Yield Reliability: Room for Improvement

With limited resources to cope with weather variability, smallholder farmers in Sub-Saharan Africa (SSA) are facing highly unreliable crop production from year to year. In this modeling exercise we quantified how much area is under such condition and what are their production potential under intensification. The simulation result indicates that, without further intensification, only 15% of current maize growing area has the potential to reliably produce more than 3 t/ha, a yield level suggested as being sufficient to sustain the cereal needs of a typical smallholder household. However, with well-managed intensification, 82% of the current maize area showed the potential to reliably produce 3 t/ha or more.

How (Un)Reliable?

Current cereal crop yield in smallholder farmers' field is below 1 t/ha in many parts of Sub-Saharan Africa (SSA) largely due to the degraded soil quality and lack of adequate management practices and investment. Worse, with very limited resources to cope with weather variability, such as this year's devastating drought in the Horn of Africa region, smallholder farmers in Sub-Saharan Africa (SSA) face not only the low levels of yield but also highly unreliable crop production from year to year. In this modeling exercise we attempted to quantify how much area is under such unreliable condition and what are their potential, in terms of the average yield as well as its reliability, under the production intensification - once inorganic fertilizer and hybrid varieties become available to those areas.

Data and Method

Simulation was run for the current maize growing areas in SSA countries at 5 arc-minute spatial resolution, based on Spatial Production Allocation Model (SPAM; see http://mapspam.info for more detail) for 50-year period (1951-2000). HarvestChoice's spatial datasets of synthesized long-term daily weather (SLATE; see previous post for more detail) and the gridded soil distribution database (see previous post for more detail) were used as the model input. The extent of maize growing areas was fixed over the simulated time period. To assess the yield variability under different crop management intensification assumptions, two input systems were defined as follows:

• Low-input: Farmers rely on the land's inherent and no fertilizer or manure is applied; soil fertility degrades over time. OPV variety is used.
• High-input: Farmers use inorganic fertilizer at 40 kg[N]/ha rate and hybrid variety.
• As we're interested in the yield variability caused by rainfall variability, we only focused on the rainfed systems; supplementary irrigation was not simulated in this study. For each grid cell 50-year sequential simulation of maize cultivation was run for each soil. In post-processing of the result, area share of each soil was used to compute the area-weighted average of simulated yield for each grid cell per year.

We defined the "reliability" based on the probability of achieving a predefined threshold level of yield for at least three out of five years (i.e., > 60% of years). A range of the threshold yield level was set from 1, 2, 3, ..., 7 t/ha to construct the probability function across the region. In addition, we set the target yield threshold level as 3 t/ha, following the discussion by the Hunger Task Force of the UN Millennium Project and the African Green Revolution that set the cereal yield need to increase from the current level of 1 t/ha to 3 t/ha to achieve the Millennium Development Goal (MDG) on halving the number of people suffering from hunger in Africa.

Results and Discussion

Average potential yield under low-input and high-input systems were simulated as 2,271 and 4,270 kg/ha, respectively. In reality, however, actual yields achieved by farmer can be much lower than the potential level as the simulation does not take into account all biotic and abiotic constraints that may exist in the field.

Result at the lowest yield threshold level, 1 t/ha, indicated that there were 74% of maize area that reliably (more than 3 out of 5 years; hereafter) produce more than 1 t/ha. In other words, there are about a quarter (26%) of maize area produce less than 1 t/ha in most of the years. Map 1 shows the distribution of such low-potential areas across the region (red colored areas); they are distributed in much of the Horn of Africa countries (Ethiopia, Somalia, and Kenya), as well as Southern Africa countries (e.g., South Africa and Zimbabwe) and arid and semi-arid areas in West Africa (e.g., Ghana, Burkina Faso, and Nigeria). If we shift our focus to the target yield level of 3 t/ha, there are only about 15% of areas that can reliably produce yield higher than the level. Map 2 shows such areas are sparsely distributed in mostly humid and highland agro-climatic areas, such as Western Ethiopia, South-Eastern Ghana, Rwanda, and Burundi. With intensification, however, such areas could vastly expand to about 83% of maize areas, as shown in Map 3, except arid and semi-arid regions in Eastern Ethiopia, Northern Kenya, and Chad. That is, the simulated intensification methods in this study, inorganic fertilizer and hybrid variety, not only increased the yield levels but also contributed to improve the reliability of maize production over time.

This quick study assessed the status of (un)reliability of maize production in SSA region under two management practice scenarios of low-input/subsistence and high-input/intensified systems on the current maize growing areas. However, the improvement of production at regional or country level could also come from the expansion of crop land to new areas, whose land quality for maize cultivation could be better (in newly cleared land) or worse (in marginal land under arid climate). As we did not consider such changes in the land use, our estimation of land areas maybe underestimating, especially for the case of high-input/intensified systems. In addition, we only focused on the output side of food production system (i.e., yield), not considering the input (e.g., fertilizer and seed cost, labor charges, transportation cost to the market), such that the reliable production does not necessarily translate into the economic profitability. There maybe areas where fertilizer is cost-prohibitively expensive that the marginal yield improvement does not justify the upfront investment.

Despite the caveat, the reliable potential yield improvement under the intensification scenario in 83% of maize growing area is highly encouraging. This finding also supports the interim outcome from the Millennium Villages Projects that reported the achievements of average maize yield of more than 3 t/ha in all villages across SSA where maize is the major crop, through subsidized fertilizers, improved germplasm, and intensive training on appropriate agronomic practices.