Implications for Future Research

With the exception of Bengtsson et al. (2005), who compared the biodiversity of conventional and organic farms, and Mondelaers et al. (2009) who analyzed several environmental indicators including GHG between CFS and OFS, this meta-analysis represents the first attempts to empirically identify the relationship of independent structural variables and performance variables of EE or GHGE by previous studies on the comparisons of conventional and organic farming systems.

Findings from the empirical validation of structural variables with organic superiority of EE or GHGE will improve the reliability of energy use and GHGE studies in the future.

This meta-analysis may contribute to developing the research model of energy use and GHGE studies at the farm-level.

There have been a number of hot controversial viewpoints about the methodological approaches of comparative studies on EE and GHGE between CFS and OFS. Without defining structural variables which determine the difference of EE and GHGE between CFS and OFS, it is considered difficult to get reliable data from the EE and GHGE studies. For this reason, meta-analysis of previous EE and GHGE studies was performed.

In conclusion, on the basis of research framework of meta-analysis (Figure 2-2) and findings obtained from meta-analysis, the following research model for EE and GHGE studies in comparisons of OFS and CFS is developed as shown in Figure 2-7.

First, the findings obtained from meta-analysis manifest that among the 4 categories of the 12 structural variables, just 5 variables are significantly influential variables in determining the differences of environmental performances between CFS and OFS. The influential structural variables are the unit class of dependent variables, duration of data collection, sample size of surveyed farm, the type of farm product, and selection of energy coefficients.

Second, since the results of EE and GHGE in both systems can be shown to be significantly different according to the selection of units of dependent variables – EE and GHGE, the unit class should be defined clearly in independent EE and GHGE studies. Taking into account the efficiency of land use and energy productivity, dependent variables of product-based unit (ton, kg or liter) are better suited to

assess EE and GHGE results than those of area based unit (ha). Therefore, it is recommended that research results using both units of per ha basis and per ton basis should be reported for application for future studies as well as for utilizations of policy making data.

Third, Bertilsson et al. (2008) asserted that organic systems often have lower nutrient inputs and rely on nutrients previously added to soils before conversion to organic agriculture. It may take decades until yields decline to levels reflecting true organic practices. Thus, there is a risk that energy outputs from organic systems can be overestimated.

Their assertion assumes that the longer the duration of organic farming, the lesser the organic superiority of EE or GHGE. However, contrary to their assertion, the result of this meta-analysis indicates that the longer duration of data collections in both farms leads to positive results in organic EE and GHGE. Therefore, to obtain the reliable data from EE and GHGE studies on the comparisons of CFS and OFS, the longer duration of data collection and the larger sample size of surveyed farms are recommendable as far as possible.

It would be better if researchers gather larger samples comprising of longitudinal data to assess the lag effects of organic conversions. Since most of the organic farms do not originate from virgin lands and are cultivated by conversion from conventional farms, the longitudinal data is better in evaluating the genuine effects of organic farms in comparative studies on EE and GHGE.

Fourth, the result of meta-analysis shows that the differences of EE and GHGE between CFS and OFS can

clearly vary according to the type of farm products. For this reason, the comparisons of EE and GHGE between both systems should be conducted between same types of farm products, and farm product type compared in both systems should be defined clearly. Especially when multi-culture is compared, the kinds of farm products included in multi-culture should be homogeneous between both systems.

Fifth, the results explain that the type of dependent variables (EE or GHGE), data source, studied country, studied date, farm size and type of culture are not influential in the comparisons of EE or GHGE between CFS and OFS. Hence, in comparative studies on EE and GHGE between CFS and OFS, it is not so necessary to take into consideration these variables.

Sixth, as being debated in EE studies, the selection of energy coefficients of inputs appears strongly associated with the result of EE. Hence, the recent or reasonable values of energy coefficients are recommended for the calculation of energy use.

Finally, for the advancement of future EE and GHGE studies in agriculture, one of the urgent requirements is the presentation of sufficient data and information as far as possible through the advanced and appropriate statistical approaches.

Most extant studies of EU (energy use) and GHGE in agriculture have reported just mean amounts of EU or GHGE to assess the environmental performance in agriculture. As a result, even though the numbers of studies on the comparison of conventional and organic environmental

performance have increased vigorously to date, they can not provide successive researchers with sufficient and detailed data. This insufficiency appears as one of the limitations for more developed follow-up studies. Improved follow-up studies can also provide the appropriateness of EU and GHGE studies for future researchers and policy makers.

To facilitate future meta-analyses, therefore, EU and GHGE studies should explicitly report sample sizes, independent and dependent variables, correlation coefficients and their statistical significance, etc. In addition, future studies should present more clearly the amounts of input or energy output, energy coefficients of inputs, applied GHG conversion factors, and the results of both units.

And to assess the differences of environmental performances between CFS and OFS, the comparisons of more various environmental indicators as well as EE and GHGE, will be required in future studies. Such advancement in the conduct of studies will develop the research model for the effective EU and GHGE studies in agriculture. As a result, such a development in studies will be able to provide more abundant and reliable data in searching for solutions to improve the efficiency of energy use and to mitigate GHGE at the farm and national levels.

III. An Empirical Field Study : Comparisons