Table 2.

Summary of prominent systematic and meta-analytical reviews of conservation agriculture (CA).

	Study
Criteria	Van den Putte et al. (2010)	Rusinamhodzi et al. (2011)	Zheng et al. (2014)	Corbeels et al. (2014b)	Pittelkow et al. (2015a)	Rusinamhodzi (2015)	Pittelkow et al. (2015b)	Lundy et al. (2015)	Huang et al. (2015)	Steward et al. (2017)	Knapp and van der Heijden (2018)
Type of study`	Meta regression	Meta-analysis	Meta-analysis	Meta-analysis	Meta-analysis	Meta-analysis	Meta-analysis	Meta-analysis	Meta-analysis	Meta-regression	Meta-analysis and meta-regression
Research question justification and framing	Soil degradation and erosion are important problems. CA is as a proposed solution, but systematic yield assessments are limited.	Soil degradation and erosion are important problems in smallholder farming systems. CA is as a proposed solution, but systematic yield assessments are limited.	CA is a recommended practice for sustainable crop production, but yield variability has been inadequately assessed	Soil degradation and erosion are important problems in smallholder farming systems. CA is as a proposed solution, but systematic assessment of yield stability is lacking.	CA is proposed as a method to address growing food security and development challenges, though impacts of no-tillage on yield remain contested.	Crop rotation and residue retention are important for yield and yield stability, but inadequately studied under CA.	NT may be important for feeding a growing world population while providing environmental and economic benefits, though impacts of no-tillage on yield remain contested.	CA is actively promoted in Africa and described as erosion controlling and ‘climate smart’, though yield outcomes may be limited by farmers’ ability to manage soil fertility.	Rice is critical for food security. Laborsaving and soil conserving technologies are needed. Environment and management effects on NT yield are poorly understood.	Climate change threatens food security in Africa. CA may be a ‘climate smart’ management option, but yields under different stresses, soils, and management practices have not been systematically assessed.	Population growth increases global food demands. Production increases must be sustainable. Yield stability under CA is inadequately addressed.
Primary response variable	Yield	Yield and yield stability	Yield	Yield and yield stability	Yield and yield stability	Yield	Yield and yield stability	Yield and yield stability	Yield and yield stability	Yield and yield stability as a function of precipitation and heat stress	Yield and yield stability (relative stability ratio)
Crop(s)	Fodder maize, grain maize, potato, sugar beet, spring and winter wheat	Maize	Cereals	Cereals, legumes, cotton	Multiple cereals, legumes, roots and tubers, tree crops, vegetables	Maize	Multiple cereals, oilseeds, legumes, roots and tubers, tree crops, vegetables	Cereals, legumes, roots and tubers, tree crops, vegetables	Rice	Maize	Multiple cereals, oilseeds, legumes, roots and tubers, vegetables
Paired comparisons (n)	563	364	123	261	5463	688	6005	2759	265	1042	2453
Geography	Europe (100%)	North America (50%) Africa (19%) Latin America (12%) S. Asia (<0.1%) Asia (<0.1%) Europe (<0.1%) Oceania (<0.1%)††	Asia (100% China)	Africa (100%)	North America (57%) Europe (12%) Africa (6%) Latin America (6%) S. Asia (9%) Asia (4%) Oceania (4%) Middle East (2%)	North America (33%) Africa (50%) Latin America (5%) S. Asia (5%) Asia (2%), Europe (2% Oceania (2%)	North America (38%) Europe (17%) Africa (9%) Latin America (8%) S. Asia (9%) Asia (8%) Oceania (8%) Middle East (3%)	North America (46%) S. Asia (18%) Europe (12%) Africa (8%) Latin America (6%) Asia (6%) Oceania (3%) Middle East (1%)	Asia (100%, all China)	Africa (72%) North America (16%) Latin America (6%) Asia (5%) Oceania (1%)	North America (60%) Asia (15%) Europe (10%) Oceania (7%) Africa (4%) Latin America (3%) Middle East (2%)
