Forage-based livestock systems are complex, and interactions among animals, plants and the environment exist at several levels of complexity, which can be evaluated using computer modelling. Despite the importance of grasslands for livestock production in Brazil, tools to assist producers to make decisions in forage–livestock systems are scarce. The objective of this research was to use the CROPGRO-Perennial Forage model to simulate the irrigated and rainfed growth of Marandu palisade grass (Brachiaria brizantha (A. Rich.) Stapf. cv. Marandu), the most widely grown forage in Brazil, by using parameters previously calibrated for the tall-growing cv. Xaraes of the same species, under non-limiting water conditions. The model was calibrated for the irrigated experiment and then tested against independent data of the rainfed experiment. Data used to calibrate the model included forage production, plant-part composition, leaf photosynthesis, leaf area index, specific leaf area, light interception and plant nitrogen (N) concentration from a field experiment conducted during 2011–13 in Piracicaba, SP, Brazil. Agronomic and morpho-physiological differences between the two grasses, such as maximum leaf photosynthesis, N concentration and temperature effect on growth rate, were considered in the calibration. Under rainfed conditions, the simulations using the Penman–Monteith FAO 56 method gave a more realistic water stress response than the Priestley and Taylor method. After model parameterisation, the mean simulated herbage yield was 4582 and 5249 kg ha^–1 for 28 days and 42 days irrigated, and 4158 and 4735 kg ha^–1 for 28 days and 42 days rainfed, respectively. The root-mean-square error ranged from 464 to 526 kg ha^–1 and the D-statistic from 0.907 to 0.962. The simulated/observed ratios ranged from 0.977 to 1.001. These results suggest that the CROPGRO-Perennial Forage model can be used to simulate growth of Marandu palisade grass adequately under irrigated and rainfed conditions.