The objective was to analyze the economic viability of using different sources and doses of nitrogen in Mombaça grass (Megathyrsus maximus). The design was in randomized blocks with a 2x4 factorial arrangement, with two sources of nitrogen (urea and ammonium nitrate) with four nitrogen doses (0; 25; 50 and 75 kg / ha) per cycle totaling eight treatments and four repetitions, distributed in 32 plots of 9m2 each. 50 kg / ha of P2O5 were applied in the form of simple superphosphate and 50 kg / ha of K2O in the form of potassium chloride throughout the experimental area, including treatments that did not receive nitrogen. The cuts took place at fixed intervals of 28 days. A 1.25m x 1.25m frame with an area of 1.56 m2 was used to evaluate the production of forage that was harvested at a height of 10 cm in relation to the soil. Using forage dry mass data, economic viability assessments were performed for the use of the nitrogen source and its optimized fertilization as presented by the different dosages offered in plots, and also of the nitrogen source and dose associated with phosphoric and potassium fertilization. There was no interaction (P> 0,05) between doses and nitrogen sources for any of the variables analyzed. Regardless of the doses applied, the sources used did not significantly influence production costs (P> 0,05). The kg cost for PMSF and PMST with nitrogen fertilization adjusted in a positive linear way (P <0,05) to the applied doses. There was no effect (P> 0,05) of nitrogen doses on the cost of kg for PMSF with N + P + K fertilization. In relation to the cost of kg of PMST being the sum of the costs of N + P + K, it presented a quadratic behavior (P <0,05). Nitrogen fertilization is essential for maintaining pasture productivity, as it results in increased production of vegetable mass. Thus, the decision on the source to be applied will depend on the unit cost of nitrogen aiming at greater bioeconomic viability.
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