Marginal Fertilizer Response Across African Maize Research Domains
Marginal Fertilizer Response Across 10 African Maize Research Domains
This ranking compares marginal maize yield response to nitrogen at a common application rate of 50 kg N/ha. The metric shows how much additional maize the fitted response curve predicts from the next kilogram of nitrogen at that point. Higher values rank higher.
The entities are research or inference domains rather than national averages. Country names provide geographic context only because OFRA inference domains reflect agronomic and environmental similarity rather than national borders alone.
The table is a compiled research dataset based on 4 sources, with row-level source and method notes shown in the ranking table. Two sources provide the coefficients used in the calculations, while two additional OFRA references document the broader dataset and methodology.
Coverage: 10 confirmed maize-N research domains across 9 African countries. Unit: kg maize per additional kg N at 50 kg N/ha. Status counts: 10 calculated values, 0 directly published values, 0 forecasts and 0 modeled projections.
Achefer, Ethiopia has the highest point estimate in the 10-domain comparison.
Kapchorwa, Uganda ranks tenth within this confirmed research sample.
Calculated from the fifth- and sixth-ranked raw values.
Nine OFRA-IT locations plus the Sudan Savanna maize-N domain in Nigeria.
Two coefficient sources plus two supporting OFRA dataset and methodology references; reviewed as a 2026 research snapshot.
All ranking values are derived from published response-function coefficients; none is a forecast or modeled future value.
What Marginal Fertilizer Response Measures
Crop response to nitrogen is nonlinear. An additional kilogram of nitrogen can produce a different yield gain depending on how much nitrogen has already been applied. Comparing marginal response therefore requires the response curves to be evaluated at the same nutrient rate.
Metric used in this ranking
MPP50 is the instantaneous marginal physical product of nitrogen at an application rate of 50 kg N/ha. A value of 15 kg/kg N means the fitted curve is rising by about 15 kg of maize for the next additional kilogram of nitrogen at that point.
Nine entries come from the OFRA-IT comparison published by Wortmann and coauthors: Achefer, Bougouni, Farako-ba, Kapchorwa, Mlingano, Mwanza, Nyakunguru, Siaya and Wenchi. The tenth uses the Sudan Savanna maize-N response function published in the OFRA Nigeria chapter.
Keeping the domains separate avoids an arbitrary country-level aggregation. Tanzania, for example, appears twice because Mlingano and Nyakunguru have separate published response functions and produce different MPP50 estimates.
Top 5 Point Estimates Within the Confirmed 10-Domain Sample
Achefer has the highest calculated MPP50 at 15.64 kg maize/kg N, followed by Mwanza at 14.66. Bougouni, Farako-ba and the Sudan Savanna domain complete the five highest point estimates in this sample.
Five highest MPP50 values within the confirmed comparison
| Rank | Research Domain | MPP50 | Source / Method Note |
|---|---|---|---|
| 1 | Achefer, Ethiopia | 15.64 kg/kg N | Calculated value. Wortmann et al. (2017), Achefer OFRA-IT domain. Published inputs: b=2.279, c=0.972. MPP50 calculated with the derivative formula at 50 kg N/ha for the 2026 research snapshot. |
| 2 | Mwanza, Malawi | 14.66 kg/kg N | Calculated value. Wortmann et al. (2017), Mwanza OFRA-IT domain. Published inputs: b=2.135, c=0.972. MPP50 calculated with the derivative formula at 50 kg N/ha for the 2026 research snapshot. |
| 3 | Bougouni, Mali | 12.95 kg/kg N | Calculated value. Wortmann et al. (2017), Bougouni OFRA-IT domain. Published inputs: b=1.859, c=0.973. MPP50 calculated with the derivative formula at 50 kg N/ha for the 2026 research snapshot. |
| 4 | Farako-ba, Burkina Faso | 12.55 kg/kg N | Calculated value. Wortmann et al. (2017), Farako-ba OFRA-IT domain. Published inputs: b=1.802, c=0.973. MPP50 calculated with the derivative formula at 50 kg N/ha for the 2026 research snapshot. |
| 5 | Sudan Savanna, Nigeria | 11.69 kg/kg N | Calculated value. IFDC OFRA Nigeria chapter (2017), Sudan Savanna maize-N function. Published inputs: b=1.760, c=0.970. MPP50 calculated with the derivative formula at 50 kg N/ha for the 2026 research snapshot. |
Chart: Marginal Maize Response at 50 kg N/ha
The chart uses the same calculated point estimates as the full ranking table.
Methodology
The ranking uses published maize response functions of the form Y = a − bcr, where Y is grain yield in Mg/ha and r is elemental nitrogen applied in kg/ha.
Metric
Instantaneous marginal physical product of nitrogen evaluated at exactly 50 kg N/ha.
