Energy consumption in mixed crop-sheep farming systems: what factors of variation and how to decrease ?
Résumé
Prompted by current concerns about energy resources and greenhouse gas emissions, we sought to assess the impact of certain key factors on energy efficiency in sheep-for-meat production and to evaluate the main directions for improvement. We used a modelling approach to simulate the functioning and performances of sheep-for-meat production systems integrating an energy balance calculation module. In the first step of this study, we reconstructed system functions and technical and economic results of four typological groups of farms in plainland areas. This served as a basis for calculating their energy efficiency in order to focus on the main factors of energy efficiency, such as high levels of fodder self-sufficiency (low concentrate consumption) and high ewe productivity. The Graze system presented the highest energy efficiency (EE) for sheep unit (EEs50.62) with the lowest consumption of equivalent fuel litres requirements (FuReq) per kilogram of lamb carcass produced (1.47), while the ‘sheep and cash crop’ system had the lowest EEs (0.36) and the highest FuReq per kg carcass (2.54). We then took the ‘mixed-farming system’ (a 130 ha farm, including 610 ewes and 40 ha of cropland) and studied three adaptations designed to increase the EEs: improvement of feed self-sufficiency (increased proportion of concentrate produced on-farm), introduction of legumes into the rotation (removal of bought-in nitrogen fertilisers), and production of fuel-oil (from rapeseed) with the flock using oil cakes. The most effective adaptation was the removal of the nitrogen fertilisers. The successive adaptations make it possible to cut energy consumption from 2.2 FuReq/kg carcass down to 0.98 after the optimisations, thereby increasing EEs from 0.42 to 0.93. Finally, we went on to study the energy impact of four factors influencing flock functioning and farm structure, i.e. ewe productivity, lamb weight, distances between plots, and flock size. Ewe productivity and lamb weight had a strong positive impact on EEs. When ewe productivity switched from 0.80 to 1.70, EEs increased from 0.29 to 0.48 while FuReq per kilogram carcass dropped from 3.39 to 1.88. When flock size was increased to over 1000 ewes, there were little or no energy-related economies of scale, as farm area also increased and most of the systems required more equipment.