Rumen inoculum was collected by stomach tube from 6 young calves (Holstein, with a live weight of 40e55 kg) at 20, 40 and 60 d of age using stomach tube. Calves were fed daily one time at 11:00 h on a total local formulated mixed ration containing (/kg dry matter (DM)): 200 g canola, 625 g sorghum grains,150 g soybean meal, and 25 g mineral salts. The diet contained (/kg): 200 g crude protein, 230 g neutral detergent fiber, 50.3 g acid detergent fiber and 35.6 g either extract. The diet was formulated to cover their nutrient requirements. Calves were received 2 L of milk at 07:00 h and other 2 L at 16:00 h with a free grazing time from 8:00 to 16:00 h during the day on ryegrass and white clover ad libitum with free access to fresh water at all times during rumen contents collection phaseThe present context was aimed to determine the in vitro gas production (GP), mitigation of methane (CH4) and carbon dioxide (CO2) emission, and ruminal fermentation from nine different tree leaves as suitable alternatives for alfalfa hay. Tree leaves and alfalfa hay were incubated with rumen inoculum from calves at different ages (20, 40, and 60 d). The level of asymptomatic GP [mL/g dry matter (DM)] differed (P < 0.05) between different tree leaves compared with alfalfa hay. Most of the plant leaves showed differed (P < 0.05) asymptomatic CH4 production (mL/g DM), fractional rate of GP and lag time. The asymptomatic CO2 production and fractional rate of CO2 production decreased effectively (P < 0.01) with the different tree leaves. In vitro rumen CH4 and CO2 productions (mL/g incubated DM) by species incubated in the rumen liquor of calves collected at 60 d of age increased effectively. Fermentation pH (P < 0.001) was found to be significant parameter but DM degradability (P > 0.05) was not significant by dose and substrate dose interaction. Tree leaves showed lower production of CH4 (mL/g incubated DM) and proportional CH4 emission when compared to alfalfa hay after the required period of incubation at significant level (P < 0.05). However, different tree leaves had no influence (P > 0.05) on CO2 production (mL/g incubated DM and mL/g degraded DM) and proportional CO2 production. The incorporation of tested tree leaves in diet would be a valuable alternative of alfalfa hay with sustainable reduction properties of CH4 and CO2 productions. These potent tree leaves can be used as valuable cleaner product and feeding stuffs for the environment and ruminants respectively due to their in vitro fermentative properties.