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A rumen simulation technique (RUSITEC) apparatus with eight 940ml fermentation vessels was used to study the effects of the steroidal saponins in Yucca schidigera extract (YE) on ruminal microbial activity and saponin degradation. The YE contained approximately 4.4% (w/w) saponin, as smilagenin equivalents, and was included at 0 (control) or 0.5mgml⁻¹ (n=4) in the McDougall's buffer infused continuously into the vessels (dilution rate=0.75 day⁻¹). Each vessel received 5g chopped alfalfa hay and 5g concentrate (as-fed basis) daily for 22days. Ammonia concentrations were lower (P<0.05) in effluent from vessels receiving YE than from controls for the first half of the study, but did not differ thereafter. Total amounts of VFA in effluent were not affected (P>0.05) by YE, but molar proportions of iso-butyric and iso-valeric acids were lower (P<0.05) in the YE vessels than in the controls in the first half of the …