This work studies the effects of forestation on forage mass and its chemical composition, as well as mean air temperature and their impact on beef cattle performance and grazing behaviour when compared to a natural grassland system. The systems comprised 100% natural grassland from the Campos biome (NG) and forested land (FL) 60% 6 y.o. Pinus taeda (500 trees per hectare) and 40% of natural grassland. We found that the crude protein composition of the pasture growing under the trees was higher, while mean air temperature was lower during the hot season. This changed the grazing pattern of the cattle, and was associated with higher average daily gain of the animals grazing in FL. We conclude that the introduction of trees in natural grasslands changed the environmental conditions, providing a better thermic and nutritive situation for growing grazing cattle that ultimately results in an increase in their productivity.