Effects of nitrogen supplementation with and without starch supplementation on nutritional performance and metabolic characteristics of cattle fed low and medium-quality tropical forages were evaluated using ruminal and abomasal cannulated steers. Four European × Zebu bulls (381 kg BW) were distributed according to a 4 × 4 Latin square. Each experimental period of 28 d was divided into 2 sub-periods: in the first sub-period 2 animals received low-quality hay and 2 animals received medium-quality hay (Brachiaria decumbens); the following supplementation schemes were evaluated in the second sub-period: low-quality hay with nitrogen supplementation (300 g CP/d); low-quality hay with nitrogen (300 g CP/d) and starch (225 g/d) supplementation; medium-quality hay with nitrogen supplementation (300 g CP/d); and medium-quality hay with nitrogen (300 g CP/d) and starch (225 g/d) supplementation. In the absence of supplementation, the medium-quality forage provided higher (P < 0.05) intake, digestibility, nitrogen balance (NB), and efficiency of nitrogen utilization (EFNU). The greater performance of animals fed medium-quality forage was attributed to anabolic stimuli, supported by higher (P < 0.05) serum concentration of IGF1. Comparing sub-periods, the supply of supplements depressed (P < 0.05) medium-quality forage intake, but did not affect (P > 0.05) low-quality forage intake. There was no effect (P > 0.05) of supplementation on NDF digestibility as compared with the sub-period without supplementation. However, comparison between supplements indicated depression (P < 0.05) in NDF digestibility when starch was included. Supplementation increased (P < 0.05) NB, EFNU, and serum concentration of IGF1 in animals fed low-quality forage. Nitrogen supplementation increases nitrogen retention in animals, an effect attributed mainly to anabolic stimuli. However, this effect is more prominent when animals are fed low-quality forages. No positive impact on animal metabolism was obtained with the combination of supplemental nitrogen and starch.