Rainfed crop energy balance of different farming systems and crop rotations in a semi-arid environment: Results of a long-term trial

This study was conducted to determine how energy balances of crop production are affected by three farming systems (conventional, conservation with no tillage, and organic) and four barley-based crop rotations (barley followed by fallow [B-F], barley in rotation with vetch [B-V] or sunflower [B-S], and barley monoculture [B-B]), under the semi-arid conditions of central Spain over a 15-year period (1993/94-2007/08). As inputs, the factors supplied and controlled by farmers were considered. The energy balance variables considered were net energy produced (energy output minus energy input), the energy output/input ratio, and energy productivity (crop yield per unit energy input). The total energy inputs were 3.0-3.5 times greater in the conservation (10.4 GJ ha(-1) year(-1)) and conventional (11.7 GJ ha(-1) year(-1)) systems than in the organic system (3.41 GJ ha(-1) year(-1)). With respect to the crop rotations, the total energy inputs varied from 6.19 GJ ha(-1) year(-1) for B-F to 11.7 GJ ha(-1) year(-1) for B-B. The lowest energy use corresponded to B-F in the organic system (2.56 GJ ha(-1) year(-1)), and the highest to B-B in the conventional and conservation systems (16.3 and 14.9 CJ ha(-1) year(-1), respectively). Energy output was lowest in the organic system (17.9 GJ ha(-1) year(-1)), a consequence of the lower barley grain and vetch hay yields. With respect to the crop rotation, the order followed B-B (19.1 GJ ha(-1) year(-1)) approximate to B-F < B-S < B-V (29.3 GJ ha(-1) year(-1), 53% higher). All the energy efficiency variables analysed had the highest values for the organic system (net energy of 14.5 GJ ha(-1) year(-1), output/input ratio of 5.36 and energy productivity of 400 kg GJ(-1)). No differences were recorded between the conventional and conservation managements. This indicates that, in terms of energy efficiency, the viability of organic systems (low-input practices) under semi-arid conditions, compared to farming systems requiring agrochemicals (conventional and conservation), would appear more recommendable. Cereal monoculture (B-B), independent of the crop management employed, is an energetically unfavourable practice, especially in the driest seasons. However, crop rotations, especially those including a leguminous plant, increase energy efficiency. (C) 2011 Elsevier B.V. All rights reserved.