The ability of the horse to efficiently utilize fiber and roughages due to the presence of fermentative microorganisms in their hindgut and the use of fibrous feeds as the main component of the mature horse diet have been documented [1,2]. Forages are important primary natural component of horse diet needed for normal function of their digestive system and to suppress certain metabolic disorders like hindgut acidosis, laminitis, and colic occasioned by feeding high-starch diets [3]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 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 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, CH4 and CO2 productions (mL/g DM), 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 aguamiel dose interactions were significant (P < .05) for asymptotic, rate and lag time of gas, and 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 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 on CH4 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, CH4 and CO2 productions from these substrate