Sustainable intensification of African agriculture: a necessity, but not yet reality

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Worrying indicators
Agriculture intensification indicators [1] such as yield, mechanization, and agriculture land use since 1960 show sub-Saharan Africa lagging far behind the rest of the world (Fig. 1). With few exceptions, there has been little positive transformation and few signs of needed intensification. In contrast, African rural landscapes are most often poverty traps for unlucky inhabitants, who depend almost exclusively on lowyield agriculture for food security and livelihoods. Africa has generally addressed increasing food demand by putting more land under low-input, labor-intensive farming ( Fig. 1(c,d)). While providing some livelihood options, this has not set the continent on a positive development pathway, nor triggered development of secondary and tertiary sectors to process and sell farm products, nor led to fewer people in rural areas ( Fig. 1(e)). Farmland expansion has bled into less productive and marginal land, causing deforestation, loss of wildlife habitats and an overall erosion of once plentiful land resources. African agriculture is actually in a downward spiral, ever more vulnerable to climate change, degrading soils, erratic rainfall, droughts, destructive pests and crop diseases. Barring immediate large and well-conceived development investments, the region will suffer severe crises from agriculture system failures in coming 1 decades.

Low adoption of intensification measures by smallholder farmers
Why is African agriculture creating more poverty than wealth? Most answers to that question lie outside the agriculture sector. Technologies, improved seeds and better agronomic practices developed in Africa or elsewhere have shown their potential largely to reduce yield gaps and agricultural risks but, outside of research-for-development projects, adoption remains relatively low. There are many reasons: lack of knowledge, lack of access to inputs or to cash/credit to buy them, lack of input and output markets, limited financial incentives for enhanced productivity, risk aversion, and uncertain land tenure and land endowment; finally, extension and scaling approaches have fallen short in Africa [5,6] . 3 Can the land support a fast-growing rural population?
An eye-opening study by Harris and Orr [7] concluded that, given that rainfed farms in sub-Saharan Africa are so small (80% are less than 2 ha), even the best yield improvement practices will not provide enough revenue to lift farmers and their families out of poverty 2 .
Rapid demographic growth and the scarcity of livelihood options outside of farming will exacerbate rural land fragmentation, further reducing average farm size and the ability to make a living through farming alone. Off-farm agricultural labor or jobs outside the farm sector, along with transfers and remittances, may support some [8] but will seldom provide significant economic advancement. If the massive clearing of natural habitats for farming is to be avoided, while maintaining the current 80% food self-sufficiency as food demand triples over the next 30 years, van Ittersum et al. [4] suggest the need to transform agriculture, adding irrigation and practicing double cropping.

Low productivity of rural labor
The low total-factor productivity of African smallholding derives in large part from the absence of mechanization ( Fig. 1(e)). Labor may appear abundant in the region, but shortages at periods of peak demand (sowing and weeding) delay operations and reduce yields [9][10][11] , and farm labor entails considerable drudgery, especially for women and children. State-led efforts in several African countries to mechanize agriculture mostly failed, due to their governance models, lack of proper use and maintenance, and the type of equipment selected [12] . Recent interest in mechanization for the region is building on lessons from the success of scale-appropriate mechanization in Asia [13] . Alternate cropping practices that feature reduced tillage require smaller, less powerful, more affordable tractors-including two-wheel tractors-and are suited for small farms [14] . Even so, mechanization will prove expensive for individual farmers, so publicprivate partnerships and mechanization service provision models, whereby machine owners contract their services to farmers, will be required [15] .

