Abstract Botanical composition of cattle seasonal diets (faecal sampling with microhistological analysis) was determined in a four-pasture, one-herd grazing system during a 3-year period. Diets were classified using cluster analysis. Three diet groups were identified: beginning of rainy season (spring), peak of rainy season (summer) and dry season (fall and winter). Contribution of grasses and woody species to these diet groups was significantly different ( p< 0·001). Grasses were the most important component of cattle diets throughout the rainy season, while shrubs and trees were important in the dry season. The major shifts in diets occurred in response to changes in grass phenology.