LCA Food 2012 in Saint Malo, France! - Manifestations et colloques ...

KEYNOTE SESSION 8 th Int. Conference on LCA in the
Agri-Food Sector, 1-4 Oct 2012
(QB). A significant QB indicates that there are differences in the effect sizes between different classes of a
categorical variable (Rosenberg et al., 2000). All statistical analyses were carried out in MetaWin 2.0
(Rosenberg et al., 2000). For representation in graphs effect sizes were backtransformed to response ratios.
3. Results
The overall organic-to-conventional yield ratio is 0.75 (with a 95% CI of 0.71 to 0.79), meaning that
across the 316 yield comparisons organic yields are 25% lower than conventional yields (Fig. 1a). This result
only changes slightly (yield ratio of 0.74) if the analysis is limited to studies following high scientific quality
standards (Fig. 2).
Figure 1. Influence of different crop types (a), plant types (b) and crop species (c) on organic-to-conventional
yield ratios. Only those crop types and crop species are shown that were represented by at least 10 observations
and 2 different studies. Values are effect sizes with 95% CIs. A significant response is when the CI
does not overlap 1. The number of yield observations in each class is shown in parentheses. The dotted line
indicates the cumulative effect size across all classes.
The performance of organic systems varies substantially across crop types and species (Fig. 1a-c). Only
categories with a significant QB are presented in Figures. Organic yields of fruits and oilseed crops show a
small, but not statistically significant, difference to conventional crops (their CI overlap zero), whereas organic
cereals and vegetables have significantly lower yields (-26% and -33% respectively) (Fig. 1a). These
differences seem to be related to the better organic performance (referring to the relative yield of organic to
conventional systems) of perennial over annual crops and of legumes over non-legumes (Fig. 1b). Marked
differences can, however, also be observed between crop species of the same crop type -- maize outperforms
wheat and barley yields under organic management (Fig. 1c).
Part of the yield response can be explained by differences in the amount of nitrogen (N) input received by
the two systems (Fig. 3a). When organic systems receive higher quantities of N than conventional systems,
organic performance improves, whereas conventional systems do not benefit from more N. In other words,
organic systems appear to be N limited, whereas conventional systems are not. To achieve yields that are
comparable to conventional systems, organic agriculture thus appears to require higher N inputs. This could
be due to organic N inputs being less readily available to plants. Even if the total amount of N in soils managed
with organic or conventional methods do not differ, the composition of the N pools often do (Stockdale
et al., 2002). Soils under organic management often have high organic matter and organic N pools but low
mineral N content (Stockdale et al., 2002). The release of plant-available mineral N from these organic pools
is slow and does often not keep up with the high crop N demand during the peak growing period (Berry et
al., 2002; Pang and Letey 2000). Nitrogen availability has thus been found to be a major yield-limiting factor
in many organic systems (Berry et al., 2002; Clark et al., 1999).
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a
Crop type
0.6 0.8 1 1.2
Organic:conventional yield ratio
All crops (316)
Fruits (14)
Oilseed crops (28)
Cereals (161)
Vegetables (82)
b
Plant type
0.6 0.8 1 1.2
Organic:conventional yield ratio
Legumes (34)
Non-legumes (282)
Perennials (25)
Annuals (291)
c
Crop species
0.6 0.8 1 1.2
Organic:conventional yield ratio
Maize (74)
Barley (19)
Wheat (53)
Tomato (35)
Soybean (25)