The shade of the trees projected on the herbaceous stratum that grows under the canopy represents an undesired effect for livestock farmers for forage production in the silvopastoral system. In order to determine the dry matter production and the floristic composition of the herbaceous stratum with different amounts of photosynthetically active radiation, three plots with low tree density (BDA; 50 to 200), moderate tree density (MDA; 201 to 350) and high tree density (ADA; 350 to 500) trees ha-1 were selected. In each plot, photosynthetically active radiation (RFA) that affects the herbaceous stratum was estimated and the dry matter production (DM) of the forage herbaceous plants present during 15 weeks of the rainy season was determined. The intensity of light under the canopy and the production of herbaceous forage biomass in BDA, MDA and ADA were 8 594, 6 437 and 3 801 MJ m2 year-1 and 2 859, 1 988 and 1 679 kg of DM ha-1, respectively. The number of herbaceous species in BDA, MDA and ADA was 38, 48 and 27, respectively, although the dominant species, with more than 80% of the botanical composition, were only six. It is concluded that the tree density modifies the amount of light that falls on the herbaceous stratum, affecting the biomass production and the floristic composition of forage herbaceous plants that grow under the tree canopy in a traditional silvopastoral system in the Low Deciduous Forest.