The aim of the present study was to evaluate the effect of peppermint (Mentha piperita) essential oil (PE, 40 or 80 µl/g DM) on in vitro gas production potential of wheat straw (WS) supplemented with glucose (GLU), fructose (FRU), sucrose (SUC) or starch (STA) as 0.0, 30, 60 and 90 mg/g DM. Supplementation of WS with FRU, GLU (60 mg/g DM), STA and SUC (60 and 90 mg/g DM) caused a significant increase in the volume of gas produced (b). However, PE caused to reduce the b fraction of the samples evaluated (p<0.05). The fractional rate constant of gas production (c) was significantly (p<0.05) increased when FRU, SUC and PE was added to WS. In addition, c was significantly increased when PE as 40 µl/g DM was added to WS samples supplemented with both SUC and STA. However, there was no significant effect of PE when applied as 80 µl/g DM on c parameter of those treatments (except for WS supplemented with SUC at 90 mg/g DM).

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