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Greenhouse gases mitigation in equine is a new developing area of interest. Methane (CH4) emission is associated with its negative impacts on the environment and its energy loss processes in foregut and hindgut fermenter where it is produced. Hence, this study was designed to examine the influence of two vegetable oils on carbon dioxide (CO2), CH4, and hydrogen (H-2) in vitro. Five total mixed rations (TMR) were formulated and used as incubation substrates. Steam rolled corn was replaced by corn oil or soybean oil at 0% (control), 2.4% (low level), and 4.8% (high level) of TMR with 0 and 4 mg of yeast culture per gram dry matter of TMR. The use of yeast and soybean oil resulted in the increased (P < .05) pH during fermentation. Inclusion of corn oil without yeast resulted in the highest (P < .05) dry matter digestibility, and a high level of corn oil had the highest (P < .05) lag time required for CO2 production, which is one of the contributor greenhouse gases for global warming. However, high soybean oil produced the least fecal CO2 despite its low lag time. Similarly, inclusion of high soybean oil resulted in the highest (P < .05) fecal gas production, whereas high corn oil had the lowest gas production/dry matter degraded. High level of corn oil in equine diet may be used to reduce fecal greenhouse gas emission (44.5, 36.0, and 54.6% for CH4, CO2, and H-2, respectively). Low level of corn oil and high level of soybean oil may be used when digestibility is the primary concern, which resulted in 54.9% and 31.1% increase in fecal dry matter digestibility, respectively. Overall, low level of corn oil seems to be having general effectiveness on fermentation in horses. (C) 2018 Elsevier Inc. All rights reserved.