This study aimed to evaluate the effect of different dose levels of aguamiel (Agave atrovirens) on in vitro cecal gas, methane (CH4) and carbon dioxide (CO2) productions of five forage species (Avena sativa (hay)), Moringa oleifera, Caesalpinia coriacea, Salix babylonica and Eichhornia crassipes) using inocula from horse. The forage samples were incubated with three doses of aguamiel: 0, 34 and 68 μg of aguamiel/g dry matter (DM) of substrate. Cecal inocula were collected from 4 adult female Criolla horses (3 to 4 years of age and weighing 300 ± 15.0 kg) grazed on native grasses for about eight hours without supplementation. Forage type affected (P < .001) cecal asymptotic, rate and lag time of gas, CH4 and CO2 productions (mL/g DM) and pH and DM degradability. Aguamiel dose had linear and quadratic effects (P < .05) on the asymptotic and rate of CH4 productions and rate and lag time of CO2 productions (mL/g DM). Forage type x aguamiel dose interactions were significant (P < .05) for asymptotic, rate and lag time of gas, CH4 and CO2 productions (mL/g DM). Forage species effects were pronounced (P < .05) on CH4 and CO2 productions (mL/g incubated and degraded DM) and proportional CH4 production at all hours of incubation, except for CO2 production (mL/g incubated DM). Aguamiel dose affected (P < .05) CO2 production (mL/g incubated DM) and proportional CO2 production at the incubated hours. Forage type x aguamiel dose interactions were observed (P < .05) for CO2 production (mL/g incubated DM) and proportional CO2 production at the incubated hours but had no impact of CH4 production. It is concluded that addition of aguamiel to five forage species affected fermentation kinetics of GP resulting in different in vitro cecal gas, methane and carbon dioxide productions from these substrates.