Global S&T Development Trend Analysis Platform of Resources and Environment
| DOI | 10.1126/science.abf5413 |
| Agricultural productivity must improve in sub-Saharan Africa | |
| Thomas S. Jayne; Pedro A. Sanchez | |
| 2021-06-04 | |
| 发表期刊 | Science
 |
| 出版年 | 2021 |
| 英文摘要 | In the first two decades of the 21st century, sub-Saharan Africa (SSA) has changed rapidly for the better in many ways, counter to many outdated narratives. Many of these improvements—including those in gross domestic product (GDP) per capita, poverty rates, health, life expectancy, education, and agriculture—have been mutually reinforcing ([ 1 ][1], [ 2 ][2]). SSA achieved the highest rate of growth in agricultural production value (crops and livestock) of any region in the world since 2000, expanding by 4.3% per year in real [inflation-adjusted US dollars (USD)] between 2000 and 2018, roughly double that of the prior three decades. The world average over the same period was 2.7% per year ([ 1 ][1]). Agricultural value added per worker in real 2010 USD rose from $846 in 2000 to $1563 in 2019, a 3.2% annual rate of growth. But to assert that Africa is rapidly developing does not mean that all livelihood indicators are improving, though most are in most countries ([ 1 ][1], [ 3 ][3]). SSA faces many major challenges. We focus below on one such challenge, which we see as a precondition for sustaining livelihood improvements in the region: transitioning from area expansion to productivity growth as the source of Africa's agricultural development.
Most African countries show a strong correlation between agricultural growth and GDP. Even for the region as a whole, the degree of correlation is notable (see the figure, top ), confirming the reinforcing synergies between agriculture and African economies. When agriculture grows, its extensive linkages with off-farm stages of the agrifood system and nonfarm sectors expand employment and livelihoods in the rest of the economy ([ 2 ][2], [ 4 ][4]). Rapid agricultural growth since 2000 encouraged new private investment and employment in agricultural value chains and nonfarm sectors of African economies, pulling labor from farming into off-farm jobs that provide considerably higher returns to labor than semisubsistence farming ([ 5 ][5]). More than 40% of SSA's labor force, mainly young people, is now engaged in off-farm jobs ([ 2 ][2], [ 5 ][5]). The bottom line is that high farm production growth in SSA has contributed to high overall economic growth and improvements in the welfare of most Africans ([ 2 ][2], [ 4 ][4]).
Notwithstanding the region's impressive agricultural growth since 2000, roughly 75% of its crop production growth came from the expansion of area under cultivation and only 25% from improvements in crop yield (metric tons per harvested hectare) (see the figure, middle ). Cereal yields in SSA rose by 38% in the 38 years between 1980 and 2018, roughly half that of South Asia and Southeast Asia ([ 3 ][3]). There remains great unmet potential for crop yield improvement—or, more appropriately, increases over time in the ratio of agricultural output to inputs, hereafter “productivity.”
Pivoting from area expansion to sustained productivity growth on existing farmland is becoming increasingly urgent for several reasons.
### Mounting land pressures
Most smallholder households' farm sizes have been gradually shrinking for decades because of rural population growth and limited potential for continued area expansion in relatively densely populated areas where most rural Africans live. At the continental level, estimates show that 52% of the world's remaining arable land is in SSA ([ 6 ][6]). Yet, most of this land is concentrated in just eight countries, whereas many of the region's remaining 41 countries contain large rural populations clustered in notably small areas ([ 6 ][6]). Of SSA's arable rural lands that receive more than 400 mm average rainfall per year, 20% contains 74% of its rural people ([ 6 ][6]). Many young rural Africans today cannot obtain additional land through inheritance or customary land institutions as they used to. Hence, even though most of SSA might be considered “land abundant,” a relatively large proportion of rural Africans face land scarcity. Land prices in the region are also rising rapidly, even in areas once considered remote ([ 2 ][2]). For these reasons, improving livelihoods for African farmers increasingly depends on raising the productivity of existing farmland.
### Environmental costs of area expansion
Continued reliance on area expansion as the main source of agricultural growth is not a viable option on environmental grounds, including biodiversity conservation and destruction of natural vegetation. The goals of feeding Africa's growing population and conserving the planet's natural resources, diverse ecosystems, and the services they provide will be more effectively achieved through productivity improvements on existing farmland instead of area expansion ([ 7 ][7], [ 8 ][8]).
Achieving higher rates of agricultural productivity growth will require technical innovation, that is, doing things differently and doing existing things more efficiently. Greater and more efficient use of improved seed, mineral fertilizers, and organic inputs are widely recognized as preconditions for achieving productivity growth on African farms ([ 4 ][4], [ 7 ][7]–[ 9 ][9]).
