Anaerobic mesophilic batch tests of energy crops (forage barley, rye, Italian ryegrass, and oats) were carried out to evaluate their ultimate biodegradability and two distinctive decay rates (k1 and k2) with their corresponding degradable substrate fractions (S1 and S2). Graphical statistical analysis and biochemical methane potential (BMP) tests showed that the ultimate biodegradability was 82 ~ 88% for forage barley, 70 ~ 77% for rye, 66 ~ 79% for Italian ryegrass, and 59 ~ 67% for oats. The readily biodegradable fraction (S1) of 73% of forage barley biodegradable volatile solid (BVS, S0) degraded within the initial 40 days at k1 of 0.055 day−1, whereas the slowly biodegradable fraction (S2) of BVS degraded over more than 80 days with long term batch reaction rates of 0.002 day−1. For oats, this readily biodegradable portion (S1) was 57%, which degraded with a k1 of 0.023 day−1 for the initial 60 days. The minimum hydraulic retention time (HRT) for the anaerobic digestion of forage barley, rye, and Italian ryegrass were estimated in the range 45 ~ 55 days and 65 days for oats.