Crop yield responses to Conservation Agriculture practices in sub Saharan Africa a meta-analysis

1. Crop yield responses to
conservation agriculture practices
in Sub-Saharan Africa: a meta-
analysis
Raymond Sakyi, Marc Corbeels, Ronald Kühne, Anthony Whitbread
Department of Crop Production Systems in the Tropics, Georg-August
Universität, Göttingen, Germany
Agro-ecology and Sustainable Intensification of Annual Crops, CIRAD, Montpellier, France
corbeels@cirad.fr
18 -21 March, Lusaka, Zambia

2. Introduction
• Growing number of studies have been carried out
comparing the practices of conservation agriculture
(CA) to conventional tillage (CT) in Sub-Saharan
Africa
• Conducted under a range of conditions (climate, soil,
crop management, cropping systems)
• The effects of CA on crop yield compared to CT are
diverse

3. Aim of the study
• Better understanding of crop responses to CA
• Identifying the agro-ecological and management
conditions that favor positive crop responses to CA
• Contributing to better targeting the investments with
CA development and research

4. Methods
• Search of scientific literature on the effects of the CA
(no-tillage, crop residue mulching and rotations) on
crop yields in SSA
• 42 (peer-reviewed) papers were selected for the final
dataset with 61 independent studies
• Meta-analysis: random effects model to calculate
effect sizes
• Weighted mean difference in grain yield between the
CA and CT treatment
• Weight given to each study was calculated as the
inverse of the variance

5. Overall results
• Large variability in grain yield responses (from positive to negative) to CA
compared to CT
• NTM had the largest positive mean (378 kg ha-1) followed by NTR with a
positive mean of 142 kg ha-1, and then NT with a negative mean (- 24 kg
ha-1)

6. Short-term yield responses
• Positive responses: under conditions where water
stress occurs (e.g. dry spells), since mulching
increases soil water availability (e.g. Mkoga et al.
2010; Mupangwa et al. 2012; Thierfelder and Wall
2009).
• Negative responses: i) under high rainfall, as
mulching may exacerbate waterlogging (Thierfelder
and Wall 2012); ii) increased weed competition and
problems with seeding (Mashingaidze et al. 2012).

7. Crop responses in time
• Yield benefits are expected to accumulate over time, because CA is known to
gradually improve biological, chemical and physical properties of the soil
• Results from a meta-analysis of existing crop yield data from long-term
experiments in sub-Saharan Africa do not confirm this hypothesis, and show large
variation in the data
Data from: Vogel (1993), Lal (1997), Nehanda (2000), Moyo (2003) and
Thierfelder et al. (2013)

8. Rotations with legumes!
• meta-analysis of results from long-term experiments in sub-humid and
semi-arid regions of the world
Rusinamhodzi L, Corbeels M, van Wijk M, Rufino MC, Nyamangara J, Giller KE. Long-term effects of
conservation agriculture practices on maize yields under rain-fed conditions: lessons for southern Africa.
Agron Sustain Dev 2011;31:657-673.

9. Underlying reasons: soil carbon?
Location Soil
texture
Soil
depth
(cm)
Duration (yr) Soil C (Mg ha-1) ΔC
/year
Reference
CT CA
Mulching including rotation or intercropping
Monze, Zambia loamy sand 0-30 5 23.4 32.3 1.78 Thierfelder et al.
2013c
Henderson,
Zimbabwe
loamy sand 0-30 4 18.4 24.7 1.57 Thierfelder et al.
2012
Malende, Zambia sand 0-30 3 Non-significant differences Thierfelder et al.
2013c
Lemu,
Malawi
sandy clay
loam
0-30 6 Non-significant differences Ngwira et al. 2012
Zidyana, Malawi sandy loam 0-30 6 Non-significant differences Ngwira et al. 2012
Kayowozi, Zambia loamy sand 0-20 3 Non-significant differences Thierfelder et al.
2013c

