Types of study considered

Most of the studies considered within this review have taken what JonesReference Jones²⁰ describes as a ‘mechanistic’ or ‘process analysis’ approach, i.e., assessing the fossil energy use associated with the various production stages of an agricultural product. This includes the assessment of energy associated with production processes on case study farmsReference Cobb, Feber, Hopkins, Stockdale, O'Riordan, Clements, Firbank, Goulding, Jarvis and Macdonald^21, Reference Alföldi, Schmid, Gaillard and Dubois²² or through the application of LCA. This is a method used to calculate the burdens associated with one unit of a food commodity, e.g., 1 kg of wheat, area of land or livestock unit (LU) defined as the ‘functional unit’²³. Within the LCA approach, inputs to the system are usually traced beyond the farm gate to the primary resource. For example, this can include the coal or uranium used to generate electricity or the energy required to produce steel, plastic and other materials required for the manufacture of tractorsReference Williams, Audsley and Sandars²⁴. LCA has the distinct advantage of being able to determine efficiency within supply chains in a manner that can be easily understoodReference Wegener Sleeswijk, Kleijn, van Zeijts, Reus, Meeusen-van Onna, Leneman and Sengers²⁵. In addition, the broad principles for the application of LCA have been standardized, e.g., through the International Organization for Standardisation 14044 standard²⁶. This has helped to make LCA the most widely used method for the assessment of energy use within supply chains in the agriculture sectorReference Pelletier, Audsley, Brodt, Garnett, Henriksson, Kendall, Kramer, Murphy, Nemecek and Troell²⁷. It is important to note, however, that these standards are not prescriptive about boundary conditions, the functional unit or the purpose of the study, which can make comparisons between studies difficult.

Other studies considered here have followed a ‘thermodynamic approach’Reference Jones²⁰ through the adoption of emergy accountingReference Odum²⁸. Emergy has developed as an alternative to the ‘traditional’ fossil energy focused approach of energy accounting. It takes an eco-centric approach that accounts for the contribution of natural services (e.g., rain, pollination, soil formation) in delivering agricultural productsReference Bakshi²⁹. In a similar manner to LCA, the emergy approach measures the energy previously used in the creation of a product. However, it also accounts for the amount of available energy that sits within the assessed product or system. The units of energy are expressed in a common unit (i.e., ‘solar energy’ or ‘emjoules’)Reference Odum²⁸. The emergy approach also takes into account natural/ecological inputs and human activities. It calculates natural inputs, based on the distribution of solar energy in the biosphere and the energy output potential of the various processes (e.g., rainfall, total wind energy and total wave energy)Reference Brown and Herendeen³⁰. Human labor and services can also be accounted for, both in terms of the energy used to support human life and the energy associated with the accumulation of informationReference Odum²⁸. In this sense, emergy allows for an assessment of ‘energy quality’ through considering the importance of inputs and outputs in a web of relationshipsReference Pizzigallo, Granai and Borsa³¹. A limited number of studies have used the emergy approach to assess the efficiency of organic and conventional agriculture. The results from these studies will be described in a separate section below.

A number of studies within this review have also taken the nominally dimensionless ‘energy ratio’ approach to determine the efficiency of production systems (i.e., dividing the energy output in food sold by the energy input of fossil fuels). This approach is nominally dimensionless in that the gross energy of fuels is compared with the metabolizable energy of foods or feeds. LampkinReference Lampkin¹⁵ highlights that this method can be a useful determinant of the efficiency of agricultural systems in capturing solar energy and transforming this into feedstuffs for growing populations. Halberg et al.Reference Halberg, Verschuur and Goodlass³² also highlight the potential of this approach to allow farmers and advisors to compare the efficiency and environmental impacts of crop and livestock enterprises, in order to identify areas for improvement.

A limitation of the study is that there are insufficient data to perform a statistical analysis. The wide variation in the scale of the studies and the methods used prevents this. In addition, the wide geographical variation in the studies and the resultant wide range of soil types and climates makes it difficult to draw definitive conclusions that will apply to each country or region (Appendix 1 shows the list of studies, their location and energy assessment method used).

