Função de produção de etanol de cana-de-açúcar sob diferentes lâminas de irrigação e doses de nitrogênio

Abstract

Sugarcane is a crop of relevant importance for the Alagoas state, mainly due to the high potential for ethanol production that is currently one of the main products of the Brazilian energy matrix. Therefore, in order to evaluating ethanol yields as a function of irrigation levels (L) and nitrogen (N) rates, was carried an experiment in the region of Rio Largo, AL. The experimental design was a completely randomized block in the scheme of subdivided plot, with seven irrigation levels, five nitrogen doses and four replicates. At irrigation levels were determined based on reference evapotranspiration (ET0): 0,0 ET0; 0,25 ET0; 0,50 ET0; 0,75 ET0; 1,00 ET0; 1,25 ET0 and 1,50 ET0 and the doses of N were: zero 50, 100, 150 and 200 kg of N per hectare. The average gross income of ethanol obtained was 14,14 m3.ha-1. The irrigation level of maximum economic efficiency was 307 mm and the maximum physical yield of ethanol (15,14 m3.ha-1) was achieved with 573 mm. In relation the nitrogen rates, the one of maximum economic efficiency was 118 kg of N.ha-1 and the highest physical yield of gross ethanol (14,51 m3.ha-1) was 95 kg of N.ha-1. The square root polynomial function represented well the yield of ethanol in relation to water and nitrogen. The marginal physical product of nitrogen being below zero between 100 and 125 kg of N.ha-1, and The marginal physical product of nitrogen being below zero between 500 and 600 milimeters. The marginal rate of replacement of water by nitrogen varies according the price of the millimeter of water, the kilogram of nitrogen and the cubic meter of ethanol and this rate provides the minimum cost at each yield level when it equals 1,53 mm.kgN-1. The region of rational production was located at the beginning of the tangency of the isoquants (86 kg of N and 510 mm) until the crossing at the level of 15,60 m3.ha-1.