This document discusses estimating returns to fertilizer from crop simulation modeling in Malawi. It provides an overview of different data sources and methods used to measure yield response and their reliability. Crop simulation modeling was used to estimate yield responses under smallholder conditions. The modeling found that hybrid maize varieties, early planting, good agronomic practices, and the application of fertilizer can all increase yields and returns. Specifically, the modeling estimated local maize varieties had a yield response of 18 kg of grain per kg of nitrogen applied, while hybrid varieties responded with 22 kg of grain per kg of nitrogen.

1. 1
Returns to fertilizer
and program efficiency:
Estimation techniques & results
from crop simulation
Andrew Dorward & Ephraim Chirwa,
School of Oriental and African Studies, University of
London
Wadonda Consult

2. Outline
 Background – policy importance
Returns to fertiliser (kg grain yield per kg fertiliser
or nutrient N &/or P) are critical to the production
benefits & economic returns to the programme
Therefore …..
 What is the yield response?
Estimation techniques’ results & reliability
 How can it be improved?
Crop simulation modelling results & policy
implications
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July 2014

3. Returns to fertiliser: yield response
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July 2014
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0.50
1.00
1.50
2.00
2.50
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50
100
150
200
250
300
30% 40% 50% 60% 70% 80% 90% 100% 110% 120%
US$mill
% of Simulation NUE (23.4 kg/kg hybrid, 18.0 local)
NPV (US$ mill)
BCR
Fiscal Efficiency

4. What is the yield response?
 Difficult to obtain reliable information on smallholder
yields and yield responses
 Different data sources & methods with different
reliability and bias
 Variability across years (rainfall, population growth,
changes in varieties & cropping methods)
 Variation between areas & across farms (rainfall,
population density, varieties, soils & cropping
methods)
4
July 2014

5. Data sources – methods & reliability
5
Farm
surveys
• General random errors from farmer & enumerator
errors, especially small plots
• (Omitted variables over-estimate yield response)
• Farmer
estimates
area
• Under-estimates of smaller plot areas & over-
estimates of larger plot areas – average over-
estimate
• Clustering in acre fractions
• GPS area • Should be reliable
• Farmer
estimates
production
• Possible over- or under- estimates from farmer
reporting & measurement units.
• Possible under estimates from omission of green
maize & multiple respondents
• Differential bias effects on yield response?
• Crop
cutting
• Over estimates of yield: production & yield
response affected by bias in area estimates
• Fertiliser
use
• Under reporting of sales & purchases leads to
under-estimates of yield response
On farm
trials
• Common over-estimate of yield & yield response
due to farmer selection & crop management
Crop
simulations
• Model over- or under- estimates yield response
• Over estimate of yield response, no pest & disease

6. Yield response estimates
Study
Response
kg/kgN
Yield (kg/ha)
Local Hybrid Local Hybrid
Chibwana et al (2010) (subs) 2008/9 12 13 1,312 1,510
Holden & Lunduka (2010,3) 2006,7,8 9? 14? 1,609
IHS2 – AISS2 (adjusted yield) 2004,6,8 6.6 – 13.9 682
NACAL (pure stand only) 2006/7 n.a. 928 1,803
Crop simulation (with subs) 2012/13 18 23 1,392 1,921
Makumba (2013) hybrid 2010/11 16.8
0 Fert 2,483
Fert 4,003
Kamanga (2013) CR hybrid 2003/4 29
1 weed 900
2 weed 1,300
SOAS (2008) Summary/review ‘median’ 15 22

7. National food security: consumption,
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2008/9
2009/10
2010/11
2011/12
2012/13
2013/14
'000MT
Total consumption (MT)

8. National food security: consumption,
production with subsidy
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2009/10
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2011/12
2012/13
2013/14
'000MT
Total consumption (MT)
Production with subsidy (MT)

9. National food security: consumption,
production & surplus/deficit with subsidy
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2002/3
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2008/9
2009/10
2010/11
2011/12
2012/13
2013/14
'000MT Domestic surplus (deficit) before subsidy (MT)
Domestic surplus (deficit) with subsidy (MT)
Total consumption (MT)
Production with subsidy (MT)

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July 2014
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2011/12
2012/13
2013/14
'000MT Domestic surplus (deficit) before subsidy (MT)
Domestic surplus (deficit) with subsidy (MT)
Domestic surplus (deficit) without subsidy (MT)
Total consumption (MT)
Production with subsidy (MT)
Production without subsidy
National food security: consumption,
production & surplus/deficit without subsidy

11. Crop simulation
 Commissioned maize simulation study under smallholder conditions
(Anthony Whitbread et al, Goettingen University, now at ICRISAT)
 Review & meta-analysis of response studies:
 limited / conflicting/ weak information
 Hybrid 30-80 kg grain/kg N @ 15-30 kg N/ha
 Local 10-30 kg grain/kg N @ 15-30 kg N/ha
 APSIM model
 calibrated for smallholder conditions (eg weeds)
 historical (1927-2004) daily climate information for Kasungu
 4 sandy or sandy clay loams (deep & shallow soils) Centre & South
 Range of management practices matching 2008 AISS2 observations
Planting date & density, N & P rates & times, good & poor weeding
 Used results to generate regression estimates of crop responses
 Evaluated crop responses using regression coefficients at observed
2012/13 crop management means
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July 2014

12. Crop simulation findings
 Influences on yield & yield response
 Mean yield and fertiliser response estimates
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July 2014

13. Illustrative N Response, HYBRID by plant
population & weeding, without & with P
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July 2014
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5000
0 15 30 60 120
Grainyield(kg/ha)
Fertiliser N rate (kg/ha)
Low Good
Low Poor
Mod Good
Mod Poor
High Good
High Poor
Chitala 0P
Variety: Hybrid
0
1000
2000
3000
4000
5000
0 15 30 60 120
Grainyield(kg/ha)
Fertiliser N rate (kg/ha)
Chitala 10P
Variety: Hybrid
Source: Whitbread et al 2013

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1000
2000
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0 15 30 60 120
Grainyield(kg/ha)
Fertiliser N rate (kg/ha)
Low Good
Low Poor
Mod Good
Mod Poor
High Good
High Poor
Chitala 0P
Variety: Local
0
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2000
3000
4000
5000
0 15 30 60 120
Grainyield(kg/ha)
Fertiliser N rate (kg/ha)
Chitala 10P
Variety: Local
Illustrative N Response, LOCAL by plant
population & weeding, without & with P
14
July 2014
Source: Whitbread et al 2013

15. Simulated yield response
 Importance of
 hybrid seed
 early planting
 good agronomy
 potential for lower N application rates
 variable returns to N
 Good potential returns to N and impact
 Nutrient responses with average smallholder
management
 Local 18 kg grain/kg N (@37 kg N/ha)
 Hybrid 22 kg grain/kg N (@47 kg N/ha)
 Hybrid without fertiliser + 600kg/ha
15
July 2014

16. 16
Returns to fertilizer
and program efficiency:
Estimation techniques & results
from crop simulation
Andrew Dorward & Ephraim Chirwa,
School of Oriental and African Studies, University of
London
Wadonda Consult