On-farm fuel use

A common criticism of organic agriculture is that reliance on mechanical tillage (e.g., for weed control) results in lower energy efficiency overallReference Hoeppner, Entz, McConkey, Zentner and Nagy³⁵. A process-oriented modeling study carried out by ADASReference Cormack and Metcalfe³⁶ supported this criticism, finding higher machinery energy use within organic systems (i.e., energy associated with the manufacture, distribution and repairs to mechanical equipment). This increase was, however, offset by higher indirect energy use under conventional management. Most of the additional fuel use within the ADAS study was associated with weed control. Organic carrot production compared particularly poorly due to the energy-intensive process of flame weeding. Organic wheat production was also associated with higher machinery energy, a potentially significant finding in view of the dominance of wheat in the European arable sector and the importance of this crop as a staple of Western diets. VenkatReference Venkat³⁷ also found higher on-farm energy use on organic farms for certain vegetable crops (see Fig. 1) within an LCA comparison, suggesting that this is due to systematically higher levels of mechanical weeding. Unlike the ADAS study, Venkat found that this difference was enough to offset the impact of fertilizer manufacture in the conventional system. Greater use of tractor diesel per liter of milk produced was also reported for an organic farm in an LCA of two large dairy units in SwedenReference Cederberg and Mattsson³⁸. Higher fuel use per functional unit on the organic farm was a result of the larger area of fodder production and lower yields within this study. Jørgensen et al.Reference Jørgensen, Dalgaard and Kristensen³⁹ also found that the levels of on-farm energy use were 28% higher for organic crop production in Denmark. This was a result of higher fuel consumption for weed control in addition to the energy-intensive practice of manure spreading (compared to spreading fertilizer). In common with the ADAS study, the authors found that the higher on-farm energy use was offset by the energy requirements for the manufacture of inputs in the conventional system.

Figure 1. Distribution of ‘direct’ (i.e., on-farm) and ‘indirect’ (i.e., off-farm) energy use from seven studies comparing organic and conventional production. Owing to variation in the scale for the products reported, a log scale has been used on the x-axis. Most studies took a ‘cradle-to-gate’ approach (i.e., considering energy use associated with production but not retail consumption and disposal); for more details on boundaries and functional unit of each study, see Table 1.

The need for moldboard plowing in organic systems, for the removal of crop residues and control of weeds, can also contribute to greater on-farm energy use in comparison to reduced tillage with herbicides, as identified in a comparison of organic, integrated and conventional farming systemsReference Leake⁴⁰. A study by Michigan State University also found a lower fuel use for a corn, soybean, wheat rotation under conventional no till, compared to the same rotation under low-input and organic conditions, although the savings were offset by the energy associated with fertilizer and lime inputsReference Robertson, Paul and Harwood⁴¹. Zentner et al.Reference Zentner, Lafond, Derksen, Nagy, Wall and May⁴² also found that gains in on-farm fuel use from reduced tillage were offset by the embodied energy associated with inputs of pesticide and fertilizer within an energy analysis of direct and indirect energy associated with nine cropping systems in Canada. Despite this, Snyder and SpanerReference Snyder and Spaner⁴³ note that high-input costs are supporting a shift toward reduced-input systems, where reduced tillage is applied, and it has been suggested that such tightly controlled conventional systems may rival organically managed farms with regard to energy efficiency, even when the costs of inputs are taken into accountReference Clements, Weise, Brown, Stonehouse, Hume and Swanton⁴⁴. However, reduced tillage is no longer exclusive to conventional farms. Recent studies show that this technique can be applied successfully under organic conditions for cereal cropsReference Berner, Hildermann, Fließbach, Pfiffner, Niggli and Mäder^45, Reference Crowley, Döring, Measures and McCracken⁴⁶ with significant energy savings as a resultReference Crowley, Döring, Measures and McCracken⁴⁶. Lockeretz et al.Reference Lockeretz, Shearer and Kohl⁴⁷ also found that organic farmers in the ‘corn belt’ of the USA were more likely to use chisel plowing methods, as opposed to the moldboard plow. This was to help conserve organic matter and water, instead of exposing the soil to wind erosion, a common problem in the area studied. It is also important to consider that reduced tillage is not always possible for farmers. The possibility for implementation will depend greatly on the soil type, topography and the available power of the machineryReference Bailey, Basford, Penlington, Park, Keatinge, Rehman, Tranter and Yates⁴⁸. Increased herbicide leaching and greater population of certain perennial weeds and grasses have also been reported in some reduced tillage systemsReference Locke, Reddy and Zablotowicz^49, Reference Tuesca, Puricelli and Papa⁵⁰, which could result in increased requirements for cultivation and fuel use to remove pernicious weeds. Reduced yields within no tillage systems have also been observed in some soil and climate conditionsReference Lal, Reicosky and Hanson⁵¹, reducing the overall energy efficiency per unit of product.

