The aim of the present study was to evaluate the effect of fennel essential oil (FE) on gas production parameters of alfalfa hay (AH) and AH supplemented with sucrose (SUC) or starch (STA). Treatments were AH, AH plus FE (40 and 80 μL/g DM), AH supplemented with SUC or STA at 60 and 90 mg/g DM plus FE (0.0, 40 and 80 μL/g DM). Approximately 0.3 g of each sample were placed in a 100 mL glass syringe containing 40 mL of buffered rumen fluid as 2: 1 (n = 4). Rumen fluid was obtained from 2 rumen canullated sheep (body weight = 45.5 ± 2 kg) before the morning feeding and immediately 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 per 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. Statistical analysis was conducted using SAS (1999) procedure. The gas production 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 theincubation 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. Supplementation of AH with FE, at both rates applied, reduced the volume of gas produced (P < 0.05; 72, 52 and 54 mL/0.3 g DM, respectively), but increased the fractional rate constant of gas production (P < 0.05; 0.07, 0.12 and 0.10, respectively). In addition, FE particularly as 40 μL/g DM, reduced (P < 0.05) the volume of gas produced from the AH samples supplemented with both SUC and STA. The rate constant of gas produced (c) from AH supplemented with SUC and STA at both levels (0.09 and 0.08/h, respectively) was increased (P < 0.05) by the adding of FE as 40 and 80 μL/g DM (0.13 and 0.09/h, respectively). It might be concluded that FE, as 40 or 80 μL/g, caused an alteration in the fermentation pote
