Five cultivars of Urochloa brizantha (Hochst. ex A.Rich.) R.D. Webster (syn. of Brachiaria brizantha) were evaluated for nutritional value and an anti-nutritional factor (protodioscin), in order to determine whether protodioscin content was correlated with reduced feed quality. We evaluated cvv. Arapoty, Paiaguas, Xaraes, Marandu and Piata and grouped the results into summer, autumn, winter and spring seasons. Protodioscin content and chemical composition of leaves, in vitro digestibility and cumulative gas production were analysed. Data were evaluated by analysis of variance as a completely randomised experimental design in a factorial arrangement (five cultivars × four seasons). There was no significant interaction between cultivar and season. All grasses showed highest protodioscin contents during autumn. Protodioscin contents ranged from 5.6 g kg^–1 (spring) to 19.2 g kg^–1 (autumn). Crude protein content varied significantly across seasons. No significant effect was detected for neutral detergent fibre (NDF) content among seasons. Cultivar Arapoty showed the highest NDF content in summer (669.3 g kg^–1) and the lowest in spring (601.9 g kg^–1). The best in vitro digestibility coefficients were observed in spring. The protodioscin content of U. brizantha cultivars can negatively affect their digestibility and some parameters of cumulative in vitro gas production.