In contrast to many of the above studies, some authors have found the similar or even lower levels of on-farm diesel use for organic production. For example, Refsgaard et al.Reference Refsgaard, Halberg and Kristensen⁵² found little difference between the amounts of diesel required for the production of conventional and organic crops. However, the organic systems within the process models used in this study tended to require more fuel for handling and spreading of manure. A farm system monitoring project in Switzerland also found very similar levels of diesel in a long-term comparison of an organic and conventional farm, although the conventional systems used as a comparator within this study were of a relatively low intensityReference Alföldi, Schmid, Gaillard and Dubois²².

Labor

With regard to human energy (or labor), organic systems have been associated with higher numbers of staff on the farm due to increased livestock, reduced machinery use and diversity in farm enterprisesReference Cobb, Feber, Hopkins, Stockdale, O'Riordan, Clements, Firbank, Goulding, Jarvis and Macdonald^21, Reference Ziesmer^53, Reference Lobley, Reed, Butler, Courtney and Warren⁵⁴. El-Hage Scialabba and HattamReference El-Hage Scialabba and Hattam⁵⁵ also report a higher share in the production of labor-intensive crops (e.g., vegetables) and on-farm marketing and processing on European organic farms. In a modeling study of four organic and conventional crops, Pimental et al.Reference Pimentel, Berardi and Fast⁵⁶ also found lower labor productivity for organically produced crops (i.e., kilogram output per hour of labor input). This was due to a need for increased cultivations, in addition to greater losses from pests and disease and high cosmetic standards, which prevent sale of certain crops, in particular organic apples. Nguyen and HaynesReference Nguyen and Haynes⁵⁷ also compared the labor productivity of three pairs of mixed cropping farms in the Canterbury region of New Zealand, with labor requirements calculated in hours per hectare for the entire rotation and the cropping part (i.e., peas, barley and wheat) separately. The labor productivity was also measured as a ratio of harvested grain to the number of hours per hectare. Although labor inputs per hectare for most grain crops were higher on the organic and biodynamic sites, the total labor use was lower as a result of the 3–4-year fertility-building period. This balanced out the higher requirement for the cropping phase. Despite this, the grain crops grown within the biodynamic and organic systems had a lower labor productivity (0.4–1.1 ton h⁻¹) compared to the conventional (1.3–1.6 ton h⁻¹), as a result of higher labor inputs and lower yields. The additional labor requirement within the organic systems was partly due to the additional field and manual operations plus the additional labor requirement for the manufacture of cow-horn manure (a homeopathic preparation for improving soil health) within the biodynamic system. Karlen et al.Reference Karlen, Duffy and Colvin⁵⁸ took a similar approach in calculating the number of fieldwork hours required for crop production and harvest in a comparison of four 40-acre fields in the ‘Corn belt’ of the USA. Within the ‘alternative’ system, labor requirements were substantially increased (between 178 and 183% of the conventional). This was primarily as a result of the additional time required for spreading manure, weed control and through the incorporation of a hay crop within the rotation, which required multiple harvests.

An attempt was also made to compare the labor requirements of organic and conventional farms by comparing calendars of work of conventional and organic farmers in addition to measuring heart rates and constructing an energy budget based on their food intakeReference Loake⁵⁹. The relatively high energy and effort expenditure on the organic farm led the author of this study to suggest that ‘the annual activity of organic farming is characterized by physical stress and fatigue’. Unfortunately the study was flawed in that it compared an organic farmer using hand tools with a conventional livestock and arable farmer who spends most of the heart rate assessment period driving a tractor. The farms were therefore not comparable, and as the author notes, the organic farmer cannot be considered representative of the sector. Having said this, the study does contribute to addressing the methodological difficulties of comparing mechanized systems with manual operations.

