In the aim to evaluate weed competition on Panicum maximum Jack grass composition, yield and structure. Massai defining the period prior to interference (PAI) and carry out the phytosociological study in pasture renewal area. A randomized block design in 3x4 m plots was used with treatments consisting of six weed coexistence periods: 15, 30, 45, 60, 75, and 90 days after emergence (DAE) and control (not living together). Forage was evaluated for fractionation (leaf, stem and dead material), natural matter (MN), dry matter (MS), organic matter (MO), crude protein (PB) and neutral detergent fiber corrected for ash and protein (FDNcp) in each cohabitation period and compared with the control treatment. At 90 days in which all treatments were evaluated, the volumetric density of dry matter (DVMS), ether extract (EE), acid detergent fiber (FDA) and lignin (LIG) were also evaluated. Data were evaluated in mixed model and split plot, with means compared by orthogonal contrast and regression adjustment. The PAI was calculated by regression assuming 5% loss of DVMS. Under the initial density of 495 m² weeds, the species Urena lobata had greater relative importance. Living with weeds decreased leaf production and increased stem production at 75 DAE, changing the leaf-stem ratio at 90 DAE which changed composition and decreased forage yields. The loss of DVMS was linear with the increase of days of living with weeds and PAI was set at 10 DAE of Massai grass.