RA 00048.pdf - OAR@ICRISAT

may often be greater under low fertility and/or
low moisture situations (Rego 1981; Chowdhury
and Misagu 1981; IRRI 1975; Reddy and Willey
1980; Natarajan and Willey 1980; Oelsligle et al.
1975). This is undoubtedly one of the mechanisms
that could result in the greater yield stability
that is often claimed for intercropping and that has
been emphasized for maize/sorghum in Central
America (Anderson and Williams 1954) and the
long season sorghum systems of West Africa
(Norman 1972; Baker 1978). Recently, a detailed
study (Rao and Willey 1980b) has been carried out
on 94 sorghum/pigeonpea experiments in India,
ranging from rainfall regimes of 408-1156 mm
and across sole crop yield levels of 310-6200
kg/ha for sorghum and 274-2840 kg/ha for
pigeonpea. If crop "failure" is measured as
monetary returns falling below a given "disaster"
level, then for an example disaster level of
Rs.1000/ha, sole sorghum fails 1 year in 8, sole
pigeonpea 1 year in 5, but the intercrop only 1 year
in 36.
Sequential Systems
Sequential cropping requires a relatively long
growing period and in the rainfed semi-arid tropics
it is not usually possible in the areas with lighter
soils and/or low rainfall. Thus on the Alfisols of
India it is only viable on the deeper soils with high
rainfall, and even then it is probably limited to
hardy "catch" crops grown after a rainy-season
sorghum (AICRPDA 1976). But in more favorable
situations, such as the deep Vertisols, there are
many reports of short season sorghum crops
being successfully followed by a wide range of
second crops (Rao 1975; AICRPDA 1980; Reddy
and Willey, in press). Studies over a number of
years at ICRISAT have indicated that productivity
can be 4 0 - 1 0 0 % greater than traditional single
crop systems. However, although this degree of
increased productivity is extremely attractive,
there can be considerable practical problems for
the farmer, mainly because of the higher inputs of
fertilizers, sprays, labor, and managerial skill required.
In particular, the success of the system
very often depends on a rapid turnaround between
the two crops to ensure adequate moisture
for establishment and subsequent growth of the
second crop; but this period is also a very critical
labor peak for harvesting and threshing the first
crop. To facilitate this turnaround it would seem
worth considering the minimum tillage systems
that have proved possible with sequential sorghum
systems in developed areas (Nelson et al.
1977; Camper et al. 1972).
Sequential systems based on a rainy season
sorghum crop also raise the dilemma of what the
optimum growing period of the sorghum should
be; earlier maturity increases the likelihood of
good moisture conditions for the second crop but
it increases the problems from head molds or wet
harvesting conditions. This is the kind of cropping
system decision that the authors are currently
trying to elucidate (in conjunction with ICRISAT
agroclimatologists) by fitting a water balance
model (Reddy, in press) to long-term rainfall data.
Table 1 gives the example of an Indian location,
Indore, which has 1000 mm of rainfall and which
is on deep Vertisols with a moisture storage
capacity of 250 mm. After a 105-day sorghum
crop, the probability of having sufficient stored
moisture and rainfall for a second crop is only 51 %
of the years. But, almost half of these years will
have insufficient showers for ensuring that the
top 2-3 inches of soil are wet enough for
germination. Thus despite the apparently favorable
rainfall and soil situation, the probability of
double cropping at Indore with a 105-day sorghum
as the first crop is only 27% of the years.
However, if the sorghum growing period is
reduced by 2 weeks to 91 days, the probability is
substantially increased to 60%. At the same time
the probability of having wet conditions at harvest
is only increased from 24 to 30%. As a first
approximation, therefore, the analysis suggests
an overall advantage for a sorghum of less than
105 days for this particular location. But the
moisture criteria tentatively used in the model still
need further consideration and refinement before
reliable conclusions can be drawn. On the whole,
however, the approach would seem to have
potential.
The above analysis also raises an important
point of comparison between these sequential
systems and a sorghum/pigeonpea intercrop. In
the latter system the pigeonpea effectively acts
as a second crop but the difficulties of having to
establish a crop after sorghum harvest are
avoided. Not surprisingly, therefore, where there
are risks associated with establishing a second
crop, the intercrop is usually a better alternative
and is certainly much more stable; in fact in the
Indore example the probability of success for the
intercrop is 97%.
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