Indirect, off-farm energy use

Indirect energy use (i.e., energy use associated with the production and transport of inputs) typically exceeds on-farm energy use within modern farming systems in developed countries, with fertilizer and imported feeds for livestock comprising the two major sources of energy inputs used for agricultural productsReference Pelletier, Audsley, Brodt, Garnett, Henriksson, Kendall, Kramer, Murphy, Nemecek and Troell²⁷. The importance given to on-farm or local resources within the IFOAM organic principlesReference Darnhofer, Lindenthal, Bartel-Kratochvil and Zollitsch¹² suggests that organic farms could be less reliant on external inputs of fertility and animal feed, and a number of studies have explored the extent to which this applies in practice.

Fertilizer inputs

The energy intensive manufacture of nitrogen (N)-based fertilizers represents the most energy expensive input for modern farming, accounting for about half of agriculture's energy use¹⁹ and approximately 1.1% of energy use globallyReference Dawson and Hilton⁶⁰. Instead of relying on manufactured fertilizers, organic farms source the bulk of their N through biological fixation by temporary, legume-based leys. The use of leys can also further the production of soil organic matterReference Leifeld and Fuhrer⁶¹ in addition to providing an energy source for the soil biota, which enables humus production through transformation of organic material. In this, sense the organic system aims to develop soil health over the long term, rather than providing a short-term nutrient supply through application of soluble plant nutrientsReference Watson, Atkinson, Gosling, Jackson and Rayns⁶². Refsgaard et al.Reference Refsgaard, Halberg and Kristensen⁵² state that in this context ‘one might think of organic farming as a systematic replacement of fossil-fuel N fertilizer production with solar-driven N fixation in legumes’, with fossil fuels being used to help this process. This was illustrated by Gomiero et al.Reference Gomiero, Paoletti and Pimentel¹⁴ who found that the main reason for increased energy efficiency under organic management was the lack of synthetic inputs, in particular fertilizers and pesticides.

Despite the reliance on biologically fixed N within organic agriculture, organic farmers still make use of mineral sources for other nutrients, in particular rock phosphate (P), which is mined from natural stores. TrewavasReference Trewavas⁶³ argues that when this aspect is taken into account, the energy efficiency of organic farming is lowered considerably, when compared to integrated no-till systems. Low solubility of rock phosphate may also make it less effective than manufactured P fertilizer (superphosphate) particularly in low rainfall areasReference Bolland, Gilkes and Antuono⁶⁴. Co-application of rock phosphate with elemental sulfur or manure could, however, help to enhance availabilityReference Agyin-Birikorang, Abekoe and Oladeji^65, Reference Evans, McDonald and Price⁶⁶. Use of rock phosphate may also help to maintain a stable supply of readily available P over time, compared to use of water-soluble phosphate fertilizerReference Randhawa, Condron, Di, Sinaj and McLenaghen⁶⁷. Pelletier et al.Reference Pelletier, Arsenault and Tyedmers⁶⁸ found in their LCA of organic and conventional wheat and soybean production in Canada, that the cumulative energy demands of producing phosphate fertilizer were on average four times higher than those associated with producing rock phosphate used in organic agriculture. Sourcing fertility from outside of the farming system also applies to farms producing large quantities of crops, which depend on external sources of compost and manure. Alonso and GuzmanReference Alonso and Guzman⁶⁹, for example, found higher energy use for organic crops grown in Spain, as a result of the energy associated with production of large quantities of compost. Karlen et al.Reference Karlen, Duffy and Colvin⁵⁸ found that without charging for the energy associated with the manure nutrients (i.e., assuming that the manure is a ‘cost’ incurred by the livestock enterprise) an ‘alternative’ system required about half of the energy of the conventional; however, if the energy costs for the nutrients were included, the alternative system used twice as much energy as the conventional (see Fig. 1). Duesing (1995) in Rigby and CáceresReference Rigby and Cáceres⁷⁰ also refer to North Californian organic farmers using manure from South Californian dairy farms, which in turn used imported feed grain from the Midwest. Rigby and CáceresReference Rigby and Cáceres⁷⁰ note that such practices have serious implications in terms of energy use and that the methods used do not necessarily sit well with some people's perceptions of organic production or the organic principles.

