Abstract The degradation characteristics of five varieties of rice straw [in vitro organic matter digestibility (IVOMD) ranging from 35 to 55%], in both the untreated and urea-treated form, were studied using the nylon bag procedure. Bag incubations up to 10 days were performed in the rumen of cows fed on a ration of 50% wheat straw and 50% hay. The degradation characteristics for organic matter (OM) and cell wall components were evaluated using a two fraction model with a fixed undegradable fraction and lag time. The variety of rice straw and urea treatment significantly ( P < 0.001) affected the potentially degradable fraction ( D), undegradable fraction ( U) and the rate constant of degradation ( kd) of all components studied. Their effects on degradation time-lag were not significant. Between varieties, the undegradable OM fraction ranged from 27 to 50%, and the rate of degradation from 2.15 to 2.91% h −1. Urea treatment not only increased the potentially degradable fraction, but also its rate of degradation. Nevertheless, even after treatment the differences between varieties were maintained. With the low quality straw (IVOMD 35%), treatment released more OM and fibre for degradation, but its use even as a maintenance ration is doubtful. The maximum benefit of urea treatment was obtained with the medium quality straw (IVOMD 48%), where the D fraction and its rate of degradation were similar to a good quality straw after treatment. With good quality straws (IVOMD 55%), the benefits achieved with urea treatment may be similar to that obtainable with nitrogen supplementation. Around 50–60% of the N in straws was released after 10 days of incubation in the rumen. Although up to 13% of the N added by urea treatment was insoluble in water, the amounts of N remaining in the rumen-incubated residues were similar for untreated and treated straws.