The agricultural sector is a pillar of the Brazilian economy, but it also serves as one of the main causes of emissions of greenhouse gases (GHG) in the world, contributing 38% of methane (CH4) from enteric fermentation. Faced with this situation, the challenge in the ruminant production system is to develop diets and management strategies that minimize CH4 production in order to reduce the impacts of global warming and increase the productive efficiency of pastures and animals. The objective of the study was to evaluate the forage mass production, chemical composition, intake, digestibility, balance of nitrogen (N) and enteric CH4 emission by crossbred dairy heifers kept in marandu grass pastures fertilized with N or mixed with forage peanut. The experiment was conducted in a completely randomized design with three treatments containing seven replications for variables related to the animals and four replications for variables related to the forage. The treatments consisted of pastures of marandu grass without nitrogen source (control), fertilized with urea (fertilized), and marandu grass mixed with forage peanut (mixed). Data were collected during two experimental periods consisting of the 2016/2017 and the 2017/2018 rainy seasons. The mixed and fertilized pastures did not mitigate enteric CH4 emission in crossbred heifers. Nitrogen fertilization did however increase the forage nutritive value as shown by increased values of crude protein, and B1 + B2 protein fraction, and a reduced NDF and protein fraction C. Additionally, nitrogen fertilization of marandu grass pastures provided a higher forage mass, accumulation rate, and allowances of forage and leaves. The introduction of legumes to the system, represented as the mixed treatment, increased the crude protein and the fraction A of the forage but reduced the B3 fraction. It also increased the supply of leaves to the animals and the forage accumulation rate. There were no significant differences between the treatments for intake, dry matter digestibility, and the efficiency of N use (ENU) by the heifers.