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The main characteristics of these systems may be summarized as follows:
Progressive clearing of land; removal of stumps, if any.
'Half' shifting cultivation: The villages are permanent, only the
fields change. The cropping cycles are broken by fallows, the
duration of which is decreasing more and more.
Livestock may be present, but there is no true mixed farming. Owing
to the lack of animal power, it is not possible to have deep tillage and to incorporate harvested residues or grass fallow into the soil.
Owing to inadequate equipment, there is poor control of weeds.
Mineral fertilization is absent or inadequate.
The number of plants which can be cropped is limited, species and varieties having high requirements for water; and mineral elements cannot be utilized.
Rotations are mor e or less without definite plan.
Association of plants on the same field (multiple cropping) is
frequent. >
Selected varieties are little used.
Control of diseases and parasites is inadequate.
As noted above, despite low yields, such systems can no longer maintain soil fertility, in contrast to former traditional systems based on short cropping cycles and long resting periods.
1.2 Improved Traditional Systems
As a first step, agronomists tried to improve the peasant systems of cropping without upsetting the basic system and using only techniques
adapted to the very low financial resources of farmers. Efforts were
made successively on the following:
Use of high-yielding and disease-resistant varieties.
Disinfection of seeds, which is an inexpensive and beneficial operation; in some cases (mainly for cotton), control of diseases and pests during the vegetative cycle.
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Effects of length, nature and various treatments of grass fallows on crop yields were studied in m a n y long-term experiments, especially by
IRHO in Senegal (IRHO-Senegal, 1960-1968; I R H O - Senegal, 1962-68). From
these data, it seems that the cffects of burned short-term grass fallows on crops yields would be of some importance in the poorest and driest regions (northern and central Senegal) and of little importance in the regions where soils and climates are m or e favorable to agriculture, as in southern Senegal (Charreau, 1972-73).
2.2 Rotations in the Semi-intensive Systems of Cropping
In the semi-intensive systems of cropping, the duration of grass
fallow is reduced to one year. It can, however, be two years, in the
driest regions. As the grass fallow is incorporated in the soil by
plowing at the end of the rains, effects on soil properties are much
more beneficial than in the case of burned grass fallow. Soil m a c r o
structure is largely improved for two or three years, resulting in
beneficial effects on rooting of the succeeding crop. Water stored in
the soil profile by the end of the rainy season is conserved throughout
the dry season. Control of weeds is easier. Effects on the humus
balance are likely to be mor e marked, although there is little evidence
of this. Grass fallow can be replaced by a green manure or an annual
forage crop. Pearl-millet and sorghum, planted at high density, are
most commonly used for this purpose. By the end of August, the forage
is mowed and the regrowth is plowed under by the end of September or in
October. It is thus possible to combine the benefits of soil improvement
and of forage production. Such green manures or annual forage crops
have about the same benefits on soils as grass fallow but they usually have a higher production in dry matter and are more easily mowed and
plowed under than natural grass fallow. On the other hand, their
extension to farmers is m o r e difficult (Charreau and Nicou, 1971).
After plowing under the fallow or green manure, the cropping sequence which has been recommended for a long time in Senegal is the same as in the improved traditional systems of cultivation, that is: one year of fallow or green manure plowed under - groundnut - cereal - ground
nut or cowpea. The cereal usually is a long-season pear1-millet or
sorghum.
2.3 Rotations in the intensive systems of cropping
In these systems, recently studied, the resting period is eliminated and plowing under grass fallow or green manure is replaced by plowing
^ Institute de Recherches pour les Huiles et Oleagineux or Research Institute for Oil and Oil Crops.
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a au jB n nu pu no aS UB qa aa ^a aq s ao p x^ 3^ 30 E S V •a nx BA g o s A bm xb s T ^n up un oa S - xB 9: is o - an up un oa S - aa nu Bu i ua aa S sb ^ ax xT ™-!^ 3^ i 0 MO TI Bá tuo T^ B^ oa a sa A anoj ot ss bx o aq x *uo x3Ba oa aqa jo S uT uu TS aq a q^ 3B aa pu n pa wo xd mo xx ^J ss sa S b a AB q ao T St 7Two kinds of rotations are presently practised:
(1) A four-year rotation: Fallow plowed under-maize-rainfed rice-
groundnut; or
(2) A three-year rotation: Maize (with straw incorporated into soil)-
rain-fed rice or late p e a r1-millet - groundnut.
Studies on rotations are still going on in Senegal and other
countries of dry West Africa. Special attention should now be paid to
two topics which might become of increasing interest due to present
changes in economic conditions. They are: (i) The use of legumes as an
annual forage crop and/or green manure for replacing grass fallow, or cereals used as forage or green manure, and (ii) the introduction in the rotation of annual or perennial crops with deep rooting, such as Ca.janus ca.jan (Pigeon-pea) .
Legumes as green manure or/and forage were studied many years ago in French-speaking countries but for various reasons, both technical and economic, they were judged to be of secondary interest and their study
was somewhat neglected. As nitrogen fertilizers are now very expensive
in these regions, and as substantial progress has been achieved in
recent years in soil and crop science, including inoculation of legumes, this question should be examined again with a new perspective.
In the same manner, deep-rooted plants would be useful for recovering
the mineral elements lost by leaching from surface soil layers. This
would save appreciable amounts of fertilizers. Lucerne (alfalfa) plays
this role in temperate and Mediterranean countries. Pigeon-pea (Ca.janus
cajan) or other plants could play a similar role in tropical countries. More thorough studies should be undertaken on this.
3. MULTIPLE CROPPING
Multiple cropping is the general rule in traditional systems of
cropping. The most common associations of plants in the same field are:
Groundnut and cereals (pearl-millet or sorghum) and pearl-millet and
cowpea (in the northern zone). Multiple cropping has m any advantages
in the exploitation of moisture and mineral reserves of the soil by the differential rooting of the crops, mor e effective use of light, the
modification of microclimate, and the soil cover. Some experiments on
multiple cropping have been carried out in Senegal (Nicou, 1964-65;
Charreau and Nicou, 1971; Schilling, 1965) and in Niger (Nabos, 1963-65); they generally showed a slight agronomic superiority of multiple
