Nitrogen and energy utilization and methane emissions of sheep grazing on annual pasture vs. native pasture

The purpose of this study was to evaluate the differences in annual pasture and native pasture on dry matter (DM) intake, nutrient digestibility, nitrogen (N) and energy utilization, and methane (CH4) emission of grazing sheep, and to provide the basis for rational livestock grazing in salinized regions. The study used 10 male Hu sheep ♀ × thin-tailed Han sheep ♂ rams (20 ± 5 kg) aged 5 mo. Sheep grazing was conducted in annual pasture and native pasture using a 2 × 2 Latin square design. After a 15-d adaptation period for grazing, the digestion and metabolism experiment of sheep were conducted, while CH4 emissions were measured using sulfur hexafluoride tracer gas. DM intake did not differ between annual pasture and native pasture (P = 0.386). Meanwhile, the digestibility of DM (P < 0.001), neutral detergent fiber (P < 0.001), acid detergent fiber (P < 0.01), crude protein (P < 0.001), and ether extract (P < 0.001) of sheep grazing on native pasture was significantly higher than that of annual pasture. Sheep grazing on native pasture had increased N intake (P < 0.001) and N retained (P < 0.001) compared with those grazing on annual pasture. Digestion energy (P < 0.05) and metabolic energy (P < 0.01) of sheep grazing on annual pasture were significantly improved compared with those on native pasture, while fecal energy (P < 0.001), urine energy (P < 0.001) and CH4 energy (CH4-E) output (P < 0.001) and CH4 emission (P < 0.001) of sheep grazing on annual pasture were significantly decreased. The CH4-E/gross energy (GE) values of sheep grazing on annual pasture and native pasture were 0.09 and 0.10, respectively. In conclusion, grazing sheep have higher N utilization on native pasture, whereas grazing sheep have higher energy utilization and low CH4 emissions in annual pasture. In conclusion, annual pasture has a lower CH4-E/GE compared to native pasture, which helps in reducing environmental pollution.