Competition between crops and weeds interfere on growth and development with harm to one or both, in different ways. To quantify these differences in competition, additive and replacement series study methods are most often used, each with different characteristics and goals. Therefore, we aimed with this work to study inter-specific competition between plants of soybean and corn against the weeds Bidens pilosa (beggartick) and Digitaria insularis (sourgrass), by two experimental methods of study: additive and replacement series (substitutive) experiments. Initial tests were performed prior to the installation of replacement series experiments to describe the intra-specific competition features for all species. For additive experiments these are unnecessary. To compare the methods of study, there were two sets of trials (an additive and a substitutive) installed under greenhouse, during the growing season 2013/14. Variables analyzed were leaf area and dry weight of shoots of crops and weeds, at 50 days after emergence. The results were analyzed according to the methods used, these being compared. It was found that the additive method supplied results most prone to be applied in the field, while the replacement series (substitutive) method was more appropriate for studies that aimed to provide detailed scientific information regarding the proportions of plants and coefficients describing the behavior of the species community.