It is generally believed that heavy metals react with proteins by combining the thiol (-SH) groups, which leads to the inactivation of the proteins (Cho et al., 2005). Silver nano-particles (Ag-NPs) are being used vastly as a strong anti-germ product. In general, Ag ions, which have antimicrobial activity, are used as an antibacterial agent. The antibacterial activity of Ag ions is inhibits intracellular enzyme activity (Cho et al., 2005). Therefore, the other possibility can be considered that remaining Ag ions in Ag-NPs solution or dissolved Ag ions might affect bacterial growth. Recently, dairy farmers have tended to apply this anti-bacterial for sanitization of the environment (as a disinfectant material) of animals. However, the environmental residual effect of this anti-bacterial on ruminant microbial activity is still questionable. In the present study, a gas production technique was applied in order to determine the effect of silver nano-particles on in vitro gas production of barley grain and lucerne hay.