a b s t r a c tThis study aimed to evaluate the effect of different dose levels of aguamiel (Agave atrovirens) on in vitro cecal gas, methane (CH 4 ), and carbon dioxide (CO 2 ) productions of five forage species (Avena sativa [hay]), Moringa oleifera, Caesalpinia coriacea, Salix babylonica, and Eichhornia crassipes using inocula from the horse. The forage samples were incubated with three doses of aguamiel: 0, 34, and 68 mg of aguamiel/g dry matter (DM) of substrate. Cecal inocula were collected from four adult female Criolla horses (3-4 years of age and weighing 300 AE 15.0 kg) grazed on native grasses for about 8 hours without supplementation. Forage type affected (P < .001) cecal asymptotic, rate and lag time of gas, CH 4 and CO 2 productions (mL/g DM), pH and DM degradability. Aguamiel dose had linear and quadratic effects (P < .05) on the asymptotic and rate of CH 4 productions and rate and lag time of CO 2 productions (mL/g DM). Forage type  aguamiel dose interactions were significant (P < .05) for asymptotic, rate and lag time of gas, and CH 4 and CO 2 productions (mL/g DM). Forage species effects were pronounced (P < .05) on CH 4 and CO 2 productions (mL/g incubated and degraded DM) and proportional CH 4 production at all hours of incubation, except for CO 2 production (mL/g incubated DM). Aguamiel dose affected (P < .05) CO 2 production (mL/g incubated DM) and proportional CO 2 production at the incubated hours. Forage type  aguamiel dose interactions were observed (P < .05) for CO 2 production (mL/g incubated DM) and proportional CO 2 production at the incubated hours but had no impact on CH 4 production. It is concluded that addition of aguamiel to five forage species affected fermentation kinetics of gas production resulting in different in vitro cecal gas, CH 4 and CO 2 productions from these substrates.