S.S. Jagtap, A.K. Chan Agricultural and Forest Meteorology 103 2000 59–72 61
highlands, for example, there is little or no opportu- nity to fallow lands. Also, as agriculture is progres-
sively transformed from the traditional, extensive fal- low systems to continuous cropping, yields of crops
and land productivity decline and sustainability is threatened. Intensification can be accelerated through
research, which could consider socio-economic, nat- ural resource, technical and institutional factors that
influence productivity and sustainability. Ecological and economic sustainability can be achieved when
the natural resources of land, crops, animals and wa- ter are used to reinforce each other. However, holistic
research on production systems in the region is weak partly due to lack of base line data, suitable method-
ology to measure benefits, shortages of qualified personnel, and institutional infrastructure.
2. Africa
The 3140 million ha land mass of Africa consists of four major agroecologies Fig. 1. The deserts 1017
million ha, the arid and semi-arid zone 30–150 days growing season, 787 million ha, the sub-humid zone
151–270 days growing season, 957 million ha and the humid zone 271–365 days growing period and oc-
cupies 379 million ha. Major soil related constraints to crop production are varied ranging from acidity,
salinity and heavy cracking to coarse texture Table 1. The sub-humid zone is mostly endowed with ma-
jor constraint-free soils and the humid zone. Acidity is the major constraint in both the humid and sub-humid
Table 1 Major soil related constraints to crop production by agroecological
zones in Sub-Saharan Africa
a
Limitation Arid
Sub-humid Humid
Total None
242 366
84 692
Acidity 13
266 226
505 Coarse
242 115
28 385
Shallow 95
72 9
176 Others
113 33
2 148
Cracking 41
53 4
98 Drainage
15 39
23 77
Saline 18
2 20
Total 779
946 375
2100
a
All areas in million ha.
zone, while coarse textured soils is the major problem of the arid zone. It is not known to what extent cul-
tivation prevails on each of these soils. There is also greater disparity in population amongst the zones. Of
the total present population in SSA, more than 43 live in West Africa. Thus, the aggregate increase in
population in that region will likely be greater than that of any other region of SSA. This rapid increase in pop-
ulation will have major implications for agriculture in all regions, but particularly so in the sub-humid zones
of West Africa. Pressures on land resources and the corresponding impetus towards intensification of agri-
culture will be brought about not only by expanding human population, but by expanding ruminant popu-
lations as well. For example, by 2025, the total num- ber of cattle in SSA will increase from 162 million in
1988 to 239 million, while sheep and goats will in- crease from 270 million to 562 million Winrock In-
ternational, 1992. Because of the existing mismatch between the feed potential of the main agroecological
zones and their respective current ruminant popula- tions, the moist savanna ecozones of West Africa are
likely to experience even more rapid increases in ru- minant populations than other ecozones, as a function
of both natural accretion and immigration. Seasonal movement of livestock from feed and water deficient
arid and semiarid areas to the sub-humid zones, where feed and water resources are more abundant, has been
historically practised by transhumant livestock pro- ducers Jahnke, 1982.
2.1. The humid zone The agroclimatic limits for the humid zone stretches
across both sides of the equator — Sierra Leone in the west to Madagascar in the east Fig. 1. Man has
modified most of the rainforest of Africa in some way. Nonetheless, the Zaire basin forest is nearly intact, and
by global standards, has a relatively slow rate of clear- ing. The present forest is a mosaic of different types of
land use, patches of secondary forest and fallow vege- tation, and small remnants of primary vegetation. The
zone is varied, embracing areas differing significantly in climate, vegetation, and soil resources. Population
densities vary considerably, with south–eastern Nige- ria recording the highest density in SSA, and the Zaire
Basin among the lowest. There are areas with large urban populations, and other areas that have a poorly
62 S.S. Jagtap, A.K. Chan Agricultural and Forest Meteorology 103 2000 59–72
developed infrastructure, with poor roads and a very uneven distribution of market outlets.
2.1.1. Production systems Manyong et al. 1995 identified over 80 traditional
farming systems in the zone consisting shifting cul- tivation and fallow rotations. In parts of the region,
the economy is largely dependent on plantation crops, such as cocoa, rubber, oil palm and kola nut. Cassava
is a major staple, and in many areas, cash crop. Fish- ing is common in the coastal areas and along the ma-
jor riverbanks in Congo, Central African Republic and Zaire. Maize is increasingly intercropped with cas-
sava in this zone because of its complementary growth habit and consumer appeal. The other major staples
are yam, plantain and banana. Food crop productivity in the humid forest zone does not have the same po-
tential as that of the moist or mid-altitude savannas be- cause of the biophysical limitations of the environment
— low insolation and infertile soils. Among the food crops, the root and tuber crops cassava, cocoyam, yam
and sweet potato together with plantain and banana have the highest potential. The highest market poten-
tial for the region remains with perennial, particularly tree-based, production systems, oil palm, cocoa, rub-
ber, timber, fruits and medicinal products. Because of widespread trypanosomiasis, cattle are less common
in this zone than in the drier zones, but goats, sheep, and poultry are common.
