RA 00110.pdf - OAR@ICRISAT

to have poor, sandy, gravelly top soils, with a low
kaolinitic clay fraction and relatively poor organic
matter content. Together these lead to low cation
exchange capacities (5 meq 100 g -1 soil), and thus
low buffering capacities (Stoop 1984b). Such soils
are often shallow (25-50 cm) and droughty since they
are frequently located over lateritic caps remaining
from the old, highly weathered landscape. Lower
slope fields are generally deeper, have higher organic
matter content, and higher fractions of swelling
clays, which together improve water-holding, cation
exchange, and buffering capacities. Experiments of
the ICRISAT Agronomy Program in Burkina Faso
have shown that there are highly significant interactions
between land type and crop on grain yields
(Stoop 1984b). Results indicate that the optimal
allocation of cereals along the toposequence is millet
on upper-slope fields, white sorghum on mid-slopes,
maize and red sorghum along lowland margins, and
rice in temporarily inundated swamp land.
Farmers' actual land-use patterns generally follow
this model, particularly in the Sudan savanna and
northern Guinea savanna zone where rainfall is sufficient
to support the widest cropping options. In
these zones, farmers use millet as a means of exploiting
their least productive land, in part to insure
additional food security in low rainfall years. Farmers
recognize that sorghum may produce more food on
such soils under good rainfall conditions, but that in
poor years millet yields tend to be more assured. An
analysis of time series data on farm-level yields for a
range of crops has confirmed that the interannual
coefficient of variation is lowest for millet (Lang et
al. 1984). The generally low input, extensive management
of millet is consistent with its role in the
farmers' strategy to reduce aggregate production
risk.
Evolution of W A S A T Farming
Systems and Implications for
Varietal Change
The below-average rainfall, which has prevailed
nearly unbroken in much of the W A S A T since the
late 1960s, has combined with a longer-term increase
in population pressure to destabilize traditional
farming systems and to create new and increasingly
urgent pressures for change. Lower mean rainfall
has been accompanied by shorter cropping seasons
and greater intraseason variability. As a result, traditional
crops, cultivars, and crop mixtures are
becoming less well adapted than in earlier periods.
Simultaneously, the rapid increase in rural populations,
currently estimated to be nearly 3% a -1 (which
will double the population in less than one generation)
has forced farmers in many regions to reduce
fallow periods and to expand cultivation to more
distant fields, and to marginal soils located on the
upper portions of the toposequence. One result is
that farmers demand cereal cultivars with shorter
maturity and greater stability under low-fertility and
drought conditions for these environments.
Diversity and Indigenous Change in
Local Millet Cultivars
I C R I S A T surveys have confirmed that farmers in
each zone test and adopt new millet cultivars better
suited to the new climate and soil conditions (ICRI-
SAT 1984, pp. 328-330). Farmers in the ICRISAT
study villages distinguished an average of between
four and seven local cultivars of pearl millet pervillage
in the three zones. Adoption histories often
showed rapid and widespread adoption of new varieties
and indicated that the pace of varietal change
has probably accelerated during the last 20 years. In
one of the Sahel study villages, for example, all but
one of the seven millet cultivars currently sown by
farmers had been introduced during the last 15
years, and of the six new cultivars, five were locals
which had been introduced informally by farmers
acting independently of the agricultural extension
system. It is significant that the characteristic common
to all of the most recently adopted local cultivars
was their shorter maturity.
A statistical analysis of farmers' planting dates by
cultivar confirmed that farmers in the Sudan and
northern Guinea savanna zones exploit this genetic
diversity by changing to relatively shorter-duration
millet cultivars as the planting season progresses.
Employing an analysis of variance, highly significant
(P