The aim of the present study was to evaluate the agronomic traits in the pre- and post-grazing and the nitrogen use efficiency of Piatã grass that was subjected to nitrogen fertilization throughout the seasons of the year. The experiment was performed in a randomized block design with four treatments (0, 150, 300, and 450 kg ha−1 N). Grazing was performed with sheep when the forage canopy reached 95% light interception up to 20 cm height of post-grazing. The pre-grazing forage mass of the Piatã grass showed a behavior (p < 0.05) linear to nitrogen fertilization, with higher and lower values in spring and winter, respectively. The post-grazing forage mass responded linearly (p < 0.05) to nitrogen fertilization, with an increase of 43.12% for the dose of 450 kg ha−1 N in relation to pastures without nitrogen fertilization. When the non-fertilized pasture was compared with the highest nitrogen dose, a reduction of 21.79% dead tissue was observed in the post-grazing forage mass. The highest nitrogen use efficiency was obtained with the dose of 450 kg ha−1 N. The forage mass production of the Piatã grass in the pre- and post-grazing conditions are positively influenced by nitrogen fertilization and the seasons of the year, with lower values in winter.