This study was conducted at the College of Agricultural Engineering Sciences/university of Baghdad. Were used wild reeds and barley straw inoculation with cumulative levels of  rumen fluid, Which aimed to calculate in vitro total and methane gas production using five treatments, as follows:-

T1: 1 ml of buffalo rumen fluid + 9 ml of lamb rumen fluid.

T2: 2 ml of buffalo rumen fluid + 8 ml of lamb rumen fluid.

T3: 4 ml of buffalo rumen fluid + 6 ml of lamb rumen fluid.

T4: 6 ml of buffalo rumen fluid + 4 ml of lamb rumen fluid.

T5: 8 ml of buffalo rumen fluid + 2 ml of lamb rumen fluid.

The results showed a highly significant decreases (P≤0.01) in total gas and methane production for the treatments T1, T2, and T3 after 24, 48, and 72 hr. of incubation periods compared with, T4 and T5 for wild reed and barley straw samples. The results also showed a highly significant decreases (P≤0.01) in the production of total gas and methane production after, 48 hr. of incubation periods of barley straw samples compared with wild reeds. The results also showed a highly significant increases (P≤0.01) in the values of the organic matter digestibility, metabolizable energy, and net energy for milk production for treatments T4 and T5 compared to treatments T1, T2, and T3 after 24 hr. of incubation period. Conclude be that addition buffalo rumen fluid didn't decrease of total gas and methane production in in vitro, so this means that buffalo are less efficient than sheep in utilizing feed energy.