Cassman and Connor also suggested that high YRRs were an artefact of contrasting intensified organic management with resource-constrained subsistence agriculture in developing nations. Further critique focused on the application of unbalanced nutrient application rates where organic systems receive organic manure but conventional crops do not, and on the need to quantify the effect of caloric yield per unit area and time in organic rotational systems with cover crops, rather than yield per hectare in a single season. The first framed their analysis in response to the emerging debate and as an assessment of the role of OA in the future of world agriculture. de Ponti et al.utilized 362 organic-conventional yield comparisons, collected exclusively from peer-reviewed sources in which OA treatments met IFOAM standards . They therefore rejected 86% of the data presented by Badgley et al. , and concluded that OA yields are on average 20% lower than yields under conventional management, but with large variance. Exponential regression showed that the gap between organic and conventional yields grows as conventional yields increase and OA becomes nutrient limited,growing blackberries in containers addressing earlier critique that OA should be compared with conventional best management practices rather than subsistence systems .
Seufert et al.framed their meta-analysis in response to the proposition that OA can be a solution to major challenges in the global food system, including the need to minimize environmental impacts and agricultural land expansion from low-yielding farming systems. Sixty-six primary studies, 90% peer-reviewed, were used to generate 316 yield comparisons in which OA conformed to commercial organic certification standards . Only primary studies reporting means and variance were included, with cumulative effect size weighted by individual or multiyear observations of variance in the hierarchical, categorical mixed model employed. Seufert et al.’s results indicated 5 to 34% lower organic yields, depending on cropping system and site characteristics. They however cautioned that yield is but one of many metrics by which OA should be judged, while also suggesting that ideologically charged debate should be minimized in favour of systematic evaluation. Nature also published a Forum on Seufert et al. , in which Reganold positively interpreted their meta-analysis as evidence that can ‘…underscore the potential for organic farming to have an increasing role in a sustainable food supply’ , while Dobermann considered the problematic nature of field-scale experimentation and suggested that ‘ It is time to accept that various types of agriculture can have a place in feeding the world, depending on the availability of land, the degree of self-reliance of agricultural systems in terms of critical inputs to value chains , the scale of food production, and the desired and feasible trade in agricultural goods’ .
Connor conversely argued that Seufert et al.were misguided and lacked critical thinking that ‘…adds confusion to the current debate of which food production systems can best feed and green a world expected to reach 9.2 billion human inhabitants by 2050’, while proposing that ‘The solution must be found in greater yields and cropping intensity’ . Framing OA as the most feasible ecological option that responds to the imperative to ‘adopt resilient and sustainable agricultural practices as soon as possible’, Ponisio et al.aimed to provide new evidence and argued that all previous organic conventional comparisons were methodologically flawed. They specifically critiqued Badgley et al.for not accounting for variance or applying probability statistics. While Seufert et al.did account for variance, Ponisio et al.nonetheless critiqued the study for combining nested data from multiple trials without accounting for hierarchy in ways that introduce pseudo-replication and the risk of type-1 statistical error. Ponisio et al.therefore assembled an ‘…extensive dataset including over three times more yield comparisons than previous studies’ represented by 1071 comparisons from 115 studies contrasting conventional from organic and ecological agriculture . A hierarchical regression model was used to account for between- and within-study random variation, as well as between and within-year variation, with random effects nested within studies. Ponisio et al.found average organic and ecological yields to be 19% lower than conventional systems. Where diversified crop rotations and multi-cropping were practiced, this yield gap was declined to 9±4% and 8±5%.
They concluded that investment in analytically rigorous research aimed at eliminating this yield gap is justified given the urgent need for more sustainable and resilient production systems that overcome improve livelihoods of the rural poor. Responding to these debates and recognizing the lack of information on LEI agriculture as an alternative to both conventional and OA, Hossard et al.provided a more nuanced analysis. Their study analysed LEI maize and wheat systems in the US and Europe. While organic maize yields were 1.71 Mt ha–1 lower than conventional yields, LEI had a YRR averaging 1.25 times more than OA, while being statistically indistinguishable from conventional agriculture. Pesticide and fertilizer N use was reduced by 50 and 36%, respectively, in LEI compared to conventional systems. Organic winter wheat yields were also lower than with conventional practices, but with 70 and 28% less pesticide and mineral N inputs. LEI winter wheat conversely yielded 1.43 times more than OA. Framing their work as a comprehensive response to the promotion of OA an ‘environmentally friendly’ method proposed to meet population growth and food security challenges, Knapp and van der Heijden presented what they termed as a ‘global meta-analysis’ of yield stability over time in both OA and CA compared to conventional agriculture. Utilizing data provided by Ponisio et al. , they analysed 443 multiple-year observations, 86% of which were derived from studies in developed nations . Considering OA, they concluded that OA has 15% lower temporal yield stability than conventional systems, and suggested increased emphasis on studies of the resilience of cropping systems considering growing population and food demands.Framed considering the need to arrest soil degradation in different environments, Van den Putte et al.provided the first meta-analysis described as an evaluation of CA by assembling 563 European comparisons for five crops. They concluded that RT without crop residue retention leads to significant yield reductions of 13 and 4% in maize and winter cereals, respectively. NT with residues retained resulted in a 8.5% reduction in yield relative to conventional tillage, although residue management practices under the latter were not clearly defined. RT with surface residues conversely reduced yields by ca. 4.5%. Van den Putte et al.also unpacked environmental and management influences on NT and RT performance,square pot concluding that biotic stresses and deep seed placement reduces yields in dry climates/years, but not on sandy or clayey soils. Rusinamhodzi et al.framed their paper in light of soil degradation concerns, while adding emphasis to smallholder farming systems. They used 364 paired comparisons to identify how long-term mulch retention, rotation, effects and precipitation regimes influence maize yield responses to CA, compared with conventional tillage under sub-humid climates. Their results, largely based on data from the Americas and sub-Saharan Africa, indicated increasing CA maize productivity over time when residues were retained with rotations and high N inputs. NT or RT without residue retention however resulted in yield depression.
