Simulação econômica de rotação de culturas irrigadas na região central do Rio Grande do Sul

Abstract

An agricultural production system is constituted by a succession of processes divided in subsystems in which it is emphasized the carriage of productive process related to the environment. In these subsystems, there are processes that can be estimated considering certain conditions such as: weather, crop adaptation and cultural practice and tillage system. The possibility of predicting dates of crop growth (phenology) and development (as harvest date, and consequently, the potential yield) has importance in the process of agricultural planning decision and crop management, not only for irrigation management, fertilizer and defensives application, but also for crop harvest planning, crop rotation, soil management, etc. The present work involved the mathematical simulation of four crop rotation systems: (i) wheat, soybean; (ii) wheat, oats, soybean and maize; (iii) wheat, oats, soybean, maize and dry beans and; (iv) wheat, oats, soybean, maize, maize in same crop sowing season, dry beans and dry beans in the same growing season. This work was conducted in Depressão Central (Santa Maria) and Planalto Médio (Cruz Alta) regions of the Rio Grande do Sul State, using meteorological data from 1990 to 2000. The crop rotation simulation systems i, ii, iii and iv for each agricultural year was conducted for five crop sowing dates of winter cultures (wheat and oats), from 15 to 15 days (first half of May; second half of May; first half of June; second half of June; and first half of July). The irrigation depth and the irrigation number applied on the crops were estimated based on soil plant available water, crop development and meteorological conditions. Production costs of the different crops were estimated adding the cultivation cost, depreciation and irrigation cost. Irrigation cost was based on an area of 380 ha of irrigated crop. The analyses of investment consisted of the determination of return and cumulative capital after 10 years of crop rotation cultivation (from 1990 to 2000) considering the following alternatives of investment of the R$ 1,258,344.00: (i) acquisition of 380 ha of center pivot irrigation system; (ii) acquisition of 134,22 ha of land and; (iii) financial application of capital. The simulated results demonstrated that the cultivation of soybean, maize, maize after a Summer crop during the same growing season, dry beans and dry beans after a Summer crop during the same growing season in Santa Maria and Cruz Alta regions, the complementary irrigation was necessary to supply water to the crops. The cumulative irrigation depth necessary to supply crop need changed according to the date of sowing and cultivation region. Dry beans cultivation after the main Summer crop during the same growing year required less amount of irrigation depth application. Among the crop rotation system tested in the region of Santa Maria and Cruz Alta, the larger cumulative net income was observed for the system IV (succession of wheat-dry beans-maize in same crop sowing season) in the third, sixth and ninth year of cultivation to the first, second and third sowing date of winter crop (wheat), respectively. The choice of the investment in irrigation resulted in investment return (payment capacity) for both regions studied in 5.5 years for the crop rotation system I, in 4.5 years for the crop rotation systems II and III, and in 3.5 years for the crop rotation system IV. The choice of the investment in land acquisition resulted in investment return (payment capacity) for both regions studied in 4.5 years. The choice of the investment in irrigation was the alternative that provided the most accumulated net income after 10 years period (R$ 3,024,395.00) compared to the choices of investment in land acquisition (R$ 1,892,259.00) and financial application (R$ 2,894,677.00) for the regions of Santa Maria and Cruz Alta.