10. Underlying reasons: soil carbon?
Location Soil texture Soil
depth
(cm)
Duration (yr) Soil C (Mg ha-1) ΔC
/year
Reference
CT CA
No rotation – mulching
Harare, Zimbabwe clay 0-20 8 18.7 26.8 1.01 Nyagumbo 2002;
Chivenge et al. 2007
Domboshawa,
Zimbabwe
loamy sand 0-20 8 13.0 20.7 0.96 Nyagumbo 2002
Madziwa,
Zimbabwe
sand 0-30 5 7.4 10.0 0.52 Thierfelder et al.
2012
Chikato, Zimbabwe sand 0-20 4 6.9 13.3 1.60 Thierfelder and Wall
2012
Hereford,
Zimbabwe
sandy clay
loam
0-20 3 37.5 43.3 1.93 Thierfelder and Wall
2012
Domboshawa,
Zimbabwe
sandy clay
loam
0-20 2 16.8 16.9 0.05 Nyamadzawo et al.
2008
Ibadan, Nigeria sandy-loam 0-10 8 13.4 24.3 1.36 Lal 1998b
Nyabeda, Kenya sandy-loam 0-30 5 Non-significant differences Paul et al. 2013
Abomey-Calavi,
Benin
sandy loam 0-15 1 Non-significant differences Saito et al. 2010

11. Effect on no-tillage
• less than 3 years: overall effect in terms of yield benefit is positive (88 kg ha-1)
more than 3 years: overall negative effect (-227 kg ha-1)
• in the longer term no-tillage without crop residue mulching triggers negative
impacts on crop production, which may be mainly due to a soil compaction or soil
surface crusting
Number of observations
0 20 40 60
Weightedmeandifference(kg/ha)
-4000
-2000
0
2000
4000
NT
Duration < 3 years
*
Number of observations
0 10 20 30 40
Weightedmeandifference(kg/ha)
-4000
-2000
0
2000
4000
NT
Duration > 3 years
*

12. Effect of mulching
• positive yield response seems to increase over time (from 294 to 487 kg ha-1)
• positive short-term effect on crop growth and productivity through increased
soil water conservation, and a positive long-term effect through enhancing soil
carbon levels and soil fertility in general.
Number of observations
0 20 40 60 80 100
Weightedmeandifference(kg/ha)
-8000
-6000
-4000
-2000
0
2000
4000
6000
8000
NTM
Duration < 3 years
*
Number of observations
0 20 40 60 80 100 120 140 160
Weightedmeandifference(kg/ha)
-8000
-6000
-4000
-2000
0
2000
4000
6000
8000
NTM
Duration > 3years
*

13. Fertilizer!
• response increases with N fertilization (from 85 to 391 kg ha-1)
• appropriate fertilization is critical for increasing crop productivity and the
availability of crop residues for mulching
Number of observations
0 20 40 60 80 100 120
Weightedmeandifference(kg/ha)
-6000
-4000
-2000
0
2000
4000
6000
N input < 100 kg/haNS
Number of observations
0 20 40 60 80
Weightedmeandifference(kg/ha)
-6000
-4000
-2000
0
2000
4000
6000
N input > 100 kg/ha
*

14. Effects of soil texture
• cop grain yields on sandy and clayey soils under CA were not significantly
different than yields under CT: 72 and 45 kg ha-1
• On loamy soils: significantly higher than that of CT:299 kg ha-1
Number of observations
0 20 40 60 80
Weightedmeandifference(kg/ha)
-6000
-4000
-2000
0
2000
4000
6000
Sandy soilNS
Number of observations
0 50 100 150 200 250
Weightedmeandifference(kg/ha)
-6000
-4000
-2000
0
2000
4000
6000
Loamy soil
*
Number of observations
0 10 20 30 40 50
Weightedmeandifference(kg/ha)
-6000
-4000
-2000
0
2000
4000
6000
Clayey soilNS

15. Effects of seasonal rainfall
• crop grain yields were overall significantly higher under CA treatments
compared to CT in all seasonal rainfall categories
• overall, crop grain yields were 143, 161 and 348 kg ha-1 higher under CA
compared to CT for < 600 mm, 600-1,000 mm and > 1,000 mm, respectively
Number of observations
0 20 40 60 80
Weightedmeandifference(kg/ha)
-6000
-4000
-2000
0
2000
4000
6000
< 600 mm*
Number of observations
0 20 40 60 80 100 120 140 160
Weightedmeandifference(kg/ha)
-6000
-4000
-2000
0
2000
4000
6000
600 - 1000 mm*
Number of observations
0 10 20 30 40 50
Weightedmeandifference(kg/ha)
-6000
-4000
-2000
0
2000
4000
6000
> 1000 mm*

16. Conclusions
• Mulching (crop residues) and rotations are required
for effective yield benefits
• (Short-term) yield benefits are variable = bottleneck
for adoption; farmers need immediate benefits from
investments
• Higher benefits in comparison with CT when fertilizer
is applied ( better-resourced farmers)
• Seems to work better on loamy soils…
• Rainfall?