The effect of semi-arid native fennel (Focniculum vulgarc) essential oil (FE) on the fennentation potential of cottonseed hulls (CT I) and wheat straw (XVS) was evaluated. Samples were ground through a I -mm screen, and then dried (66 C for 48 h). Both CH and WS were subjected to an in vitro gas production technique as un-supplemented or supplemented with FE as 40 or 80 pug DM; named CHFE4O, WSFE4O, CHFE8O and WSFE8O. respectively. Approximately 0.2 g of each feed sample (n=4) was placed in a loo ml glass syringe containing 40 ml of buffcre:rumen fluid (2: 1). Rumen fluid was obtained from two ruinen canullated sheep (body weight 45.5±2 kg) before the morning feeding and immediately strained through tiur layers of cheesecloth. Animals were fed 1 .5 kg DM alfalfa hay and 0.4 kg DM concentrate (1 65 g CP/ kg DM) per head per day. Syringes were incubated at 39 C and the volume of gas produced was recorded at 2, 4, 8, 12, 24, 36, 48, 72 and 96 h. The gas production data were fitted to an exponential equation of P= b(l_et), where 5= volume of gas produced. c= fractional rate constant (1h), 1 incubation time (h) and P— volume of gas produced at time t. The parameters of the supplemented samples were compared with the non-supplemented samples as controls using Dunnett,s test at P<0.05. Supplemented samples of CH and WS had a significant (P<0.05) less h fraction compared with the non-supplemented samples (CI 1=73, CHFE4O9. CHFF8O=1 1. WS=86, WSFE4O=7 and WSFE8OlO ml0.2 g DM). The rate constant of gas produced (c) from CH and WS (0.02 and 0.03, respectively) was significantly (P<0.05) increased by the adding of FL at the both applied rates (0.05 and 0.05, 0.05 and 0.04, respectively). The present results indicate that FE has a potential to alter the fcmcntation characteristics of the both feed samples evaluated.

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