Gas production parameters of microwave irradiated cottonseed hulls (CH) were assessed using gas production procedure of medium containing ruminal bacterial isolation. Cottonseed hulls were hydrated by distilled water as 25 g/kg DM, then, irradiated (microwave, 900 W) for 4, 6 and 8 min (mic4, mic6 and mic8, respectively). Approximately 0.3 g of each sample was placed in a 100 mL glass syringes (n = 4), then incubated into 40 mL of buffered protozoa and fungi free rumen fluid (ratio of buffer to rumen fluid was 2:1) for 2, 4, 6, 8, 12, 24, 48, 72 and 96 h. Rumen fluid was obtained from 3 sheep (body weight, 49.5 ± 2.5 kg) fitted by rumen fistulae, before the morning feeding, and strained through 4 layers of cheesecloth. The animals were fed 1 kg/d DM of alfalfa hay and 0.3 kg/d DM of concentrate (165 g CP/kg DM). Rumen fluid was centrifuged (10 min, 3000 rpm) and a solution of cycloheximide was added to protozoa free supernatant to remove the fungal population. Data of gas produced over the incubations were applied to an exponential equation of P = b(1-e)-ct where (b) is the amount of gas produced from the fermentable fraction, (c) is the fractional constant rate and (t) is time. Data were analyzed using the GLM procedure of SAS 9.1 and the means were compared by the Tukey test (P < 0.05). The amount of gas produced from the fermentable fraction (b ± SEM) was 48.1 ± 3.8, 43.3 ± 3.56 and 42.6 ± 2.5 for mic4, mic6 and mic8, respectively. The fractional constant rate parameters (c ± SEM) were 0.01 ± 0.001, 0.01 ± 0.002 and 0.01 ± 0.001 for mic4, mic6 and mic8, respectively. Data of the present study demonstrate that there is not a positive response for enhancing the CH fermentability by ruminal bacteria when microwave irradiation was applied.

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