2.1.2. Ecological constraints Cassava has a spectrum of pest problems, e.g., cas-
sava mosaic virus, cassava bacterial blight, cassava mealybug, cassava green mite, cassava anthracnose
Colleotrichum gloeosporioides, and the variegated grasshopper Zonocerus variegatus. Models based on
agroclimatic factors and insect physiology have been developed to target control methods for pest con-
trol in cassava, maize and cowpea Neuenschwander et al., 1989. The other biggest constraints are poor soil
fertility, low water holding capacity soils and weed competition IITA, 1988. The soils are susceptible to
degradation, and there is a tendency for soil productiv- ity to decline rapidly with repeated cultivation Kang,
1993. The rainfall is sufficient but the low water hold- ing capacity of the soil often results in short droughts
and low productivity. The general characteristics of
Fig. 2. Variation in 30-year annual rainfall, rainfall range and coefficient of variation ranked by latitudinal position in Nigeria.
annual rainfall shown in Fig. 2 between 1961–1990 over Nigeria reveals that the variability in the rainfall
increases with latitude. The coefficient of variation being less than 20 south of 11
◦
N while it is above 25 north of 11
◦
N latitude. Also, the rainfall range maximum–minimum in the south, the humid zone,
is three-times that of the drier areas in the north. Thus the yields vary dramatically due to rainfall variability
as shown in Fig. 2. Twenty years of daily historical weather data were used at Lagos in the humid zone
Fig. 3 and Kaduna in the sub-humid zone of Nigeria to generate probabilistic estimates of maize yields as-
sociated with the nitrogen N and variety technology using validated CERES MAIZE model Jagtap et al.,
1993, 1999. At Lagos with 200 kg Nha, the mean yield was about 5000±2100 kgha for the January
planting and increased to 7430±100 kgha for the March planting. The yield declined rapidly thereafter
due to dry August conditions during the grain filling period. On the contrary, the yields at 30 kg Nha were
1600±660 kgha and 2640±260 kgha for the January and March planting. Planting maize early in the rainy
season is becoming popular amongst farmers due to high demand for green maize from large cities and
the revenue it generates. Due to the high risks associ- ated with climatic variability, there has been virtually
no small farmer adoption of improved soil and crop
S.S. Jagtap, A.K. Chan Agricultural and Forest Meteorology 103 2000 59–72 63
Fig. 3. Planting date and nitrogen input effects on the mean and standard deviation of 20-year simulated maize yields at Lagos in
Nigeria.
management technologies recommended by research and extension systems. As clearly hypothesized by
Anderson World Bank 1993, without risk reducing information, most of the recommended technologies,
including chemical fertilizer use, are not likely to be more profitable than existing practices, given the con-
strained resources of affected farmers. Most farmers get about 500 kgha or less with no fertilizer input.
Currently several countries in the region are devel- oping potential for supplying weather information to
consumers.
2.2. The sub-humid zone The sub-humid zone characterized by a growing
season of 151–270 days occupies about 1000 million ha extending in a band across West and Central Africa
and includes coastal lowlands in eastern and southern Africa. The sub-humid zone can be subdivided into
four zones, based on their length of growing period LGP, vegetation, and agricultural potential. The
northern guinea savanna LGP of 151–180 days, the southern guinea savanna LGP of 180–210 days;
the derived savanna LGP of 211–270 days; and the coastal savanna LGP 211–270 days. The rainfall
pattern is monomial in the northern and southern guinea savanna, but bimodal in the derived and coastal
savanna. The zone is suited to medium-duration an- nual crops, such as maize, cotton, groundnut, and soy-
bean. In the more humid areas of the zone, insolation declines but total biomass production is greater. This
provides a comparative ecological advantage for live- stock pastures and longer-duration crops, such as rice,
yam and cassava in the southern and tree crops in the derived and coastal savannas. The sub-humid zones of
eastern and southern Africa are climatically more fa- vorable for agricultural production than West Africa.
Insolation is greater, increasing photosynthesis; tem- peratures are cooler, extending crop growth duration
without inhibiting photosynthesis, so that net crop as- similation of photosynthate is increased; and humidity
is lower, limiting the importance of fungal diseases.
