Due to the increase in greenhouse gases from burning fossil fuels, there is increased attention on renewable energy sources from specialized crops. These crops should not compete with food security, and it is important to select plant resources which can produce methane-rich biogas efficiently. The most commonly used energy crops are planted and managed intensively with high inputs in productive land, and this negatively affects land use and sustainable use of resources. The main purposes of this study are to: (a) determine the best cropping system for optimal biogas and methane production from sole crops of winter pea, triticale and out and their mixtures at two different maturity stages (first stage: full-flowering stage of winter pea and beginning of milky stage of cereals; second stage: emergence of firsts pods for pea and milky/waxy stage of cereals); and (b) to develop and use a surface model to determine the best combinations of various mixtures that result in highest biogas and methane. The used pure or mixtures of pea, oat and triticale in two seed weight ratios (50\%:50\% and 75\%:25\%) produced different green mass, dry matter, solids, biogas and methane yields. The experiments showed that maximum green mass was produced by the mixture of pea and oat at the seed ratio 75\%:25\% and when crop was harvested at the full-flowering stage of winter pea and beginning of the milky stage of cereals. After quadratic model analyses, the combination ratios of the oat and triticale were, respectively, 30\% and 8\%, with a maximum green biomass yield of 61.48 t ha(-1), while the corresponding values were 28\% and 38\%, with maximum solids yields of 25.64 t ha(-1). As the model was set at 100 for all three independent variables (oat, triticale and pea), the pea should be at 62\% (100-30-8) and 34\% (100-28-38), respectively, for green mass and organic solids yields. The results of surface analysis and multivariate analysis of variance showed that the mixture of oat and triticale had great potentiality for biogas and methane yields. The optimal mixture of oat with triticale was 27 similar to 35\% with 73 similar to 65\% for producing biogas and (or) methane.