Economic indicators for ethanol production from starch crops under different irrigation managements/Indicadores economicos para producao de etanol proveniente de culturas amilaceas sob diferentes manejos de irrigacao

HIGHLIGHTS: The market value of ethanol influences its production viability. Full irrigation (100% of ETo) reduces the unit cost of ethanol production. The scenarios are favorable for maize and grain sorghum in ethanol production, with selling prices above R$2.50 [L.sup.-1]. The study aims to quantify the production costs of maize and sorghum crops under rainfed and irrigated conditions for the generation of ethanol and to evaluate the economic feasibility in different scenarios of marketing price. Two experiments were carried out in the years 2019/2020 and 2020/2021, in an experimental area at the Federal University of Santa Maria, Santa Maria, RS, Brazil. The experiment was in strip plots, in a randomized block design and four replicates. Three irrigation depths, 0, 50 and 100% of the reference evapotranspiration (ETo), as first factor, and two maize cultivars and one grain sorghum cultivar as second factor, using 45 scenarios. The use of full irrigation (100% of ETo) reduces the production costs of the liter of ethanol by 15 and 17.89% and increases ethanol production by 44.18 and 48.25% for maize and grain sorghum crops, respectively. For the market price of R$ 2.00 [L.sup.-1], the grain sorghum does not show good performance, with negative net present value (NPV) and internal rate of return (IRR). The sale price of R$ 4.00 [L.sup.-1] and full irrigation is the best economic scenario, with values of NPV of R$ 90,356.93, IRR of 33.83%, benefit/cost ratio (B/C) of 2.28, profitability index (PI) of 3.99 and payback of three years. Maize is economically viable for ethanol production in all scenarios. Full irrigation and market price of R$ 4.00 [L.sup.-1] represent the best economic scenario, with mean values for NPV of R$ 204,381.68, IRR of 63.35%, B/C of 2.96, PI of 8.67 and payback of 1.58 years, among the cultivars. Key words: grain sorghum, maize, economic viability, water management, biofuel O estudo visa quantificar os custos de producao das culturas de milho e sorgo em condicoes de sequeiro e irrigado para a geracao de etanol, e avaliar a viabilidade economica em diferentes cenarios de preco de comercializacao. Foram realizados dois experimentos nos anos de 2019/2020 e 2020/2021, em area experimental da Universidade Federal de Santa Maria, Santa Maria, RS. O delineamento experimental foi em faixas, com blocos ao acaso e quatro repeticoes. Foram avaliadas tres laminas de irrigacao: 0, 50 e 100% da evapotranspiracao de referencia (ETo) como o primeiro