Nitrogen fertilization is an efficient practice to improve pasture productivity and quality. However, the efficiency of Nitrogen (N) application depends on favorable climatic conditions. Thus, in the present work, a metanalytic study was used to summarize and analyze data on nitrogen fertilization in forage species: elephant grass (Pennisetum purpureum) and panicum grass [Megathyrsus maximus (Syn. Panicum maximum)], in order to determine the critical levels and recommended doses of nutrients for the rational use of fertilizers in forage plants, whether in pastures or weeds. Therefore, in this study, a systematic review was used, which is a type of investigation, for which research works carried out in Brazil between 2010 and 2020 were considered, including only scientific articles and dissertations. Based on the data from these studies, the relative increments of dry matter and crude protein in relation to the control treatment and the efficiency of nitrogen use in the production of dry matter and crude protein were calculated. All data collected were submitted to analysis of variance for linear and quadratic effects on each of the variables and, in case of significance, regression analysis was performed. Nitrogen fertilization in Pennisetum purpureum and Megathyrsus maximus provides consistent increases in dry matter and crude protein production, however above 1000 kg of nitrogen ha-1 there is a compromise in the efficiency of dry matter production. With a view to reducing losses due to the lower efficiency in crude protein production, it is important to verify that above 500 kg of nitrogen, there is a drastic reduction in the efficiency of crude protein production by Pennisetum purpureum. For all nitrogen doses, there is a drastic reduction in crude protein production efficiency by Megathyrsus maximus in relation to the control.