This study was conducted with the objective of quantifying the effect of zero grazing on the biogas and slurry production in Fogera woreda through grouping cattle in to two main categories, i.e., zero grazers and free grazers having four cattle in each groups and collecting data accordingly. Both biogas and slurry production were measured by taking two biogas plants that fed with manures of zero grazing and free grazing cattle separately. The parameters on which data were collected are manure production, manure fed to biogas plants/influent, slurry and biogas production. The interpretation and discussion were carried out based on descriptive statistics through comparing and contrasting the data/ the parameters between zero grazing and free grazing. The result shows that zero grazing has significant effect on the production of manure, slurry and biogas. There was significant (P<0.05) difference between manure and slurry from zero grazing cattle and that of free grazing ones, i.e., the manure collected from zero grazing cattle was about 38% larger in amount than that of free grazers. This difference results the same amount of slurry and 36.4% of biogas productions. More over, it was investigated that the dry matter content of manure and slurry of zero grazing cattle is significantly greater than that of the same amount of manure and slurry from free grazers. On the other hand, there was no significant (P>0.05) difference between the amount of liquid manure fed to the two biogas plants which resulted equivalent amount of biogas production per a unit of manure from the two groups. Therefore, it can be concluded that, in addition to the direct impact through over grazing and over stocking, free grazing management system of cattle has significant indirect impact on the environment by affecting the production of slurry and biogas which may be the silent impact of cattle management systems on the environment. Key words: Biogas, slurry/bio-slurry, feedstock/influent, zero/free grazing