The aim of this study was to determine the energy levels of some energy source grains commonly used in ruminant nutrition by in vivo and in vitro methods. To achieve this objective, corn, barley, wheat, rye, oat, and triticale (4 replicates) were utilized. The in vivo digestibility of grains was determined with a metabolism trial using 4 animals per sample in an incomplete block design. The in vitro digestibility of grains was determined using an enzyme technique. For this purpose, cellulase, hemicellulase, amylase and pepsin were used. The in vivo dry matter and organic matter digestibilities of corn, barley, wheat, rye, oat and triticale were 83.81, 79.86, 81.34, 83.05, 75.50 and 84.00%; and 83.90, 81.50, 82.33, 83.84, 76.95 and 84.81%, respectively. Digestible energy, metabolizable energy, net energy for lactation content of grains, in the same order, were 15.94, 15.49,15.64, 15.93, 14.62 and 16.12 MJ/kg DM; 13.52, 12.76, 12.98, 13.12, 12.39 and 13.23 MJ/kg DM; and 8.41, 7.84, 7.99, 8.13, 7.52 and 8.21 MJ/kg DM (P < 0.05). The in vitro dry matter and organic matter digestibilities of the grains, in the same order, were 81.21, 80.04, 81.04, 81.72, 62.05 and 84.17% 82.27, 81.07, 81.82, 82.66, 63.25 and 84.76%. Digestible energy, metabolizable energy and net energy for lactation calculated by the in vitro method were 15.27, 14.98, 15.22, 15.07, 11.88 and 15.61 MJ/kg DM; 12.90, 12.51, 12.68, 12.68, 9.90 and 13.04 MJ/kg DM; and 8.41, 8.11, 8.23, 8.30, 6.79 and 8.43 MJ/kg DM (P < 0.05). In this study: a) in vivo and in vitro dry matter digestibility and organic matter digestibility were similar in all grains, except for oat; b) the enzyme technique can be used instead of the in vivo method for the calculation of metabolizable energy and net energy for the lactation content of barley, wheat and rye; c) the enzyme technique can replace the in vivo method for the calculation of the net energy for the lactation content for corn, and the metabolizable energy content for triticale; and d) the enzyme technique cannot be used instead of the in vivo method for oat.