Unit
Kilograms of additional maize per additional kilogram of elemental nitrogen.
Direction
Higher MPP50 ranks higher. Raw point estimates determine order before display rounding.
Coverage
Ten research domains with compatible published maize-N response functions.
Calculation Formula
The factor 1,000 converts Mg of maize to kg. The plateau parameter a disappears when the derivative is taken, so marginal response at 50 kg N/ha depends on b and c.
Published Inputs
- Achefer: b=2.279, c=0.972.
- Mwanza: b=2.135, c=0.972.
- Bougouni: b=1.859, c=0.973.
- Farako-ba: b=1.802, c=0.973.
- Sudan Savanna, Nigeria: b=1.760, c=0.970.
- Siaya: b=1.473, c=0.965.
- Wenchi: b=1.144, c=0.975.
- Nyakunguru: b=1.246, c=0.967.
- Mlingano: b=0.987, c=0.975.
- Kapchorwa: b=1.094, c=0.959.
Inclusion Rules
Only maize response to nitrogen is included. Nine OFRA-IT locations from the same cross-country study are retained as separate entities. The Sudan Savanna function is included because the OFRA Nigeria chapter publishes compatible maize-N coefficients using the same curvilinear-to-plateau framework.
Other crops, phosphorus and potassium functions, national fertilizer-consumption statistics and studies without compatible response coefficients are excluded. Missing locations are not assigned zero values.
Uncertainty and Statistical Interpretation
The ranking compares point estimates. Wortmann et al. also report uncertainty for the fitted response coefficients, together with sample sizes and model-fit statistics. This ranking does not perform pairwise significance tests between domains.
Close ranks should therefore be interpreted cautiously. Achefer ranking above Mwanza, or Bougouni above Farako-ba, describes the ordering of fitted point estimates and does not establish that the underlying responses are statistically different.
Limits
MPP50 is a biological response measure, not a profitability measure. Fertilizer price, maize price, rainfall, soil properties, crop variety, management and other nutrient constraints can all change the value of applying additional nitrogen in practice.
OFRA inference domains are based on agronomic similarity. The evidence supporting a domain can include response functions from locations outside the country containing the named query point, so country labels should be read as geographic context rather than national estimates.
Main Ranking Table
Search, filter or reorder the 10 confirmed research domains below.
10 confirmed research domains · MPP evaluated at 50 kg N/ha
| Rank | Research Domain | MPP50 | Source / Method Note |
|---|---|---|---|
| 1 | Achefer, Ethiopia | 15.64 kg/kg N | Calculated value. Wortmann et al. (2017), Achefer OFRA-IT domain. Published inputs: b=2.279, c=0.972. Source year: 2017. Target: 2026 research snapshot. Formula basis: MPP50 = −1,000 × b × c50 × ln(c). |
| 2 | Mwanza, Malawi | 14.66 kg/kg N | Calculated value. Wortmann et al. (2017), Mwanza OFRA-IT domain. Published inputs: b=2.135, c=0.972. Source year: 2017. Target: 2026 research snapshot. Formula basis: MPP50 derivative at 50 kg N/ha. |
| 3 | Bougouni, Mali | 12.95 kg/kg N | Calculated value. Wortmann et al. (2017), Bougouni OFRA-IT domain. Published inputs: b=1.859, c=0.973. Source year: 2017. Target: 2026 research snapshot. Formula basis: MPP50 derivative at 50 kg N/ha. |
| 4 | Farako-ba, Burkina Faso | 12.55 kg/kg N | Calculated value. Wortmann et al. (2017), Farako-ba OFRA-IT domain. Published inputs: b=1.802, c=0.973. Source year: 2017. Target: 2026 research snapshot. Formula basis: MPP50 derivative at 50 kg N/ha. |
| 5 | Sudan Savanna, Nigeria | 11.69 kg/kg N | Calculated value. IFDC OFRA Nigeria chapter (2017), Sudan Savanna maize-N function. Published inputs: b=1.760, c=0.970. Source year: 2017. Target: 2026 research snapshot. Formula basis: MPP50 derivative at 50 kg N/ha. |
| 6 | Siaya, Kenya | 8.84 kg/kg N | Calculated value. Wortmann et al. (2017), Siaya OFRA-IT domain. Published inputs: b=1.473, c=0.965. Source year: 2017. Target: 2026 research snapshot. Formula basis: MPP50 derivative at 50 kg N/ha. |
| 7 | Wenchi, Ghana | 8.17 kg/kg N | Calculated value. Wortmann et al. (2017), Wenchi OFRA-IT domain. Published inputs: b=1.144, c=0.975. Source year: 2017. Target: 2026 research snapshot. Formula basis: MPP50 derivative at 50 kg N/ha. |
| 8 | Nyakunguru, Tanzania | 7.81 kg/kg N | Calculated value. Wortmann et al. (2017), Nyakunguru OFRA-IT domain. Published inputs: b=1.246, c=0.967. Source year: 2017. Target: 2026 research snapshot. Formula basis: MPP50 derivative at 50 kg N/ha. |
| 9 | Mlingano, Tanzania | 7.05 kg/kg N | Calculated value. Wortmann et al. (2017), Mlingano OFRA-IT domain. Published inputs: b=0.987, c=0.975. Source year: 2017. Target: 2026 research snapshot. Formula basis: MPP50 derivative at 50 kg N/ha. |
| 10 | Kapchorwa, Uganda | 5.65 kg/kg N | Calculated value. Wortmann et al. (2017), Kapchorwa OFRA-IT domain. Published inputs: b=1.094, c=0.959. Source year: 2017. Target: 2026 research snapshot. Formula basis: MPP50 derivative at 50 kg N/ha. |
All values are calculations from published response-function coefficients. None is a forecast or a national statistical estimate.