Despite evidence that some organic farmers are importing fertility and are therefore ‘robbing Peter to pay Paul’, Alonso and GuzmanReference Alonso and Guzman⁶⁹ point out that inputs of manure and compost help to promote the long-term health of the system, and cannot be compared in the same way to non-renewable energy sources. They also highlight that organic farmers are able to reduce levels of compost application as soil humus levels develop. Moreover, when a comparison was made of non-renewable energy use (i.e., fossil fuels) within this study, the energy use was significantly lower within all of the organic production systems. El-Hage Scialabba and Müller-LindenlaufReference El-Hage Scialabba and Müller-Lindenlauf⁷¹ also highlight that the pollution and soil degradation problems associated with landless livestock production systems can be reduced through the co-operative use of farmyard manure between crop and livestock operations on organic farms. With landless livestock production systems currently supplying over 50% of pig and poultry meat worldwideReference Steinfeld, Gerber, Wassenaar, Rosales and de Haan⁷², the relative advantages of a more integrated approach to production are an important consideration. Reviews comparing nutrient budgets on organic and conventional farms have also found that nutrient surpluses and N leaching are generally smaller for organic farms. This suggests a more efficient use and recycling of nutrients between enterprisesReference Shepherd, Pearce, Cormack, Philipps, Cuttle, Bhogal, Costigan and Unwin^73, Reference Tuomisto, Hodge, Riordan and Macdonald⁷⁴.

Livestock feed

As mentioned above, organic farms try to maintain a closed production system as far as possible with regard to all inputs, not only those relating to soil fertility. Assessments of energy use within beef and dairy production by SchaderReference Schader⁷⁵ and Haas et al.Reference Haas, Wetterich and Köpke⁷⁶ found that this approach manifests through a reliance on home-grown sources of feed for livestock (see lower energy inputs associated with imported feed within these studies in Table 1). Lower energy use associated with concentrate feed has also been reported in comparisons of organic and conventional dairy production in Sweden, Denmark and the NetherlandsReference Cederberg and Mattsson^38, Reference Jørgensen, Dalgaard and Kristensen^39, Reference Thomassen, van Calker, Smits, Iepema and de Boer⁷⁷. Within an assessment of the environmental impacts of a 1996 ‘baseline’ and a number of 100% organic conversion scenarios in Denmark, Dalgaard et al.Reference Dalgaard, Halberg and Porter⁷⁸ also found that domestically produced, organic grass/clover was energetically cheaper than conventional forage, due to a lack of fertilizer application. The increased efficiency contributed to lower energy use overall per LU.

Table 1. Distribution of ‘direct’ (i.e., on-farm) and ‘indirect’ (i.e., off-farm) energy use from nine studies comparing organic and conventional productions.

For poultry, most organic production systems have longer production cycles. This not only can have a positive effect in terms of animal welfare (e.g., lower prevalence of limb disorders, through use of slow-growing breedsReference Castellini, Berri, Le Bihan-Duval and Martino⁷⁹), but also results in lower energy efficiency through higher levels of feed use per unit of product (e.g., Leinonen et al.Reference Leinonen, Williams, Wiseman, Guy and Kyriazakis⁸⁰ see Fig. 1). In addition, mortality rates of caged poultry systems have been shown to be lower than organic or free-range systemsReference Leinonen, Williams, Wiseman, Guy and Kyriazakis^80, Reference Leinonen, Williams, Wiseman, Guy and Kyriazakis⁸¹. For pig meat production, recent studies have shown that organic systems tend to import not only less feed, which contributes to lower energy use and greater efficiency per unit of land, but also lower levels of output and a possible increased energy use per kilogram of product, depending on the assessment method usedReference Basset-Mens and van der Werf^82, Reference van der Werf, Tzilivakis, Lewis and Basset-Mens⁸³. Williams et al.Reference Williams, Audsley and Sandars²⁴ also reported a considerable increase in the area of land used for the production of pig feed within organic systems, in an LCA study of UK production. This led to a reduced energy output per hectare, compared to conventional production.

