The aim of this study was to evaluate the effect of peppermint essential oil (PE) and non-fiber carbohydrates (NFC) including sucrose (SUC) and starch (STA) on gas production parameters of alfalfa hay (AH). Treatments were AH, AH plus PE (40 and 80 μl/g DM), AH supplemented with SUC or STA at 60 and 90 mg/g DM plus PE (0.0, 40 and 80 μl/g DM). Approximately 0.3 g of each sample (n = 4) was placed into a 100 mL glass syringe containing 40 mL of buffered rumen fluid (buffer to rumen fluid was 2:1). Rumen fluid was obtained from 2 rumen cannulated sheep (body weight = 45.5 ± 2 kg) before the morning feeding and strained through 4 layers of cheesecloth. Animals were fed 1.5 kg DM alfalfa hay and 0.4 kg DM concentrate (165 g CP/ kg DM) per head/ day. Syringes were incubated at 39°C and the volume of gas produced were recorded at 2, 4, 8, 12, 24, 36, 48, 72 and 96 h. Data were fitted to an exponential equation of P = b (1-e-ct), where b is the volume of gas produced, c is the fractional rate constant of gas production (/h), t is the incubation time (h) and P is the volume of gas produced at time t. The gas production parameters of the supplemented samples were compared with AH as control using Dunnett’s test at P < 0.05. The parameters for AH were b = 72 mL and c = 0.07/h. Supplementation of AH with PE reduced the volume of gas produced, however, peppermint essential oil caused to reduce (P < 0.05) the volume of gas produced from the samples of AH supplemented with the NFC sources. The rate constant of gas produced (c) from AH was significantly (P < 0.05) increased by the adding of the NFC and PE (except PE as 80 μL/g DM, which had no significant effect on c). The fractional rate constant was significantly increased when PE as 40 μL/g DM was added to the AH supplemented with the SUC and STA (0.13 and 0.10, respectively). However, at 80 μL/g DM of PE, c was significantly decreased (P < 0.05) for those treatments (0.06 and 0.05, respectively). It was concluded that PE at th