Insights From the 10-Domain Comparison
Key Insight
Achefer has the highest point estimate at 15.64 kg maize/kg N, about 6.7% above Mwanza.
Notable Pattern
The first five domains remain above 11 kg/kg N. Tanzania appears twice, with Nyakunguru at 7.81 and Mlingano at 7.05, showing meaningful variation between domains in the same country.
Source Concentration
Nine entries use coefficients from the same 2017 OFRA-IT comparison. The Sudan Savanna row uses a compatible OFRA Nigeria response function.
Outlier
Kapchorwa has the lowest point estimate at 5.65 kg/kg N, about 64% below Achefer. The table alone does not establish the agronomic causes of this gap.
What These Results Mean
Higher MPP50
A higher value means the fitted maize response curve remains steeper at 50 kg N/ha, so the next kilogram of nitrogen is associated with a larger incremental yield gain.
Lower MPP50
A lower value indicates a flatter fitted response at the same nitrogen rate. It does not by itself show whether fertilizer use is economically attractive.
Why Domains Matter
Soil, rainfall, crop variety, management and other nutrient constraints vary spatially. OFRA inference domains reflect similar production conditions rather than political borders alone.
How to Read Close Ranks
Small differences between neighboring point estimates should not be treated as proof of statistically different underlying responses because the fitted coefficients carry uncertainty.
FAQ
What is marginal fertilizer response?
It is the additional crop yield associated with one more unit of nutrient at a specific fertilizer rate. Here the crop is maize, the nutrient is nitrogen and the response is evaluated at 50 kg N/ha.
Which domain has the highest marginal maize response?
Achefer, Ethiopia has the highest point estimate in this 10-domain comparison at 15.64 kg maize per additional kg N.
Why is this not presented as a country ranking?
The source functions represent inference or agroecological research domains rather than harmonized national averages. Country names identify where the named domains are located.
Why evaluate every curve at 50 kg N/ha?
Marginal response changes with nitrogen application. Using one common rate ensures that every fitted response curve is evaluated at the same point.
Are the MPP50 values published directly by OFRA?
No. The sources publish fitted response-function coefficients. The values shown here are calculated from those coefficients using the derivative formula disclosed in Methodology.
Why does Tanzania appear twice?
Wortmann et al. report separate OFRA-IT functions for Mlingano and Nyakunguru. Keeping them separate avoids imposing an unsupported national average.
Does a higher MPP50 mean farmers should apply more nitrogen?
No. Fertilizer decisions also depend on fertilizer and maize prices, soil conditions, rainfall, other nutrients, management and production risk.
Are these differences statistically significant?
Not necessarily. The table ranks fitted point estimates and does not perform pairwise statistical tests between domains. Close ranks should therefore not be interpreted as statistically distinct without further analysis.
Sources
Wortmann et al. — Maize-Nutrient Response Information Applied Across Sub-Saharan Africa
Primary coefficient source for nine ranking rows. The study reports b and c parameters for the selected OFRA-IT maize-N response functions, together with uncertainty and model-fit information.
IFDC — Optimizing Fertilizer Use in Nigeria
Primary coefficient source for the Sudan Savanna, Nigeria row. The maize-N function reports a=3.000, b=1.760 and c=0.970.
https://ifdc.org/wp-content/uploads/2026/01/OFRA-Chapter-12_Nigeria_HR.pdf
Dryad — Geo-Referenced Crop-Nutrient Response Function Dataset for Tropical Africa
Supporting dataset reference for the broader OFRA collection of georeferenced crop-nutrient response functions.
IFDC — Optimizing Fertilizer Recommendations in Africa
Supporting methodology and resource hub for OFRA response functions, fertilizer optimization tools and country materials.
