Effects of Energy Intake on Nutrient Digestibility, Nitrogen Metabolism, Energy Utilization, Serum Biochemical Indices, and Rumen Microbiota in Lanzhou Fat-Tailed Sheep

This study sought to investigate the impact of different levels of dietary maintenance energy metabolism on nutrient digestibility, rumen microbiota composition, and serum biochemical parameters in Lanzhou fat-tailed sheep rams. A total of twenty rams, each aged eight months and with an initial mean body weight of 27.81 ± 3.38 kg, were selected and randomly assigned to one of four experimental groups. These groups were administered with different levels of metabolizable energy (MEM): low energy (LE), intermediate energy (IE), high energy (HE), and extra high energy (EHE), corresponding to 6.77, 7.22, 7.72, and 8.20 MJ/d, respectively. The results showed a linear increase (p < 0.001) in average daily gain (ADG), dry matter (DM) intake, apparent DM digestibility, and crude protein (CP) digestibility. Conversely, the intake of nitrogen (NI), fecal nitrogen (FN), and manure nitrogen (MN) exhibited a significant linear decrease (p < 0.001). The N utilization efficiency rations of FN/NI and MN/NI linearly decreased (p < 0.001), while RN/NI linearly increased (p < 0.001). Additionally, the intake of gross energy (GE), methane energy (CH4-E), digestible energy (DE), and metabolizable energy (ME) exhibited a linear increase, whereas the ration of FE/GE intake linearly decreased (p < 0.001). The efficiency of energy utilization expressed as a proportion of GE intake (DE/GE intake, ME/GE intake, ME/DE intake, and CH4-E/GE intake) showed linear alterations (p < 0.05) with the increase in the dietary energy supplementation level. The dietary energy level did not exert a significant impact on serum biochemical indices (p > 0.05). At the phylum level, the average abundances of Verrucomicrobiota were significantly reduced in the EHE group compared to the IE group, while the average abundances of Desulfobacterota were significantly lower in the EHE group relative to the LE group. At the genus level, the average abundances of Succiniclasticum were significantly higher in the HE and EHE groups compared to the LE group. In conclusion, the energy level (8.20 MJ/d) significantly enhanced nutrient digestibility, energy, and nitrogen metabolism, and it significantly increased the relative abundances of Succiniclasticum.