This experiment was conducted to evaluate in vitro effects of equine fecal inocula fermentative capacity on 9 fibrous forages in the presence of Saccharomyces cerevisiae. The fibrous feeds were corn stover (Zea mays), oat straw (Avena sativa), sugarcane bagasse and leaves (Saccharum officinarum), Ilanero grass leaves (Andropogon gayanus), Taiwan grass leaves (Pennisetum purpureum), sorghum straw (Sorghum vulgare), and steria grass leaves (Cynodon plectostachyus). Fibrous feed samples were incubated with several doses of S. cerevisiae; 0 (control), 1.25 (low), 2.5 (medium) and 5 (high) mg/g dry matter (DM) of a commercial yeast product containing 1 x 10(10)/g. Fecal inoculum was collected from 4 adult horses were fed on an amount of commercial concentrate and oat hay ad libitum. Gas production (GP) was recorded at 2, 4, 6, 8, 10, 12, 24, and 48 hours post inoculation. An interaction occurred between feeds and yeast dose for fecal pH (P<.01), asymptotic GP (b, ml/g DM); rate of GP (c, /hr); initial delay before GP began (L, hours), GP at 4 hours and 48 hours (P<.01), and GP at 8 hours (P<.01) and at 24 hours (P <.01). Differences in fecal fermentation capacity between the tropical and template grass (P<.05) occurred for fecal pH, c, and GP during first 12 hours, whereas differences occurred (P<.05) between the agriculture byproducts and the grasses for fecal pH, b, and GP from 8 to 48 hours. Fermentation capacity between straws versus not straws (P<.05) differed for fecal pH, b, and GP after 12 hours between straws versus not straws. Addition of S. cerevisiae to Z. mays stover reduced (P <.01) fecal pH and the c fraction with a higher (P <.01) b fraction versus the other feeds. From 4 to 24 hours, S. officinarum bagasse improved GP to the highest values versus S. officinarum leaves. After 24 hours, Z mays stover had the highest GP, whereas C plectostachyus leaves had the lowest. There were no differences among the yeast doses for all measured parameters with the exception of L values (linear effect; P <.01). The Z. mays stover had the highest nutritive compared to the other fibrous feeds. However, addition of S. cerevisiae at 2.5 to 5.0 g/kg DM improved fecal fermentation capacity of low-quality forages. (C) 2014 Elsevier Inc. All rights reserved.