Effect of functional unit when comparing studies

As found by Lynch et al.Reference Lynch, MacRae and Martin¹³, the unit of comparison affects the performance of organic farming systems with regard to environmental assessment criteria such as energy use. In common with this study, we have found that for most product types, organic performs better than conventional per unit of product, with over 75% of the product comparisons in Figure 2 reporting lower energy use. In particular, Figure 2 illustrates the efficiency of organic grazing systems due to the lower energy impacts associated with forage production for beef and sheep production (organic energy use ranges from 21 to 94% of conventional for these systems, depending on the system intensity). In common with Lynch et al.Reference Lynch, MacRae and Martin¹³, we have also found that organic systems tend to compare less favorably for poultry systems. Energy use under organic management was found to range from 125 to 160% of conventional for broilers. For egg production, energy use also tended to be higher, between 120 and 127% of the conventional barn and cage-based systems, respectively. There was less difference between the energy requirements of organic and conventional free-range systems (with organic requiring 103–105% of the energy used on the conventional systemsReference Williams, Audsley and Sandars^24, Reference Leinonen, Williams, Wiseman, Guy and Kyriazakis⁸¹).

Figure 2. Organic versus conventional energy use per unit of product with expanded selection. Organic performs better below the line, worse above the line. Please note the ‘trend-line’ is x=y for the purposes of illustrating the relative performance for each product type and is not a line of best fit. Production units were not constant across the studies compared.

With regard to crops, most organic systems perform better than conventional in energy use terms, mainly as a result of an absence of manufactured N fertilizer. Energy use for cereal cropping is approximately 80% of conventional per unit of product, despite the lower yield. Vegetable production energy requirements also tend to be lower on organic farms, requiring approximately 75% of the energy used under conventional. There are some exceptions, in particular glasshouse vegetables, apple and potato production exhibit reduced yields and similar levels of energy inputs, which can result in more energy use per unit weight of product overall. In particular, this is a result of greater losses from insect pests and diseases for potatoes and apples. Reduced yields in organic vegetable production glasshouse systems were partly due to an increase in specialty cropping (e.g., vine tomatoes) on organic farmsReference Williams, Audsley and Sandars²⁴.

It is also clear from Figure 3 that the difference between conventional and organic systems is greater when comparisons are made on a per hectare basis, over 80% of the comparisons showing a lower energy use associated with organic production. This is to be expected due to the lower intensity of production on most organic holdings, resulting in fewer inputs, and a reduced yield. Despite this, organic performs less well when the energy content of the organic matter/compost used on organic holdings is taken into consideration. Average energy inputs per unit of land area were approximately double that of the conventional farms when this was taken into accountReference Karlen, Duffy and Colvin^58, Reference Alonso and Guzman⁶⁹. For the reasons outlined above, however, this renewable energy input cannot be compared in the same way to fossil-fuel-based energy.

Figure 3. Organic versus conventional energy use per hectare with expanded selection. Organic performs better below the line, worse above the line. Please note the ‘trend-line’ is x=y for the purposes of illustrating the relative performance for each product type and is not a line of best fit.

A number of studies have compared organic and conventional systems in terms of energy efficiency (energy out/energy in). A range of approaches to measuring energy have been used, with some authors expressing production of organic/non-organic systems in terms of combustion energyReference Pimentel, Berardi and Fast⁵⁶ and other authors using metabolizable energy output valuesReference Cormack and Metcalfe³⁶. In addition, some studies have included energy use associated with the production of farm infrastructure (e.g., buildings and machinery), whereas others have only focused on energy use associated with feed, fertilizer and other variable inputsReference Alonso and Guzman^69, Reference Helander and Delin⁸⁴. Despite the variation in methods, it is possible to see that organic production outperforms conventional for nearly all of the products listed in Table 2. Again, lower levels of inputs are the main reason for the increased efficiency of organic farming within these studies. There are some exceptions, however, for instance the Cormack and MetcalfeReference Cormack and Metcalfe³⁶ study found that the lower yield and the inclusion of fertility-building crops within stockless arable farms resulted in a lower energy efficiency overall. Guzmán and AlonsoReference Guzmán and Alonso⁸⁵ also found that net efficiency is lower in organic olive production, mainly due to incorporated organic material originating from other ecosystems, although the organic systems performed better in terms of non-renewable energy use efficiency. Nguyen et al.Reference Nguyen, Haynes and Goh⁸⁶ also reported greater machinery use for weed control in organic pea production, which resulted in a lower energy efficiency overall, in a comparison of mixed farming systems in New Zealand.

