| Summary: | This study investigated the effects of mannan oligosaccharides (MOS) on <i>in vitro</i> and <i>in vivo</i> gas emission, utilization of crude protein (CP) and energy, and relative parameters of sheep under fasting metabolism conditions. <i>In vitro</i> gas productions were evaluated over 12 h in sheep diets containing different amounts of MOS (from 0% to 6.0%/kg, the increment was 0.5%). A control experiment was used to assess the gas emission, utilization of CP and energy, and fasting metabolism in control sheep and sheep treated with 2.0% MOS over 24 days (d). The results showed that 2.0% MOS supplementation led to the lowest <i>in vitro</i> CO<sub>2</sub> production and less CH<sub>4</sub> production, while also leading to decrease <i>in vivo</i> nutrients intake, CP and energy excretion, digested and retained CP, and energy released as CH<sub>4</sub> (<i>p</i> < 0.05). Furthermore, 2.0% MOS supplementation appeared to decrease <i>in vivo</i> O<sub>2</sub> consumption and CH<sub>4</sub> production per metabolic body weight (BW<sup>0.75</sup>), and increase the CP retention rate of sheep (<i>p</i> < 0.074). MOS did not affect other parameters, along with the same parameters of sheep under fasting metabolism conditions (<i>p</i> > 0.05). The findings indicate MOS has only slight effects on the gas emission and nutrients and energy metabolism of sheep.
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