The effect of nitrogen (N) fertilization on the dry-matter (DM) yield and nutritional value of sorghum (Sorghum sp., cv. Jumbo) and black oat (Avena strigosa cv., IPR 61) was investigated in the context of forage and livestock production in southern Brazil. Sorghum was cultivated with 0, 37·5, 75, 150, 225, 300 and 375 kg N ha−1 during the summer crop seasons of 2010/11 and 2011/12. Black oat received 0, 40, 80, 120, 160, 200 and 240 kg N ha−1 in the winter of 2011. According to the adjusted polynomial regression, sorghum DM yield increased in response to N up to 288 (12·9 t ha−1) and 264 kg ha−1 (5·6 t ha−1) in 2010/11 and 2011/12 respectively. Crude protein (CP) content of sorghum was highest at 349 and 328 kg N ha−1, but in vitro dry-matter digestibility (IVDMD) was highest at 212–207 kg N ha−1 in 2010/11 and 2011/12 respectively. Sorghum neutral detergent fibre (NDF) and acid detergent fibre (ADF) were not affected by N fertilization. In black oat, the maximum DM yield (6·0 t ha−1) was obtained with 187 kg N ha−1; the IVDMD, NDF and ADF were not affected by N fertilization, but the CP content increased up to 220 kg N ha−1. It is concluded that these forage species can improve the year-to-year amount and quality of forage produced but high rates of N fertilizer are required to achieve high yields. Fertilizer N rates of 210–280 kg N ha−1 in sorghum and 180 kg N ha−1 in black oat in the crop rotation provide the greatest responses in DM yield consistent with good nutritional quality for livestock production.