Effects of Dietary Protein and Fat Levels on Growth Performance, Nutrient Digestibility, Serum Indexes, and Rectal Fecal Microbiota of Sika Deer (Cervus nippon) Fawns in Early Wintering Period

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 (<i>p</i> < 0.05), along with greater apparent digestibility of dry matter, crude protein, calcium, and fiber fractions (<i>p</i> < 0.05). Serum urea content was significantly lower in this group, indicating improved nitrogen utilization (<i>p</i> < 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 <i>Bacteroidota</i>, 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.