Agricultural inputs
Use of agrochemicals, such as mineral fertilizers ( Fig. 1(f)), herbicides and pesticides, underpinned agricultural intensification in other regions but has not been widely adopted by African smallholders. Their use and abuse are at the core of current debates and reflections regarding sustainable intensification versus agroecological approaches. The two paradigms largely overlap in aiming to increase resource use efficiencies but differ regarding the use of external inputs. Agroecological approaches do not totally reject use of external inputs but favor capitalizing on endemic resources and aiming for high cycling and recycling efficiencies. In contrast, most proponents of sustainable intensification approaches for Africa cite the lack of plant nutrients applied in a balanced way as the chief constraint, along with a lack of water in the arid and semiarid regions, and believe that nutrient cycling alone will not be enough to boost productivity in the region's soils. The agroecological push in Europe reflects a reality where lands are rich in nutrients, external inputs have long been applied in excess, and the decoupling of crop and livestock production has generated major nitrogen pollution. In contrast, in African farming systems, recycling solutions without external inputs have often shown their limits and have a low factor productivity/higher labor demand. The debate should be about whether low-input agriculture will feed a rapidly growing population and provide decent living conditions, health, education for farmers and their families. Today African farmers are not agroecological by choice but for lack of viable intensification options. Their constraints are for the most part not technical but socioeconomic and due to the absence of an enabling environment.
There are technical issues. Poor soil fertility has been amply documented as the main cause of large yield gaps throughout sub-Saharan Africa [16] . Other causes for stagnant or declining yields include a lack of quality seed, poor management practices, lack of crop protection, poor or delayed weeding, erratic rainfall, dry spells and drought [4,17] . Understanding these agronomic problems is an important first step but deploying solutions at scale has been much more challenging. Clear, long-term, integrated approaches are needed and must include improved policies, functional markets, socioeconomic analysis, and strong, coordinated public and private support. 6 Detrimental economic policies Beginning in the 1960s, improving agriculture in Africa was seen as a stepwise exercise, first to fix the most critical constraint, soil fertility, by easing access to mineral fertilizer through Input Subsidy Programs (ISPs). Two decades of such programs achieved mixed results and much less impact on land productivity than expected [18] . To continue receiving World Bank loans, in the 1980s and 1990s, many countries pursued structural adjustment programs to reduce government spending and promote free trade, leading to the suspension of ISPs. Structural adjustment programs are now widely recognized as unsuccessful, failing to contribute to sustainability goals, poverty reduction or increased wealth. Economic policies under those programs did little to further agricultural development but rather weakened African agricultural research capacity. A second generation of ISPs, labeled as smart subsidies, were implemented in several African countries. They gave lower returns than expected [19] but at least demonstrated that access to fertilizer did boost productivity at scale, as seen in Malawi (Fig. 2). 7 Fertilizer alone will not do the job Ethiopia provides a striking example of strong public support for agriculture. Cereal yields have doubled there over the past 15 years with a relatively low use (around 20 kg·ha −1 ·yr −1 ) of fertilizer. One important lesson learned from the ISPs was that the lower than expected returns on fertilizer investments [19] were largely due to farmers not applying better crop management practices (timely weeding, optimum planting density, crop protection and use of quality seeds) in conjunction with fertilizer. In addition, large areas of crop land in sub-Saharan Africa have low inherent fertility, are physically and chemically degraded from long use for low-input farming, and have low organic matter content, making them unresponsive to fertilizer [20] . So, it appears that farmers need to apply a suite of improved technologies and practices, to get the best return on their investments in fertilizer. Moreover, a sequential adoption over years of improved management practices, starting with use of small quantities of mineral fertilizer [21] , might not be the best strategy in many contexts.
8 An underfunded public sector and a growing but timid private agrifood sector engagement In Maputo, Mozambique, in 2003, the African Union (AU) Summit made the first declaration regarding a Comprehensive Africa Agriculture Development Programme (CAADP), as an integral part of the New Partnership for Africa's Development. The CAADP is an African policy framework for agricultural transformation, wealth creation, food security and nutrition, economic growth and prosperity for all. At the time, two broad targets were set: 6% annual growth in agricultural GDP and an allocation of at least 10% of public expenditures to the agricultural sector. Main objectives included developing a continent-wide framework to boost job creation and increase employment opportunities, especially for women and youth, as well as guaranteeing food security and improved nutrition and strengthening resilience. The importance of a cross-sector approach was also explicit. In 2014, during an AU summit in Equatorial Guinea, African heads of state and governments repledged to the Malabo declaration on Accelerated Agricultural Growth and Transformation for Shared Prosperity and Improved Livelihoods, restating that agriculture should remain high on the agenda for the development of the continent and is a critical policy initiative for African economic growth and poverty reduction. The declaration set a series of ambitious goals to be achieved by 2025 through concrete and targeted actions. Facing strong budgetary challenges, many countries failed to fund agriculture at the levels originally envisaged by CAADP [22] and, according to macro-level indicators, most countries are far from achieving the 2025 targets.
Many consider the private sector as an important, potential driver for agricultural development in Africa. Private growth and engagement, including those of small-and medium-scale regional and national enterprises, have grown steadily but remain relatively low. Governments and societies must foster an enabling environment through larger incentives, supportive policies and adequate infrastructure, to capture private sector investments and lift agriculture. 9 Agricultural research is much needed to guide development The diversity of biophysical, socioeconomic and market conditions across the vast African continent demands agricultural solutions tailored for specific environments. As part of a sustainable intensification framework, research must embrace a multi-criteria approach including social, economic and environmental components and across multiple levels and scales. It must also be multidisciplinary to reflect past tradeoffs and synergies, address current realities, and come up with integrated solutions in context. Biological, environmental and human science knowledge should be integrated to develop relevant solutions. Also, only strong coordination between research, policy and development will improve the likelihood of development investments being designed, targeted, and implemented to provide the desired return on investment.
Integrated methods are essential but still need to be built around strong disciplinary science, such as field agronomy and breeding. Breeding must in turn expand its focus beyond mere yield enhancement to include yield stability, resistance or tolerance to biotic and abiotic stresses, and nutritional quality.