Van den Putte et al.and Rusinamhodzi et al.therefore both suggested targeting and adapting CA to specific biophysical environments where these practices are most appropriate. Zengh et al.open their paper indicating the need to resolve uncertainties limiting the ‘smooth and wide application’ of CA in China. Their meta-analysis of 123 comparisons characterized environmental influences on yield response in 5+ year duration maize, rice and wheat trials. NT was found to increase yield by 6.3% when residues were retained, particularly in environments with low or temporally unstable precipitation, though crop responses differed, and rotational effects were not considered. Corbeels et al.conversely focused their meta-analysis in sub-Saharan Africa to study yield patterns with differing rotation, mulch and N rates. Their work was framed as an assessment of proposals that CA can limit soil degradation in smallholder farming systems and confirmed the importance of N fertilization, mulch and crop rotation to avoid yield depression. They found that NT without residues or rotation depresses yield, and that CA treatments responded best to high nitrogen rates. Although the suitability of CA for smallholder farmers in developing countries had been questioned for several years , it was not until a study in Nature by Pittelkow et al.that debate over the relevance of meta-analysis surfaced. Pittelkow et al.describe their work as a ‘global meta-analysis’ , which included 5463 yield comparisons from 43 crops across 63 countries with a robust methodological approach. Measured across all data, they concluded that NT lowers yields by an average of 5.7% relative to tillage for a variety of crops, although positive effects were found in more arid climates when rotations and residue retention were applied. Across climates and observations, the addition of rotations and residue retention to NT also reduce yield loss by 2.5%. In comments sent to Nature, So et al.critiqued Pittelkow et al.as geographically biased – and hence not globally representative – because 69% of observed data points were from North America and Europe alone. Others emphasized that measurements of yield under CA may not be as important as yield stability and resilience over time, while also suggesting that CA has crucial environmental benefits 1. Fried rich et al.criticized Pittelkow et al.for insufficient details regarding how NT plots were managed in primary studies, leading to mis-classification of CA treatments, a topic for which earlier guidelines had been proposed by Derpsch et al. . Friedrich et al.did not however attempt to quantify the number of studies which may have been misclassified using the public database provided by Pittelkow et al. . Khun and Hu conversely commented in Nature’s online comments that ‘…increased yields in drylands with conservation tillage have been acknowledged by Pittelkow et al. , but not fully appreciated with regard to food security. The benefits of CA on crop yields is of particular significance during dry years when famine in dry lands is not caused by lack of global production, but regional access to affordable food after poor harvests’. They concluded that ‘… in the light of the actual spatial and temporal dimensions of conservation tillage impacts on crop yields, conclusions drawn from oversimplifying meta-studies like that of Pittelkow et al. , based on one global average, carry the serious risk of contributing to poorly researched policy development and agricultural practice’.Rusinamhodzi addressed these issues through meta-analysis of a smaller but similarly international dataset, though limited to maize and examining variants of CA practices . He framed this study in light of how yield stability is inadequately studied under CA, and provided evidence that mulch has greater benefits in semi-arid environments. Yield benefits from CA also were found only when maize was rotated with legumes in arid climates and with N addition. Rusinamhodzi however positioned their results in a wider context by concluding that the maintenance of permanent soil cover is a challenge in mixed crop-livestock systems where trade-offs with feed are common. They therefore suggested targeting CA by socio-ecological niche. Making use of the database initially developed by Pittelkow et al.and Lundy et al.analyzed 6005 and 2779 comparisons, respectively. The former framed their work in light of the potential environmental and economic advantages of NT in feeding a growing world population, while also carefully differentiating NT from CA in the papers’ supplementary materials. The latter strongly framed their work as informing debates around the appropriateness of CA in sub-Saharan Africa, though only 8% of their dataset included observations from this region. Each paper also heavily discussed CA, although analyses did not include CA sensu stricto. Rather, both papers concentrated on crop species, N management, and environmentally specific yield responses to NT, with or without crop residues, compared with conventional tillage with similar residue management . Huang et al.also analysed 265 comparisons of the effect of NT on rice yield patterns in China, although they avoided discussion of CA entirely, despite considering residue retention and rotation in a portion of their dataset. Framing their paper in terms of the importance of overcoming yield gaps, climate change and climate-smart agriculture in sub-Saharan Africa, Steward et al.applied meta-regression to 1042 CA and conventional tillage comparisons for maize in moisture and heat stressed environments.