RA 00048.pdf - OAR@ICRISAT

plementarity in intercropping. In particular, the
large advantages with some of the legumes
(notably soya and groundnut) compared with the
small advantages with other cereals emphasizes
the benefits that might accrue due to major
morphological or physiological differences between
the crops. Unfortunately few of the sorghum/legume
experiments reported have been
sufficiently detailed to indicate how such yield
advantages may occur, but some useful pointers
can perhaps be gained from recent detailed
studies with pearl millet/groundnut.
In an experiment with 1 row millet:3 rows
groundnut, a yield advantage of 2 6 % was
obtained (Reddy and Willey 1981). Measurement
of resource use showed some evidence of more
efficient water use, both in terms of a greater total
extraction from the profile and in terms of a
greater proportion of water passing through the
crop rather than being lost by evaporation. Nutrient
uptake was also greater, the increase being
proportional to the increased yield; however,
though it was useful to note that this greater
uptake could occur, it was not possible to determine
whether this was a cause or an effect of
higher yields. But the most striking effect was an
increase of 26% in the efficiency with which
intercepted light was converted into dry matter,
probably because of a complementary effect
between the C 4 millet and the C 3 groundnut. It
seems likely that most of these effects could
equally well occur with sorghum systems.
A further possible cause of yield advantage in
these sorghum/legume mixtures could be a
beneficial nitrogen effect from the legume, but
the authors consider this unlikely. This effect is
extremely difficult to determine in the field, but
even claims of such an effect are exceptions
rather than the rule. Moreover, cooperative studies
with the ICRISAT microbiologists on groundnut
intercropped with sorghum, maize, or pearl
millet have shown that even where groundnut
growth has not been affected, the nodulation and
fixation have been much reduced, probably because
of shading.
With Earlier-maturing Crops
Even with the short-season sorghums, some of
the very early legumes can provide an intercrop
that will mature a few weeks earlier. Such
combinations might well benefit from some temporal
complementarity as well as the possible
spatial complementarity discussed above. With
mungbean, which usually matures in 65-70 days,
a review (Rao and Willey 1979) of 12 experiments
conducted in India reported yield advantages of
3 1 % (100% sorghum and 3 1 % mung) in a 2
sorghum: 1 mung arrangement and 3 4 % (95%
sorghum and 3 9 % mung) in an alternate row
arrangement. May and Misagu (1980) also reported
very good mung yields (93% in 1 year and
8 6 % in another, averaged over 20 genotypes) but
they did not report sorghum yields. With cowpea,
six Indian experiments (AICSIP 1981) showed
similar overall advantages (34%) to those with
mung, though the cowpea proved to be slightly
more competitive (87% sorghum and 4 7 % cowpea).
Studies in East Africa with a Phaseolus bean
maturing about 1 month earlier than the sorghum
showed advantages up to 55% (Osiru and Willey
1972).
S o m e other Aspects of Intercropping
It is often suggested that intercropping gives
better control of weeds, pests or diseases.
Considering the weed control situation first, there
is good evidence that this can be improved where
the intercrop situation provides a community of
plants that are in total more competitive than the
individual crops. Thus Rao and Shetty (1976)
showed that the high populations required to give
maximum yield in sorghum/pigeonpea also gave
advantages of greater weed suppression. It has
also been emphasized that these higher populations
and the associated improvement in ground
cover can give better weed control in other
combinations (Okigbo 1981).
But with pests and diseases the situation is
much more complex. Detailed ICRISAT studies in
sorghum/pigeonpea have shown that the presence
of sorghum increases the incidence of
Heliothis pod borer on the pigeonpea, but it can
markedly reduce pigeonpea wilt disease. It is also
well known that in West Africa sorghum (or other
cereals) can reduce the thrip incidence on intercropped
cowpea. These examples illustrate that
intercropping with sorghum can produce both
beneficial and adverse effects on pests and
diseases; and while these may be associated with
cereals in general rather than sorghum in particular,
they are none the less important.
Because of the poor conditions under which
sorghum is often grown, a particularly relevant
aspect of intercropping is that relative advantages
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