Yak rumen fluid inoculum increases biogas production from sheep manure substrate

•Inclusion of rumen fluid increased biogas production in single stage anaerobic digestion.•Inclusion of rumen fluid as an inoculum in a hydrolytic reactor increases the production of VFAs.•The resulting high concentration of VFAs increased biogas production in two stage process.•As a result of the addition of rumen fluid as an inoculum hydrolytic bacteria became prominent.