The percentages of in vitro true digestibility as fed (IVTD as fed %), in vitro true dry matter digestibility (IVTDMD%), in vitro true neutral detergent fiber digestibility in dry matter basis (IVNDFDDM%) and in vitro true organic matter digestibility in dry matter basis (IVOMDDM%) of rice straw untreated and treated with cellulase enzyme + incubated at 40ºC for 30 d