Download - Maize

coherent exploratory diagnostic trials, together with trial management protocols and diagnostic
information; access to this facility enables partners, including farmers, to identify maize yield constraints
and quantify their effects under local conditions. Once established in farmers’ fields, the trials become a
resource for research while also serving as learning sites for farmers and extension agents. Results from
the international trials can be incorporated into geo‐referenced databases containing the various layers
of knowledge (e.g., soil maps, weather forecasts and market information) that are needed for a systems
approach to crop management. In addition to helping strengthen urgently needed agronomic research
capacity and enhancing information and knowledge flow, an international network of agronomists will
foster the integration of crop improvement and agronomy by providing feedback on key management
practices and plant traits needed to make maize‐based cropping systems more sustainable.
Lessons from past research
There are striking differences in fertilizer use on the maize crop in the high‐yield potential areas of Asia
and Latin America and the rainfed systems of sub‐Saharan Africa. Whereas an average of 73 kg/ha of
nutrients were applied across all crops in Latin America, 100 kg/ha in South Asia, and 135 in East and
Southeast Asia (FAO 2004), fertilizer use in sub‐Saharan Africa (excluding South Africa) in 2007 was less
than 7 kg/ha of nutrients (calculated from data of FAOSTAT—http://faostat.fao.org) although it is likely
that about 17 kg/ha of nutrients were applied to maize (based on estimated fertilizer use in maize—FAO
2002).
While it is commonly accepted that nutrients are the major limiting factor to crop productivity in sub‐
Saharan Africa, fertilizer’s agronomic potential is often unrealized because of poor land and crop
husbandry practices. Many “poor” management practices (late application or inadequate doses) often
stem from farmers’ efforts to reduce risk (Kelly 2006). It is crucial that farmers understand the factors
that increase risk and/or reduce crop responsiveness to fertilizer (seeding date, weeding, tillage, timing
of fertilizer application) if they are to achieve levels of management that allow the application of
nutrients without undue risk.
While market and political risk are important (these are addressed in SI 1), reducing the risk associated
with weather, especially rainfall, through crop and soil management options and through better
information on weather forecasts can raise the profitability of fertilizer use and crop productivity. Two
particular avenues for addressing risk are response‐farming techniques and simulation models; lessons
from these tools need to reach many more farmers (Kelly 2006).
In higher‐productivity environments of Asia and Latin America fertilizer use is often excessive; this
reduces the profitability of crop production and also increases environmental risks of nitrate leaching
and eutrophication. The efficiency of use of nitrogen is commonly 30% or less, even though levels of
over 80% are technically feasible (Raun and Johnson 1999). Highly‐efficient systems will depend on
concurrently improving several avenues to efficiency—including application methods and fertilizer
formulations, use of crop varieties that are more efficient in nitrogen absorption, and methods to
diagnose and apply needed levels of fertilizer (taking into account spatial variability in soils).
While present recommendations for economic phosphorus fertilizer applications are generally good
once laboratory analyses have been calibrated with field studies, results with nitrogen
recommendations are not as good or precise. In recent years researchers have worked with different
methods to directly assess crop nutrient requirements; these methods range from observing leaf color
(in rice), use of chlorophyll sensors, and use of remote sensing devices (either satellite‐installed or handheld),
to predict nitrogen and phosphorus responses. Results to date have been especially promising for
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