This study was carried out to determine the effects of different doses (10, 20 and 30%) of sumac shrub leaf substitution instead of corn silage in sheep rations on in vitro gas and methane production, metabolic energy (ME), net energy lactation (NEL) and organic matter digestion degree. Sheep ration consisting of corn silage (20%), alfalfa straw (22.5%), dry meadow grass (20%), and commercial feed (37.5%) constituted the control group. The experimental groups were formed by substituting 10 (S1), 20 (S2) and 30 (S3) percent sumac shrub leaves for corn silage in the control (C) group formed the experimental groups. The effect of sumac shrub leaf substitution on in vitro gas and methane production, metabolic energy, net energy lactation, and organic matter digestion degree was found to be significant. The 24-hour in vitro gas production values of rations ranged between 43.11- 46.77 ml/200 mg DM, methane production values 6.8-7.48 ml, metabolic energy values 8.91-9.41 MJ/kg DM, net energy lactation, 5.59-5.95 MJ/kg DM and organic matter digestion degree values found between 64.25 and 67.61%. As a result, it was determined that increasing doses of sumac shrub leaf substitute reduced gas and methane production. In addition, it was concluded that the data obtained should be supported by determining the microorganism counts, feed consumption amounts, and feed efficiency coefficients with in vivo studies.