This study examined the effects of dietary crude protein (CP: 18%, 15%) and crude fat (EE: 8%, 4%) levels, and their interactions, on growth performance, nutrient digestibility, serum indices, and rectal fecal microbiota in sika deer fawns during early wintering. A two-month 2 × 2 factorial experiment was conducted using 32 healthy five-month-old male fawns randomly assigned to four groups: P18E8 (18% CP, 8% EE), P18E4 (18% CP, 4% EE), P15E8 (15% CP, 8% EE), and P15E4 (15% CP, 4% EE). The P18E4 group showed the highest total weight gain and average daily gain (p < 0.05), along with greater apparent digestibility of dry matter, crude protein, calcium, and fiber fractions (p < 0.05). Serum urea content was significantly lower in this group, indicating improved nitrogen utilization (p < 0.05). Dominant fecal microbiota at the phylum level across all groups included Firmicutes_A and Bacteroidota, with the P18E4 group showing a unique genus composition within Bacteroidota, known for enhancing fiber digestion. In summary, a diet with 18% CP and 4% EE optimized growth performance, nutrient digestibility, and gut microbiota composition, providing a strategy for improving the health and productivity of sika deer fawns during overwintering.

This study investigated the effects of replacing soybean meal with fermented rapeseed meal (FRSM) in the diets of sika deer (Cervus nippon) during the pre-antler growth period. A single-factor experimental design was employed. A total of 24 male sika deer aged 2–3 years were randomly divided into four groups with six deer per group, including a control group (0% substitution) and three treatment groups fed diets containing 2.8%, 5.6%, and 8.4% fermented rapeseed meal (FRSM), defined as the low (L-FRSM), medium (M-FRSM), and high (H-FRSM) substitution groups, respectively. The feeding trial lasted 63 days, with measurements collected on days 30 and 63. Growth performance, nutrient digestibility, serum biochemical indices, and rectal fecal microbiota were determined. The results showed that the final body weight, total weight gain, and average daily gain L-FRSM were higher in the L-FRSM group than in the control group and other substitution groups (p < 0.05), accompanied by a reduced feed conversion ratio (p < 0.05). In addition, body height and chest circumference were improved in the L-FRSM group. Regarding nutrient digestibility, the apparent digestibility of neutral detergent fiber and dry matter at day 30, as well as calcium digestibility at day 63 were higher in the L-FRSM group compared to the control and higher-substitution groups (p < 0.05). In contrast, crude fat and dry matter digestibility were significantly lower in the H-FRSM group (p < 0.05). No statistical differences were observed among treatments in serum biochemical indices related to energy metabolism, protein metabolism, liver function, lipid metabolism, antioxidant capacity, or humoral immunity (p > 0.05). Similarly, no significant differences were detected in core microbial composition or α-diversity of rectal fecal microbiota among groups (p > 0.05). In conclusion, substituting soybean meal with 2.8% fermented rapeseed meal effectively improves growth performance and nutrient utilization without compromising health status or intestinal microbial stability in sika deer during the pre-antler growth period. The findings provide a scientific basis for optimizing dietary strategies and support the rational application of fermented rapeseed meal in sika deer production.