Table 2. Energy ratios (energy output divided by input) for conventional and organic crops and livestock. All of the studies cited here contain statistical uncertainties; some authors have calculated these and others not, where individual values are presented these represent the average energy ratio. Ranges are presented where different treatments or sites have been used to compare the production systems (e.g., Nguyen and HaynesReference Nguyen and Haynes⁵⁷).

Emergy studies

Most of the studies referred to above have concentrated on fossil-fuel use when comparing the efficiency of organic and conventional systems. A limited number of studies have taken a different approach, using the emergy method to account for all energy inputs to the system, including human activity and ecosystem servicesReference Odum^28, Reference Bakshi²⁹. Emergy accounts for these inputs through an assessment of total amount of energy used for their creation. Scienceman (1989) therefore explains emergy as a calculation of the ‘energy memory’ of systemsReference Brown and Herendeen³⁰. A common unit (i.e., solar emjoules—sej) is used within emergy assessments, to express the amount of emergy required to produce a gram (sej  g⁻¹) or joule (sej J⁻¹) of a particular resource, commodity or service. This is referred to as the ‘solar transformity’. The emergy efficiency of different agricultural production systems can be compared through their relative solar transformities, with a lower transformity value per unit indicating a greater efficiency.

In addition to exploring the solar transformities of production systems, some emergy studies have also investigated the emergy yield ratio (EYR). This is an expression of the total emergy (in sej) within a system to the emergy purchased on the market (e.g., fossil fuels). In this sense, the EYR is a ‘measure of the systems net contribution to the economy beyond its own operation’Reference Odum²⁸. Other studies have also explored the environmental loading ratio (ELR), which is the ratio of purchased and non-renewable local emergy to renewable environmental emergy. This measure can be used as an indicator of environmental stress and technological levelReference Odum²⁸. Emergy flow and emergy density are also used to explore levels of environmental stress through comparing the spatial and temporal concentration of emergy within different systems (e.g., emergy per unit of time or area)Reference Castellini, Bastianoni, Granai, Bosco and Brunetti⁸⁷.

Castellini et al. used the emergy approach to assess the efficiency of organic and conventional poultry production systems in ItalyReference Castellini, Bastianoni, Granai, Bosco and Brunetti⁸⁷. Their study found that the solar transformity was lower within the organic system assessed, despite a lower level of production. This was due to the avoidance of chemical fertilizers and pesticides for the production of feed. In addition, the study found that the emergy costs for cleaning/sanitization of buildings were lower in the organic system, as a result of organic regulations only permitting molecules for sanitization that have a low environmental impact. Through an assessment of the energy yield ratio, the same study revealed a reduction in external inputs and in ecosystem stresses under organic management. The organic system also had a higher use of renewable energy, as expressed through the ELR (see Table 3). In particular, this was through its reliance on organic sources of fertility (poultry and cow manure) as opposed to synthetic fertilizer. The emergy density within the conventional system was also approximately eight times higher than the organic, as a result of much greater use of non-renewable inputs.

Table 3. Results from five ‘emergy’ studies comparing organic and conventional production. Results expressed as ‘sej’=solar emergy joules or emjoules, i.e., units of solar energy that would be required to generate all the inputs to the farming system defined (expressed in sej J⁻¹ or sej⁻¹ or sej ha⁻¹.

Pizzigallo et al.Reference Pizzigallo, Granai and Borsa³¹ also found a higher ELR for conventional systems of wine production in Tuscany, Italy, finding that the use of non-renewable resources on the conventional farm was approximately 15 times greater than that of renewable, whereas for the organic farm this level was only 10 times greater. The higher ELR for the conventional system was a result of the increased soil erosion and the use of manufactured fertilizers. Furthermore, the conventional system used a higher amount of agricultural machinery and fuel, plus a greater amount of glass for bottling (the organic farm used bottles that were lighter). The difference is thus not intrinsic to the farming system. The organic farm also had a lower solar transformity indicating a less resource-intensive production system. However, the conventional farm was disadvantaged by a greater amount of on-farm processing and the fact that only the best grapes were harvestedReference Pizzigallo, Granai and Borsa³¹.

