| Summary: | Nitrogen (N) management affects herbage production and chemical composition; however, information on the impact of tropical herbage on N and carbohydrate fractions is scarce. A two-year study was conducted to investigate the potential use of pintoi peanut (<i>Arachis pintoi</i>) compared with N fertilization of palisade grass (<i>Brachiaria brizantha</i> cv. Marandu) by evaluating the herbage chemical composition (fractionation of protein and carbohydrate), herbage mass and accumulation rate, herbage disappearance rate, and stocking rate of pastures. The experiment was conducted in a completely randomized design with three treatments, and four replications (paddocks) were used with twenty-one non-lactating crossbred dairy heifers. Treatments consisted of pastures of palisade grass without a N source (control), fertilized with urea (150 kg/ha/year; fertilized), or mixed with pintoi peanut (mixed). Inclusion of the legume increased concentrations of fractions A (<i>p</i> = 0.009), which is the soluble N compound, and B3 (<i>p </i>< 0.001), which is slowly degraded true protein, compared with pastures fertilized with N and non-fertilized pastures. Nitrogen fertilization increased fraction B1 + B2 (<i>p </i>= 0.046), mainly true proteins, and decreased fraction C (<i>p </i>= 0.0007), indigestible protein, and neutral detergent fiber concentrations (<i>p </i>= 0.0003), contributing to increasing the nutritive value of the herbage. Additionally, N fertilization increased herbage mass (<i>p </i>= 0.004) and herbage allowance (<i>p </i>= 0.0001). Both N fertilization and biologically fixed N increased herbage allowance (<i>p </i>= 0.02) and accumulation rate (<i>p </i>= 0.02), as well as the crude protein content of herbage (<i>p </i>< 0.0001) compared with non-fertilized pastures. Nitrogen fertilization increased true protein and decreased indigestible protein of herbage and promoted a greater herbage mass production, while the inclusion of legumes increased soluble protein and decreased the slowly degraded true protein of herbage. Both N management strategies increased herbage allowance and accumulation rate.
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