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 GJ 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) ≈ 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.