La Rosa et al.Reference La Rosa, Siracusa and Cavallaro⁸⁸ also used the emergy approach to compare organic and conventional red orange production from four Sicilian farms; this study also found a higher renewable energy use on the organic farm assessed, which contributed to a higher EYR and a much lower ELR. This was the result of a greater reliance on organic sources of fertility within the organic system, compared to the energy-intensive manufactured fertilizer inputs used on the conventional farm. Furthermore, the conventional system used a greater amount of electricity per hectare. Conversely, the same study found a higher solar transformity (sej g⁻¹) associated with two of the three organic farms assessed, as a result of the lower product yield.

In a comparison of wheat production in Denmark, Coppola et al.Reference Coppola, Haugaard-Nielsen, Bastianoni, Østergård, Neuhoff, Halberg, Alföldi, Lockeretz, Thommen, Rasmussen, Hermansen, Vaarst, Lueck, Caporali, Jensen, Migliorini and Willer⁸⁹ also found a lower emergy flow in organic production systems (i.e., lower sej ha⁻¹ yr⁻¹) due to an absence of man-made fertilizers. Organic seed production was found to be more resource-intensive than conventional, and more field operations and greater machinery use were reported for the organic system. The study also reported a lower solar transformity for the organic wheat crop, suggesting a reduced efficiency per unit of biomass (straw and grain) despite the lower environmental impact, as expressed within the reduced ELR in Table 3. Ghaley and PorterReference Ghaley and Porter⁹⁰ also used the emergy method to compare two farming systems in Denmark; a conventional wheat production system and an organically managed combined food and energy (CFE) system consisting of mixed arable cropping, clover ryegrass swards and woody biomass production. The emergy use in the conventional wheat system was 7.4 times higher than in the CFE, as a result of increased use of manufactured fertilizer and higher rates of soil erosion. The multiple yield components of the CFE system resulted in a greater output and a higher EYR. A lower ELR was also reported for the CFE system due to the reliance on renewable inputs (e.g., biologically fixed N). This study concludes that the CFE system provides a greater contribution to the economy compared with a wheat monoculture. The authors also suggest that such a diverse system could provide a suitable way forward for food and energy production, if an appropriate economic and policy environment could be provided.

Emergy is clearly a useful method that presents a more complete picture of the energy and ecosystem costs and benefits associated with a range of farming systemsReference Gomiero, Paoletti and Pimentel¹⁴. Unlike energy accounting, the emergy approach allows for an assessment of a productive system's relationship with the environment, in terms of energy flows. It takes into account environmental inputs that are usually treated as ‘free’ (e.g., ecosystem services)Reference Bakshi^29, Reference Pizzigallo, Granai and Borsa³¹, assessing the amount of natural ‘labor’ required to obtain a given productReference Castellini, Berri, Le Bihan-Duval and Martino⁷⁹. Despite these perceived advantages, the emergy approach has been criticized on the basis of the subjective judgments and associations that lead to the allocation of solar energy values to inputs such as wind and rainReference Jones²⁰. The lack of a sufficiently detailed explanation behind the underlying methodology within many of the calculated solar transformities has contributed to this criticismReference Hau and Bakshi⁹¹, although recent attempts have been made to apply uncertainty calculations to the emergy approachReference Li, Lu, Campbell and Ren⁹⁴. Hülsbergen et al.Reference Hülsbergen, Feil, Biermann, Rathke, Kalk and Diepenbrock⁹⁵ also state that inclusion of solar radiation in the energy balance can mask the variation of fossil energy input influenced by different husbandry techniques, as fossil energy is often a small proportion of the total emergy use when considering solar inputs. Conversely, it can also be misleading to focus only on the use of energy on-farm (i.e., without accounting for the embodied energy associated with inputs and natural services), providing an advantage to farms dependent on external sources for the maintenance of higher levels of productionReference Topp, Stockdale, Watson and Rees⁹⁶. It has been suggested that a combined approach of using LCA and emergy analysis may help both the methods to improve, allowing LCA to account for ecosystem services, and overcoming problems with allocation (i.e., partitioning energy inputs between multiple outputs) found within the emergy approach. This combined method was adopted by Pizzigallo et al.Reference Pizzigallo, Granai and Borsa³¹, who used LCA methods to comprehend and disaggregate the productive systems assessed, together with the application of emergy to account for the energy contribution of ecosystems.