Pastures of Cynodon spp. and elephantgrass ( Pennisetum purpureum Schum.) have their productivity and forage quality directly affected by weed interference. The difficulty of controlling weeds is one of the main obstacles to growing and using these forage crops as an input in animal feeding, as well as in the production of elephantgrass biomass for energy use. Despite the progress in agroecological weed control techniques, herbicides are still indispensable, especially in crops on large tracts of land. Thus, practices for the rational use of herbicides must be improved towards quality food production with lesser impacts on the environment and human health. The overall objective of this project is to identify periods of interference of weeds, herbicides and doses selective for elephantgrass and Cynodon pastures, allowing food production free of herbicide residues or with those substances within the regulatory limits. It is expected to identify the different periods of interference between weed and Cynodon and elephantgrass crops, in order to determine the phase of the crop cycle of these forage species in which control practices should be effectively adopted; to detect differential tolerance among Cynodon species as a function of reserve structures; indicate selective herbicides for the two forage species capable of effective control of weeds and still produce quality forage, within the limits of legally allowed herbicide residues. From the economic point of view, the actions will contribute to reduce forage productivity losses for allowing the use of appropriate technologies to weed control, reducing production costs and, consequently, guaranteeing higher income. In the social aspect, there will be greater food safety and greater development of the rural sector. In the environmental aspect, there will be a lower risk of contamination of soil, water and forage crops, due to the rational use of herbicides and doses necessary to control weeds in areas with cultivation of elephantgrass and Cynodon.