2.1. Energy Analysis of Food Systems
provides the conceptual framework for analyzing energy flows and balances in agriculture including vegetal and livestock production systems. Net production is the sum of extracted vegetal biomass plus livestock production minus total feed. Total feed includes all feedstuff produced domestically or imported from abroad (essentially soybean cake in the case of France), thus it corrects domestic net production for feed trade.
Preindustrial agriculture was energy neutral. Virtually all the energy invested as an input to the production system was sourced by self-fueling which was the sum of food for farmers and feed for working animals. Farm surplus was the final agricultural produce once the system had fulfilled its functional energy requirements [4
]. Self-fueling got progressively replaced by external energy inputs used for traction machines, irrigation, greenhouses, livestock facilities, and synthetic fertilizers. At any given time, total energy invested in agriculture is the sum of self-fueling and external energy inputs. The share of self-fueling in this total accounts for the energy self-sufficiency of agriculture.
Today, self-fueling is close to zero. Farm surplus includes biomass for food and biomass allocated to first generation biofuels production. Independently on whether biofuels are returned to agriculture as an input, their production constitutes a factor of energy neutrality but competes with food. Agriculture may potentially recover energy from crop residues and manure [28
] to produce energy neutrality without competition with food. Energy recovery from agricultural residues differs from self-fueling in that it does not necessarily return to agriculture. It corresponds to the production of second-generation biofuels.
By comparing the energy recovery potential to the primary biomass equivalent of the external energy inputs, we derive a positive or negative net energy balance accounting respectively for the structural energy surpluses or deficits of agriculture. A positive balance implies that the farm surplus is structurally independent from external energy subsidies and that agriculture can, in addition to farm surplus, provide bioenergy to society. In contrast, a negative balance implies that agricultural production is structurally energy deficient and that in absence of external energy subsidies, farm surplus cannot be sustained. A reduction in farm surplus proportional to the energy shortfall is inevitable unless if energy efficiency is increased. Higher energy efficiency means lower dissipation in the production system. It can be achieved either through higher conversion efficiency between final and primary energy inputs or through cuts in the amount of the biomass used as feed.
The main feed categories are grain and by-products (including cereals and oil crops produced domestically or imported), annual fodder and grasses. Feed consumption depends on the type and size of livestock populations and on the feed to food conversion efficiency. Here we assess the effect of grain feed and annual fodder suppression and of energy recovery from crop residues and manure on the net energy balance of agriculture. We use France as a reference case in four scenarios. The results are put in common perspective with feed use and the energy invested in agricultural production in historical perspective [4
2.2. Primary Biomass Equivalent of External Energy Inputs
The primary biomass equivalent of external energy inputs is calculated by considering the type of final energy use and the associated conversion efficiency (η) between primary and final energy. The energy use in agriculture is dividable among mechanical, heat, and the energy embodied in fertilizers and other resource inputs.
The conversion of biomass to mechanical energy can be either done directly by draft animals or by motors through a two-step process. The first step is the conversion of biomass to fuel which depends on the conversion pathway [31
] of the biomass feedstock and is about 30% [31
] for a final oil-equivalent fuel (see Supplementary Materials
for detailed calculation). The second step is the combustion of fuel to mechanical power, with a typical efficiency of 40–50% [35
]. Accordingly, the energy conversion efficiency of biomass to power through motors is about 12–15% which is close to the efficiency range of draft animals over a full cycle including energy requirements for both maintenance and work [37
]. As a result, the conversion efficiency fundamentally depends on the primary energy source rather than on the converter (Figure 2
). Of course, the power level of motors is a multiple of that of draft animals, but through a number of draft animals equal to the power level gap the same cumulative power can be obtained with the same energy conversion efficiency.
The conversion efficiency of biomass to heat is considered at 90% [38
] and for the energy embodied in fertilizers at 45%, by considering a mix of heat and biomass gasification in the Haber Bosch process for ammonia synthesis, currently overwhelmingly reliant on natural gas as a hydrogen feedstock and on high pressure and temperature conditions for achieving high yields [39
shows, on the one hand, the share of fossil fuels and the distribution of total final energy use in current agriculture among mechanical, heat, and the energy embodied in resource inputs and, on the other hand, the calculated conversion efficiency and associated primary biomass equivalent per use [4
]. Machines fuel is dominated by tractors and combine harvester-threshers. Livestock facilities include heating, lighting, and ventilation. The ‘other’ category includes irrigation and the energy embodied in pesticides and imported feed.
