The objective of this study was to investigate in vitro effect of Malva sylvestris leaf extract (at 0, 25, 50 and 100 μL/30 mL of medium) on sheep ruminal cellulolytic and total viable bacteria growth, protozoa popula-tions, methane production, neutral detergent fiber degradability (NDFD) and fermentation efficiency of oat hay. The addition of Malva sylvestris leaf extract at 25, 50 and 100 μL led to a linear increase (P<0.01) in vitro truly degraded dry matter (INTDDM), NDFD and partitioning factor (PF) of oat hay and decrease (P<0.01) methane emission after 24 hours of incubation. The addition of Malva sylvestris leaf extract re-sulted in a decrease (P<0.01) in potential of gas production at 25 and 50 μL and increase in lag time (0.95, 1.01 and 1.13 h relative to 0.61 h), constant rate of gas production (b) and gas produced at half-life (c) at 25, 50 and 100 μL. The addition of this extract decreased a number of total protozoa and Entodinium, Isotri-chae, Diplodinium and Ophryoscolex species (P<0.01). The number of total and cellulolytic bacteria was not influenced by the addition of Malva sylvestris leaf extract. The result of this study demonstrated that Malva sylvestris extract had some potential for improving the rumen fermentation.
