Maclean et al. - 2002 - Rice almanac source book for the most important e

occur only by shifting out of other crops, by
reducing the fallow period, or by exploiting soils
less well suited to rice cultivation. Farm-level
yields are low, averaging about 1 t/ha, and vary as
a function of local soil and rainfall conditions.
Moderate technical potential for increasing yield
exists through improved management. Progressive
farmers applying moderate input levels can
now achieve yields of 2.5 t/ha, and yields of 4
t/ha and greater are commonly achieved on
research stations.
Rainfed lowland/hydromorphic ecosystem
The rainfed lowland ecosystem comprises floodplains
and inland valleys. Overall, rice is grown
on about 1.4 million ha of rainfed lowlands
representing 30% of the total rice area and 36.5%
of regional production. Average yield across the
rainfed lowlands is 2.0 t/ha. In the floodplains,
rice can be grown on residual and water-table
moisture in the broad, flat areas adjacent to rivers
prone to seasonal flooding. Enormous technical
potential exists to expand rice production in
inland-valley lowlands, as currently only about
10–15% of these areas are cultivated. It is
estimated that Sub-Saharan Africa has
approximately 130 million ha of inland-valley
lowlands and their hydromorphic fringes within
which rice can be cultivated. Of this total, West
Africa may have about 20–30 million ha. The
diversity of inland valleys is large. Although
regional yields average around 1.4 t/ha, they vary
according to local soil, landform, and hydrological
conditions. Potential yields in unfavorable
lowlands are around 2.5 t/ha, increasing to more
than 5 t/ha in favorable lowlands.
Lowland irrigated ecosystem
This ecosystem covers about 142,000 ha in West
Africa, representing 3% of the total area and 5.5%
of regional production. The large technical
potential for area expansion is constrained
primarily by high investment costs. Current yields
average around 3.0 t/ha. Progressive farmers
employing moderate to high input management
can obtain yields of 5 t/ha, and yields of 7 t/ha
can be achieved on research stations.
The Sahelian irrigated environment
Irrigated rice in the Sahel forms the second most
important rice-producing environment in West
Africa, covering approximately 345,000 ha (7.5%
of total rice area) and producing 20.5% of
regional production. Irrigation potential is much
greater, estimated at more than 3 million ha along
the Senegal, Niger, Black Volta, Chari, and
Logone rivers. This enormous technical potential
has attracted large public investments: the first
irrigation schemes were constructed in the Sahel
in the 1920s. Until the recent introduction of
privatization policies, the state has played a lead
role in developing and managing irrigated rice
schemes, with over 75% of the areas currently
under parastatal control. The remainder is
managed by a growing and increasingly dynamic
private sector.
Because irrigated rice is an introduced
production system in the Sahel, there are no
traditional varieties or cultural practices upon
which to build. Management skills in water
control and rice cultivation vary widely. Mean
paddy yields are currently about 4.5 t/ha, but vary
widely from as low as 1 t/ha, rising to 4–6 t/ha if
water is available throughout the season, to as
high as 8 t/ha with modern input-responsive
varieties and optimal management. Although ricerice
double cropping is currently practiced on
only about 20% of the total area, by using
varieties with appropriate duration and
adaptation, an annual yield potential of 15 t/ha
can be achieved.
The mangrove swamp rice
environment
Rice is also grown on approximately 147,000 ha
of mangrove swamps, representing 3% of the
total area and producing roughly 4% of the
region’s output. Located on tidal estuaries close to
the ocean, most mangrove swamps experience a
salt-free growing period during the rainy season
when freshwater floods wash the land and displace
tidal flows. As a result, the rice-growing
period is directly related to distance from the
ocean, varying between less than four months in
the nearest estuaries to more than six months in
those more distant. Soils are generally more
fertile than in the other environments since they
benefit from regular deposits of silt left during
annual flooding. However, the soils are also
characterized by high salinity and sulfate acidity.
Lower rainfall during the last two decades has
reduced seasonal flushing substantially, further
accentuating both problems. Although West
Africa has approximately 1 million ha of
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