Plant breeders tended to test their improved materials in Africa using state-of-the-art agronomic practices, including mineral fertilizers. These tests produced greater yields than could be obtained on most smallholder fields in SSA, which tended to have nutrient-depleted soils. Farmers generally did not return the nutrients removed by crop harvests as fertilizers and manures, creating a vicious cycle of soil fertility depletion that prevented the yield potential of improved varieties when used in most farmer fields. This contrasts with what happened during the Green Revolution in Asia where farmers found that the improved varieties doubled or tripled yields when nitrogen fertilizers were added ([ 9 ][9]). Soil fertility replenishment in SSA was finally recognized as the key entry point for increasing crop yields in Africa in the late 1990s. In 2006, farmers in SSA used 8 kg of mineral fertilizers per hectare, whereas Asian farmers used 15 times more ([ 3 ][3]). Though fertilizer use per hectare in SSA has since doubled to 17.9 kg per hectare in 2018 ([ 3 ][3]), this is still far short of what is needed to compensate for the harvested nutrients ([ 8 ][8], [ 9 ][9]).
### Raising fertilizer use efficiency
There is growing recognition that many African farmers obtain widely varying and generally low crop response to the fertilizers that are applied ([ 8 ][8], [ 10 ][10]). The agronomic efficiency of nitrogen (AEN) (i.e., the additional kilograms of crop output per kilogram of nitrogen nutrient applied to the field) in Africa averages 14.2 ([ 8 ][8]), with considerable variability across seed and soil type, rainfall, and management. With appropriate mineral fertilizers, organic inputs, fertilizer-responsive seeds, and improved soil management, crop response to fertilizers can be doubled or tripled, reaching the global average AEN of 37 ([ 8 ][8]). Raising crop response to mineral fertilizer is one of the most important steps toward achieving agricultural productivity growth in SSA because it will raise the profitability of using fertilizers, organic inputs, and fertilizer-responsive high-yielding seed varieties. This would in turn promote greater demand for these inputs and greater investment in the supply chains for all manner of agricultural inputs and support services, which in turn would support virtuous cycles of farm productivity improvements, greater competitiveness of African food and agricultural export crops, higher smallholder incomes, and increased private investment in agrifood systems. Policy environments conducive to private investment and competition in agrifood systems are also necessary to ensure that these virtuous cycles can materialize ([ 2 ][2], [ 4 ][4]).
### Promoting the use of blending facilities
Most fertilizers used in SSA are imported. In the past decade, many small fertilizer blending facilities that can produce appropriate nutrient compositions for specific areas, based on recommendations from soil tests along with digital soil maps, have sprouted in the region. Some recommendations often include sulfur, a nutrient that is commonly deficient in large areas of SSA ([ 8 ][8]). Improved blending to suit specific soil needs has contributed to the doubling of fertilizer use in SSA over the past decade ([ 8 ][8]). Still, much greater intensity of fertilizer use is needed for SSA to achieve strong agricultural growth led by productivity gains on existing cropland ([ 2 ][2], [ 4 ][4], [ 8 ][8]).
### Greater use of organic inputs
African smallholder farmers use little cattle manures, crop residues, and cover crops (planted to cover soil, rather than for harvesting), partly because such inputs are difficult to grow in fertility-depleted soils ([ 9 ][9]). Though cattle manures are used all over Africa as organic inputs, with those produced from dairy farms generally of high nutrient quality, most manures used by smallholder farmers often are of low nutrient quality, because cattle graze on low-quality grasses grown on nutrient-depleted soils and the manures are often mixed with soil ([ 8 ][8]). Crop residues such as cereal stover (leaves and stalks remaining after harvest) are mainly fed to cattle, but when cereal crop yields more than double, as commonly happens with mineral fertilizers and improved seeds ([ 8 ][8]), crop residues also increase. This provides an opportunity to satisfy feeding cattle while returning substantial quantities of crop residues containing 45% carbon to the soil. Because mineral fertilizers contain no carbon, organic inputs must be part of the equation to achieve soil health ([ 8 ][8]).
![Figure][11]
Crop yield must increase in sub-Saharan Africa (SSA)GRAPHIC: N. CARY/ SCIENCE
### Increasing the use of improved, high-yielding seeds
There are now more than 100 African-owned seed companies that sell improved seeds that can attain high yields when fertilized. Regulatory reforms that remove barriers on seed trade and allow for greater private investment in seed development and distribution are, in many cases, needed to make improved seed more accessible to African farmers ([ 4 ][4], [ 11 ][12]). For example, early generation seed production (breeder seed and foundation seed) has been a frequent bottleneck in the production and supply of certified seed, delaying farmers' access to improved seed. Most African governments formerly held a monopoly on seed production, but now many have allowed private companies to begin producing it, including in Ghana, Mali, Nigeria, Ethiopia, and Uganda ([ 11 ][12]). Improving soil health without improved seeds is a nonstarter ([ 4 ][4], [ 8 ][8]).
There is also a need to expand the coverage of SSA countries benefiting from national and international seed research. Huge increases in maize and rice yields have occurred in 10 countries that received technical and financial assistance from international and national research and development institutions on seed systems and fertilizers, in contrast to 10 other African countries that received relatively little ([ 11 ][12]) (see the figure, bottom). Although this bivariate comparison does not control for other confounding factors, the two sets of countries had very similar yield levels at the beginning of the 2000s. Today, the countries benefiting from sustained support achieve 80% higher yields.
