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This study hypothesized that urea fertilization could optimize animal performance in an agroforestry system. This study evaluated how increasing rates of N fertilization (0, 100, 200, and 400 kg N ha−1 year−1) using urea (CH4N2O) affected forage mass, nutritive value, and beef cattle (Bos indicus) performance in an agroforestry system with Massai grass (Megathyrsus maximus Jacq. cv. Massai) and legume tree (Mimosa caesalpiniifolia Benth.), in the humid tropic region of Brazil, over 2‐year period. The experiment was carried out in a randomized complete block design with four treatments (N fertilizer rates) and three replications. Forage mass was positive and linearly affected by N fertilization (p =.0236, R2 = 0.92), ranging from 1297 to 1583 kg DM ha−1 under 0 and 400 kg N ha−1 year−1, respectively. There was greater average forage mass during the rainy period (1826 kg ha−1) (p <.05). Crude protein in green forage mass increased linearly (p =.0041, R2 = 0.91) in function of the N applied. Cattle average daily gain showed a positive quadratic response to the rates of N applied (p =.0298, R2 = 0.97), ranging from 0.46 kg to 0.68 kg animal day−1; and the gain per area increased linearly from 1.84 to 2.68 kg LW ha−1 day−1 (p =.0640, R2 = 0.96). The findings indicate that applying 200 kg N ha−1 year−1, split‐applied during the rainy season, would yield acceptable outcomes in terms of forage mass, nutritive value, and animal performance for this type of agroforestry system. [ABSTRACT FROM AUTHOR]