Broom sorghum (BS) represents a suitable alternative in sheep nutrition practices in Northwest Mexico. Moreover, concerns on rumen methane production support the search for feeds to minimize environmental impacts. This in vitro assay evaluated rumen methanogenesis and degradability characteristics of mixed sheep diets with increasing levels (0, 20, 40, 60 and 80%) of broom sorghum substituting sorghum grain. Diets included soybean meal, Sudan and molasses as well. A semi-automated system for gas production (GP) using a pressure transducer was used. Ground samples (500 mg DM) were weighed into filter bags and further placed into 160-mL glass bottles. A mixture of incubation medium- rumen inoculum was added to the bottles and sealed. Nine Santa Inês cannulated sheep fed tropical grass were used as donors. Head space gas pressure was measured at 4, 8, 12 and 24 h. The CH₄ concentration was determined using gas chromatography. The truly degraded organic matter (TDOM) was estimated after 24 h incubation by refluxing the filter bags with NDF solution. The partitioning factor was calculated as the ratio of TDOM (mg) and gas volume (mL). Data were analyzed using ANOVA (GLM, SAS). Values for CP, NDF and ADF of the diets varied (P < 0.05) from 11 to 13%, 54 to 66% and 16 to 39%, respectively. Gas production (GP) varied among treatments (P < 0.05). Increasing levels of BS promoted a reduction in GP from 171 mL/g DM in the 0% (control) to 125 mL/g DM in the 80% treatment. Methane production calculated per unit of TDOM varied from 9.4 to 7.5 mL/g TDOM, although no differences were detected (P > 0.05). Nonetheless, methane production calculated per unit of degraded NDF resulted in significant variations (P < 0.05). Treatment containing no broom sorghum (0%) resulted in the highest methane production (4.7 mL/g DNDF), whereas 60% treatment showed the lowest (1.9 mL/g DNDF). No differences were detected in TDOM among treatments (mean = 351 g/kg DDM). Similarly, no effect was found in the partitioning factor (mean = 1.24). Results suggested that increasing levels of BS did not affect OM digestibility, although that might promote a reduction in methane concentration. However, further studies are to be performed to elucidate stronger effects.