Ryegrass ( Lolium multiflorum Lam.) is an important forage and suffers negative interference from weeds, like annual bluegrass ( Poa annua L.). The competition of annual bluegrass with ryegrass can interfere with crop growth and reduce the amount of forage for animal feed. We aimed to evaluate the interference of annual bluegrass in ryegrass crops through the differentiation of niche and establishment. Two experiments were carried out testing different competition strata (shoot, root, and both) and ryegrass establishment with or after annual bluegrass, with increasing populations of annual bluegrass (0, 35, 139, 279, 419 plants m ^–2). The variables were plant height, height length of the root system, stem diameter, and fresh weight of shoot and root systems of ryegrass. The negative interference of growing populations of annual bluegrass occurred when in competition for the shoot and root systems of ryegrass. The fresh weight of shoots and roots were impacted when competing for the same strata. Ryegrass, established after the population of annual bluegrass, showed less competitive capacity. The negative interference of annual bluegrass in ryegrass is greater when the plants fully compete for the resources of the environment, and when the ryegrass establishes after the competitor.