2.3. Energy Recovery Potential and the Net Energy Balance of Agriculture
The net energy balance is calculated for four scenarios using the year 2013 as a reference date for the current energy metabolism of agriculture. The four scenarios quantify how much the farm surplus would change if agriculture were to reach energy neutrality. In the context of growing food demand, the share of first-generation biofuels in farm surplus is likely to phase out for the sake of food security, especially given the ongoing loss trend of agricultural land. Indeed, total agricultural area in France decreased by 17% since 1961 [26
], mainly driven by urbanization [40
]. Given this trend, a stabilization or increase in agricultural area is doubtful as well as it is unadvisable because it would most likely compete with forestland with risks for soil carbon destorage, erosion, and loss of ecosystem services and biodiversity [41
]. Accordingly, the four energy neutrality scenarios are constructed for both current total agricultural area and farm surplus fully put aside for food production.
The scenarios are constructed on two drivers. On the one hand, the energy recovery from manure and crop residues with a medium and high variant and, on the other hand, the suppression of feed grain and annual fodder (Table 2
). Feed reduction brings down livestock production, energy consumption in livestock facilities (for heating, ventilation, and operating milking parlors), and available manure. The livestock productivity and manure excretion depend on feed (Feed) and its energy conversion efficiency (ECE) to food which varies among livestock production (i
) and rations compositions. ECEi
are calculated for the year 2013 based on Harchaoui and Chatzimpiros [29
]. The suppression of grain feed and annual fodder mainly affects monogastric animals and milk production respectively, which have the highest ECEi
. The suppression lowers the ECE of these sectors according to the new rations composition [42
] as well as the aggregate livestock ECE (Table 2
). In all scenarios, the energy share of milk and meat from cattle is kept as today at respectively 70% and 30%. The energy consumption in livestock facilities equals total livestock production times an energy intensity coefficient taken at 0.42 J of primary biomass per J of livestock production [4
] (Table 2
The energy recovery potential from crop residues, mainly straw, is calculated based on the harvest index of crops (HI), i.e., the ratio of crop yield to the crop’s total aboveground biomass, derived from literature [43
]. Total energy in crop residues is the sum of the harvested biomass per crop (harvestj
) times the share of residues (1-HIj
in total aboveground biomass. From this total, we consider a medium and high recovery rate (Rc
) of 30% and 70% respectively. Both rates are very ambitious compared to the almost zero energy recovery today. The medium and high rates inversely relate to high and medium soil carbon conservation objectives [33
Concerning manure, only the manure produced in confinement is recoverable in contrast to excretions during grazing [45
]. The average time spent in confinement per animal (ti
) is 45% for cattle, 10% for sheep and goat, and 90% for pig and poultry [33
]. The difference between feed intake and livestock production is metabolic heat losses and manure energy [34
]. We consider that manure energy (ME) is 15% to 40% of this difference to reflect variability specific to livestock and to feedstuff digestibility [34
]. Manure produced in confinement is assumed to be fully recovered, which is an optimistic assumption.
The recovery of agricultural residues has an energy cost associated to logistic and technical steps, such as compressing, picking up, pre-treating, and transporting the materials to the digester [50
]. Accounting for this energy cost in details is out of the scope of the paper. Here, we provide a simple approximation by assuming a consumption of respectively 0.17 J of primary biomass per J recovered from crop residues and 0.32 J per J recovered from manure. These consumption factors are derived from Table 1
by respectively assuming an increase in machine fuel proportional to the non-economic yield of crops (1-HI) and a doubling in the energy consumption in livestock facilities representing energy costs for collection, transportation, preparation, and pumping of manure. These energy recovery costs are within the range (low end) of a detailed review analysis of various biogas systems in Sweden [51
]. Table 2
presents the main data and calculation formulas in the four scenarios.