2.2.1. Production systems Study of Manyong et al. 1995 found a mosaic
of over 100 farming systems in West Africa. In this zone, crop enterprises is the predominant activity,
although cattle and other ruminants is an important component of the systems. Areas endowed with de-
velopment of roads and marketing infrastructure, display a tendency towards reduction in the diver-
sity of farming systems. Short–medium cycle annual crops such as maize, cowpea, sorghum, millet, and
cotton are predominant in the northern guinea sa- vanna Table 2. Photosensitive, long-cycled millets,
sorghum and maize extend to the southern guinea savanna, where long-cycled annuals yam or semi
perennials cassava become increasingly important. Cropping systems in the derived savanna include
long-cycled annuals and semi-perennials intercropped with short-medium cycled annuals, such as maize or
cowpea. Cash crops e.g., maize and cotton, that re- spond well to the use of purchased inputs and produce
high marketable surpluses, dominate in areas with ac- cess to markets. Use of fertilizer is limited to ground-
nuts, cotton and maize, little fertilizer is applied to the sorghum or millet crops; however, they may obtain
some benefit from the phosphate residues after maize, groundnuts, or cotton. Cassava is increasingly planted
where soil fertility is declining. In East Africa, where the rainfall distribution is mainly bimodal, bananas
are an important staple food at the middle altitudes in Rwanda, northern Tanzania, and Uganda, together
with sweet potato, cassava, maize, Phaseolus beans, and some swamp rice. Robusta coffee is the main cash
64 S.S. Jagtap, A.K. Chan Agricultural and Forest Meteorology 103 2000 59–72
Table 2 Relative area grown to various crops by agroecological zones in West Africa Manyong et al., 1995
a
Crop Humid zone
Derived savanna Southern guinea savanna
Northern guinea savanna Other
Maize 18.1
29.2 29.2
21.1 2.4
Cassava 30.0
49.6 14.0
3.1 3.3
Yam 27.1
48.5 18.1
4.9 1.4
Plantain 90.1
9.5 0.0
0.4 0.0
Cowpea 4.3
23.1 31.4
40.4 0.8
Soybean 0.0
80.0 14.3
1.4 4.3
a
Areas do not add to 100 due to predominance of mixed cropping.
crop, and some tea, sugarcane, and pineapple are also grown. Many different fruit trees are planted, includ-
ing citrus, loquat, avocado, breadfruit, and jackfruit. At the higher altitudes, maize is the dominant cereal,
with Phaseolus beans, sweet and Solanum potatoes, bananas, arabica coffee, tea, pyrethrum, and a wide
range of vegetables and other crops.
2.2.2. Ecological constraints Rapidly increasing human and animal population
pressures in the savanna zones are driving agriculture towards increasing intensification. Although crops and
crop varieties can fit the average LGP in each area, erratic rainfall early in the growing period can cause
drought stress. The second rainy season in the derived savanna is unpredictable, making it risky for late ma-
turing and drought-susceptible crops, e.g. maize. Con-
Fig. 4. Decline in the annual rainfall during the 1971–1980 and 1981–1990 decades relative to the 1961–1970 decade at 23 stations in Nigeria ranked by latitude.
tinuous rains after the first crop has been harvested, can cause considerable problems in drying and stor-
age of grain. Parts of the zone, particularly where soils are light or shallow, can sometimes be difficult for an-
nual crop production because, although two crops per year can sometimes be grown, there may not be suf-
ficient rain in either season to produce an optimum crop. Daily rainfall at 23 locations in Nigeria with 20
or more years of records for the 1960–1990 period were analyzed for temporal and spatial changes in an-
nual, seasonal and monthly rainfall, and the number of wet days and rainfall intensities. Annual rainfall
exhibited the persistence of below normal years and declining annual moving averages Fig. 4. Compared
with 1961–1970 years, the rainfall in 1981–1990 was over 200 mm less in the humid zone, 100 mm less in
the bimodal rainfall regions, and from 128 to 331 mm
S.S. Jagtap, A.K. Chan Agricultural and Forest Meteorology 103 2000 59–72 65
less in the northern and southern guinea savanna. The 900, 1100, 1300 and 1500 mm rainfall isohyets were
shifted south throughout the country by 167, 138, 108 and 50 km during the same 20 years. The great-
est changes occurred in the early season rainfall. The growing season shortened by nearly 1 month during
the period 1981–1990 compared with the 1961–1970 decade. The temperatures during the growing season
are typically above 23–24
◦
C. Further decline in rain- fall combined with green house warming could ad-
versely affect crop yields. Furthermore, the 1981–1990 decade recorded fewer wet days and therefore much
higher rainfall intensities Jagtap, 1995a. A high pro- portions of rains in Africa are associated with wave
disturbances giving rise to strong convective activities which results into short intense storms with peak inten-
sities sometimes exceeding 100 mmh. These intense rains frequently result in considerable compaction, soil
erosion and large runoff particularly in the absence of surface cover. Using Fournier 1962 index Jagtap
1995b estimated that one-thirds of the sub-humid zone lies in moderate erosion risk category while re-
maining two-thirds in the high-risk class. Although detailed information is still lacking on both the ero-
sivity of the rains and the erodibility of the soils in many parts of Africa, it is clear that the risks of accel-
erated erosion are very high in these areas. Improved water harvesting techniques and soil erosion preven-
tion methods are rarely used. The zone is widely con- sidered to be the zone at greatest risk of declining
agricultural production at present, and parts of it have been severely affected by erosion, soil degradation and
drought in recent years. At the same time, the parts that receive adequate rain have an enormous potential
for the expansion of rainfed agricultural production.
3. Asia
