Energy Efficiency, Monetary Costs, and Sustainability of Brazilian Rainfed and Irrigated Rice Cropping Systems

Increasing resource use efficiency to meet the world demand for food is among the greatest challenges for the sustainability of agricultural systems. Appropriate biophysical characteristics, resource renewability, and production costs are decisive for the biophysical and economic persistence of agricultural systems. In Brazil, rice is grown in flooding-irrigated (predominantly in the South) and rainfed (other regions) systems. The present study measured the energy efficiency, monetary costs, and the sustainability of characteristic examples of these two types of agroecosystems. We calculated indices of energy flow analysis, discriminating between energy fractions derived from renewable and non-renewable inputs. The irrigated system had more than double the yield, but an energy return on investment (EROI, 2.03:1) and energy productivity (EP, 0.13 kg MJ−1) that were lower than those of the rainfed system (2.86:1 and 0.19 kg MJ−1, respectively). However, the renewable EROI (EROIr) showed that the irrigated system is more sustainable: 364:1 vs 5:1 by our measure. We propose using the energy efficiency cost (EEC) and energy exchange ratio (EnER), and their renewable (R EnER) and non-renewable (NR EnER) fractions to assess the monetary costs of energy efficiency. By these criteria, irrigated systems are less efficient economically, expending more energy to produce a monetary unit of paddy rice (33.87 MJ US$−1) than the rainfed system (24.03 MJ US$−1). The pricing system can underestimate renewable resources such as water for irrigation since the irrigated system’s NR EnER (21.09) was higher than the R EnER (12.78). Thus, the pricing system underestimates the biophysical sustainability of the agricultural systems.