Abstract: The metabolizable energy contents of wheat straw (WS), barley straw (BS), alfalfa hay (AH), alfalfa silage (AS) and maize silage (MS) were predicted by the in vitro gas production technique using different regression equations obtained from previous experiments. Gas productions were measured at 3, 6, 12, 24, 48, 72 and 96 and gas production constants (c, a, b) were described using the equation y = a+b (1-ect). Metabolizable Energy (ME) contents of forages were determined using different regression equations obtained previous experiments. There were significant (P<0.001) differences between forages in terms of ME content. There were also significant difference (P<0.001) between ME contents of each forage predicted using different equations. The regression equation for prediction of ME had a significant effect on ME of forages. This resulted in different rank orders of forages based on ME. Therefore ME content of forages predicted by gas production method using different regression equations obtained from previous experiments cannot be considered absolute. The care that must be taken when comparing results obtained using different regression equations