mg/kg DM. The experiment ended when calves reached three months of age (weaning period). The increase in dietary Zn amino acid content promoted the growth of calves, and the average daily weight gain increased by 36.58% (p < 0.05) in group C compared with group A. With the increase in the content of dietary Zn amino acids, the indexes of serum immune functions initially increased and then decreased; in particular, the content of immunoglobulin M in group A and group B was higher than that in group C (p < 0.05), whereas the content of interleukin-2 in group B was higher than that in the other two groups (p < 0.05). In addition, the content of superoxide dismutase and total antioxidant capacity in the serum of calves in group B was higher than that in group C (p < 0.05), and the MDA level was lower than in group C (p < 0.05). Moreover, alpha diversity in the gut microbiota of calves in group B was higher than that in group A and group C (p < 0.05); the dominant phyla were Firmicutes and Bacteroidota, whereas the dominant genera were Unclassified-Lachnospiraceae and Ruminococcus. Linear discriminant analysis showed that the relative abundance of Bacteroides in the gut microbiota of calves in group B was higher than that in group A, and the relative abundance of Prevotellaceae-UCG-003 was higher compared to that in experimental group C. Thus, dietary supplementation of 80 mg/kg of Zn amino acids to calves could improve the immune function and antioxidant capacity, as well as enrich and regulate the equilibrium of gut microbiota, thus promoting the healthy growth of calves.

Soybean meal faces challenges of price fluctuations and limited supply, making it imperative to research cost-effective alternative protein feed ingredients in animal husbandry. This study investigated the effects of partially replacing soybean meal with flaxseed cake (FSC) in diets on growth performance, nutrient digestibility, rumen fermentation, serum biochemical parameters, and meat quality of adult Simmental cows. A total of 16 Simmental cows, weighing 650.67 ± 7.75 kg, were randomly divided into two groups: CON (fed a diet with 3.8% soybean meal) and FSC (fed a diet containing 2.28% soybean meal + 1.52% flaxseed cake, which replaced 40% of the soybean meal with FSC). The formal trial period lasted for 60 days, with a 15-day pretrial phase. The results showed that replacing soybean meal with FSC improved the average daily gain (ADG) (P < 0.05) and reduced the feed-to-gain (F/G) ratio (P < 0.05). CP apparent digestibility was enhanced in the FSC group (P < 0.05). Rumen fermentation analysis indicated increased acetate (P < 0.05) and decreased valerate concentrations (P < 0.05) in the FSC group. Serum biochemical analysis revealed the cows in the CON group had a higher albumin (ALB) content (P < 0.05). The cows in the FSC group showed significantly higher serum immunoglobulin A (IgA, P < 0.05) and IgM (P < 0.01) concentrations than those in the CON group. Regarding antioxidant indexes, the serum total antioxidant capacity (T-AOC) activity in the FSC group was higher than in the CON group (P < 0.05). Meat quality and nutritional ingredients remained similar between the two groups, while the meat's a* (redness) was significantly higher in the FSC group (P < 0.05). C15:1 content in beef was higher in the FSC group than in the CON group (P < 0.01). In conclusion, replacing 40% of soybean meal in the diet with FSC could improve the growth performance, crude protein digestibility, immunity and antioxidant status, without negatively affecting the meat quality in beef cows, suggesting flaxseed cake as a viable partial replacement for soybean meal in beef production.