| Sumario: | The study was carried out in Parnaíba, Brazil, aiming to evaluate the effects of three levels of nitrogen doses (150; 300 and 450 kgN.ha-1) and five plant age at cutting (28; 35; 42; 56 and 84 days) on height, dry matter, and biomass yield of elephant grass, cv. Napier. The experimental design was in complete randomized blocks, with 4 replications. Plant height was significantly influenced by plant age and nitrogen dose (450 kgN.ha-1). Plant dry matter, however, was influenced only by plant age. Plant dry matter yield significantly increased (P < 0.05) in line with plant age at cutting, with age at cutting of 84 days yielding the best results (18.93 t.ha-1), followed by 56 days (9.48 t.ha-1), and 42 days (5.02 t.ha-1). Plant biomass obtained by cutting at 35 days (3.96 t.ha-1) did not differ from cutting at 28 days (3.28 t.ha-1). Application of 450 kgN.ha-1 resulted in greater biomass yield (8.82 t.ha-1), and did not differ (P< 0.05) from 300 kgN.ha-1 (7.84 t.ha-1), which in turn did not differ (P < 0.05) from 150 kgN.ha-1 (7.75 t.ha-1). Nitrogen application efficiency (51.66; 26.13 and 19.6 kgDM.kg-1.N), regardless of plant age at cutting, was inversely proportional to nitrogen doses (150; 300 and 450 kgN.ha-1).
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