CA practice(s)	NT and RT	RT, NT, NT + rotation, NT + rotation +residues	CT, NT. And RT without residues, NT with residues	NT, NT + residues, NT + residues +rotation	NT + residues, NT +residues +rotation	NT, NT + rotation, NT +rotation +residues	NT	NT with residues OR without residues	NT	NT + residues, NT +residues +rotation	NT + residues, NT+ residues + rotation
Comparison system (control)	CT (residue management unspecified)	CT (residue removed)	CT residues removed	CT residues removed	CT with residue incorporated	CT	CT with residue incorporated	CT with residues OR without residues (paired with NT)	CT	CT with residue incorporated, burnt, or removed	CT with residue incorporated
Replicable procedure*	Yes§	Yes**	Yes	Yes	Yes	Yes**	Yes	Yes	Yes	Yes	Yes
References available*	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
Within- and between study analysis*	No	Yes	Yes	No	Yes	No	Yes	Yes	No	No	Yes
Sensitivity analysis*	No	No	No	No	No	No	Yes	Yes	No	No	Yes
Publication bias assessment*	No	No	No	No	No	No	Yes	Yes	Yes	No	Yes
Data weighted*	No	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes‡‡	Yes
Software described*	Yes	No	Yes	Yes	Yes	No	Yes	Yes	Yes	Yes	Yes
Dataset availability*	No	No	Yes	Yes	Yes	No	Yes	No	Yes	Yes	Yes
Clear definition of systems comparison†	¶	Yes	No	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
Quantifying and equalizing nutrients†	Unclear	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
Clarity in experiment design and replication†	Yes	Yes	No	Yes	Yes	Yes	Yes	Yes	Yes	No	Yes
Data sources	Peer reviewed (70%) Conference proceedings (30%)	Peer reviewed (88%) Grey literature (12%)	Peer reviewed (100%)	Peer reviewed (100%)	Peer reviewed (100%)	Peer reviewed (90%), conference proceedings (2%), theses (7%)	Peer reviewed (100%)	Peer reviewed (100%)	Peer reviewed (100%)	Peer reviewed (100%)	Peer reviewed (100%)
Major findings	NT reduces yield 8.5%. Strategic deep tillage and diversified rotations reduce negative effects.	Crop rotation with high N is crucial for CA. Mulch cover in high rainfall areas lowers yields by waterlogging. CA needs to be targeted and adapted.	Differential CA effects result from regional variation climate and crops. CA increases maize but reduces wheat yield. Residues are needed for maize and seasons with warm/dry climates.	NT without residues or rotation depresses yield. CA responds best to high N rates. Precipitation had no effect because most studies failed to report within season rainfall.	NT reduces yields 5.7% overall. Residue retention and rotations mitigate this effect, but not entirely.	CA yield advantages only significant with high N rates and low precipitation.	NT yields are reduced without N addition. Site-specific adaptation of NT systems is needed to attain yield goals.	Nitrogen fertilization is important in counteracting yield declines in NT systems.	NT decreased yield in rice-rice but increased in rice-upland systems, though with variation depending on climate and soils.	CA improves maize yield with increasing drought or heat stress with interaction between soil moisture and heat stress mediated by soil clay content.	Temporal yield stability under NT does not differ significantly from CT; transition to NT does not affect yield stability

CA refers to conservation agriculture. NT (without residue or rotation, unless specified), RT and CT indicate no-, reduced- and conventional-till, respectively. YRR indicates yield response ratio.

^*,†Summarized in Table 3.

^‡Grey literature includes academic and research sources lacking evidence of peer review. Conferences include those with edited published proceedings.

^§Statistical analysis replicable, literature search not replicable due to lack of clear description on search terms and databases utilized.

^¶Criteria for what qualifies as reduced tillage not clarified, with the exception of lack of soil inversion. Unclear if no-tillage treatments involve residue retention or rotation.

^**Journal databases used not clarified.

^††Percentages indicate study number rather than paired observations.

^‡‡Observations weighted by replication, plot and yield sampling area.