Investment in agricultural research and development has been consistently demonstrated to be one of the most effective ways to promote agricultural productivity growth ([ 4 ][4], [ 12 ][13]). Especially because SSA is highly vulnerable to climate change and is experiencing greater prevalence of extreme weather, plant diseases, and pests, greater investment in adaptive and site-specific agricultural research and development and extension (R&D&E) is a precondition for productivity-led agricultural growth in SSA.
Agricultural R&D&E is especially needed to respond to the growing challenges caused by climate change; leaching and runoff of nitrogen and phosphorus that result in eutrophication of water bodies; and detrimental effects on pollinators when soils, crops, livestock, fertilizers, manures, or pesticides are misused. Effective communications to farmers and the general public, such as Digital Green and Kenya's farm-focused “Shamba Shape Up” television program, can also raise public awareness of the societal benefits of agricultural R&D&E and hence bolster support for such expenditures.
Most African governments devote less than 10% of their agricultural expenditures to R&D and spend much less on agricultural R&D than governments in other regions ([ 4 ][4]). Most Asian governments spend at least four times more per farmer and per hectare cultivated than African governments ([ 4 ][4]). Cereal yields more than doubled over the past 40 years in South and Southeast Asia but rose by only 38% in Africa ([ 3 ][3]).
The Asian efforts focused first on the most obvious agricultural concern: low-yielding, tall-statured varieties of rice and wheat that, when fertilized, fell on the ground to rot. Although varieties overcoming this constraint have been bred in Africa, research institutions did not confront, until the past two decades, the first and most obvious agricultural limiting factor: the depletion of soil fertility in smallholder farmer fields ([ 4 ][4]). Now that fertilizer and organic inputs are being addressed together with higher yielding varieties, the potential for sustained crop yield increases has become realistic ([ 8 ][8]).
These are not new ideas, but they are long overdue for implementation and sustained support. Most international development organizations and foundations dedicated to promoting agricultural development in Africa have shied away from addressing the issue of adaptive local agricultural R&D. Donors and policy-makers tend to prefer low-hanging fruit that offers quick wins that are highly visible, avoiding investments that require dedicated long-term support and have long gestation periods before impacts are felt ([ 13 ][14]). But if left unaddressed, the constraints posed by the slow pace of technical innovation on African farms will retard the achievement of almost all rural development goals, including those pertaining to reducing inequalities and improving the livelihoods of rural women and youth. Sooner or later, local agricultural R&D&E will need to be addressed head-on if the region is to pivot decisively to a productivity-led agricultural growth path.
The main ingredients are sustained strengthening of adaptive R&D&E to support farm technical innovation and adaptation, supportive agricultural marketing and trade policies—including implementation of the African Continental Free Trade Agreement—and infrastructural investments that reduce costs and risks in the food system, promote investment and competition in agricultural value chains, and enable farmers to adopt these technical innovations ([ 2 ][2], [ 4 ][4], [ 12 ][13]).
### Bidirectional extension and learning
Although the importance of agricultural extension as a means for bidirectional learning between farmers and scientists is well known, these systems in Africa have suffered from chronic underfunding and variable management. Because agriculture in SSA spans a tremendous range of agroecological and economic conditions and resources and capacities of farmers, the benefits of strong agricultural science and technical innovation often cannot translate into farmer adoption and improved productivity unless the innovations can be tailored appropriately to farmers' specific conditions. Local adaptation of agricultural research to the diverse farming conditions of SSA will require bidirectional learning between farmers and agricultural scientists to identify how farmers can adapt improved management practices that fit within their resource constraints ([ 14 ][15]). Bidirectional learning is especially important when farming systems are changing, for example, because of climate change. Ethiopia is one of the few African countries to substantially increase its spending on public agricultural research, which has more than tripled in real terms since 2000 ([ 15 ][16]). Ethiopia also has half of SSA's agricultural extension workers ([ 4 ][4]). Not surprisingly, Ethiopia has enjoyed the highest rate of agricultural growth of any country in SSA since 2000 ([ 1 ][1], [ 3 ][3]). Each additional $1 of agricultural value-added in the Ethiopian economy generated an additional $0.29 in nonfarm GDP and hence contributed powerfully to the country's rapid economic transformation ([ 15 ][16]).
Ethiopia's successes provide a powerful example for other SSA countries: By committing greater investment to national and international agricultural R&D and focusing on improving the operational performance of these organizations, SSA governments will be taking one of the single most important steps to sustain Africa's economic transformation.
1. [↵][17]The World Bank, World Development Indicators; |
| 领域 | 气候变化 ; 资源环境 |
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| 文献类型 | 期刊论文 |
| 条目标识符 | http://119.78.100.173/C666/handle/2XK7JSWQ/329859 |
| 专题 | 气候变化 资源环境科学 |
| 推荐引用方式 GB/T 7714 | Thomas S. Jayne,Pedro A. Sanchez. Agricultural productivity must improve in sub-Saharan Africa[J]. Science,2021. |
| APA | Thomas S. Jayne,&Pedro A. Sanchez.(2021).Agricultural productivity must improve in sub-Saharan Africa.Science. |
| MLA | Thomas S. Jayne,et al."Agricultural productivity must improve in sub-Saharan Africa".Science (2021). |
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