Soybean (Glycine max (L.) Merrill) highlights in Brazilian agricultural sector, because of its importance as source of protein and vegetal oil for food security alongside corn, compounding the base of ration formulation for animals. Also, due to the development in human nutrition sector, soybean and corn are used in formulation of different foods. (Costa and Brum, 2008). During its growing period, soybean is subjected to abiotic stress such as herbivorous animals as well as invasive plants competition. Understanding the involved tools in competition between species is fundamental in agricultural systems, remarkably association between plants with different characteristics. Plants often compete with each other (intraspecific) and with other plants (interspecific) for the environmental resources, in many cases vith the allelopathic effect between species (Nepomuceno et al., 2017; Abdelmigid and Morsi, 2017). Duration of the competition time negatively influences on the development and on the yield of the culture (Almeida et al., 2014). This competition between plants leads to damages in its physiology, mainly in the decrease in chlorophyll index (Nogueira et al., 2010; Lisboa et al., 2017) and stomata conductance (Jumrani et al., 2017), which initiate a breakdown in the shape and development of soybean. Competition level in grass-culture associations depends on the intervention of factors linked to the weed community, which involves specific composition, distribution, specie, density, phase and duration of intercultural living, as well as depends on climatic conditions and treatments (Figueiredo et al., 2013). The competition released by weed is named grass competition, a problem that may initiates significant 90% loss of the yield. It also can cause problems in grains quality. That way, it is important to know the soybean cycle, so that, it is possible to know in which moment the greatest interference of weeds occurs. Managing weeds is an essential step to guarantee higher performances, especially in chemical control, once this is the more practical and efficient method (Franco et al., 2017). This work aimed to evaluate effects of density of competition between species of Urochloa sp. in soybean cultivation. MATHERIAL AND METHODS Installing the experiment: The experiment was carried out in the Vegetation House of the College of Technology and Agricultural Sciences, in São Paulo State University (Unesp), in Dracena, Brazil, in October, 2016. A completely randomized design was installed, in a 3x5 factorial scheme, with 3 different grasses species: Urochloa brizantha cv. Marandu and Piatã and Urochloa ruziziensis; and 5 different densities of grasses, with 5 repetitions, summing 75 plots or vases. BMX Potência soybean cultivar was used; it was sowed three centimeters deep in vases with 9.0 dm³ capacity and 490.6 cm² area. All vases were filled with sifted soil, originally classified as Argillaceous Ferric yellow red (Embrapa, 2013), with the following chemical attributes: pH CaCl2= 4.5; MO g/dm-3 = 4.5; P mg/dm-3 (re...