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Resource use in intercropping systems
Agricultural Water Management, 17 (1990) 215-231
215
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
RESOURCE USE IN INTERCROPPING SYSTt~ R.W. WILLEY School of Development S t u d i e s , U n i v e r s i t y of East Anglia, Norwich, England ABSTRACT Willey, R.W.,1990. R esource useinintercroppingsystems. Agric. Wa~rManage.,17:215-231.
Sorghum/pigeonpea and millet/groundnut intercropping systems are described to illustrate temporal and spatial complementarity and the magnitude of yield advantages that can be achieved in intercropping compared with sole cropping. The concept of considering environmental resource use in terms of 'resource capture' and 'resource conversion efficiency' is outlined and is then used to examine the resources of light, water and nutrients. Some implications for sugarcane intercropping are considered with particular emphasis on complementarity, resource use and the need for clear evaluation of objectives. i.
INTRODUCTION Intercropping
more
crops
on
is usually defined as the growing together of two or
the same
area of
land at
the
same time.
It is a very
widespread practice in the developing tropics and much recent research has shown that it can often produce substantially higher yields than sole crop systems (pure stand systems).
One of the great attractions of intercropping
is that this yield
can usually be achieved simply and cheaply,
advantage
namely by growing crops together rather than separately. Of the several mechanisms
that can bring about these higher yields
(Willey, 1979), the most widely-applicable one is that intercropping systems can make better use of environmental resources : it is this mechanism that is the subject of this paper. 2.
POTENTIAL YIELD ADVANTAGES The underlying principle
of better resource use in intercropping is
that if crops differ in the way they utilise environmental resources then, when
grown
together,
they
can
'complement'
each
other
and
make
combined use of resources than when they are grown separately. this complementarity
can be regarded
their major demands
on resources
as
'temporal',
at different
times,
where or
© 1990 Elsevier Science Publishers B.V.
Broadly,
the crops make 'spatial',
differences in resource use arise from differences in canopy or root
0378-3774/90/$03.50
better
where
216
R.W. WILLEY
dispersion.
These
illustrated
by
simple
two
annual
principles
of
complementary
intercropping
systems,
resource
use
are
sorghum/pigeonpea
and
m i l l e t / g r o u n d n u t , which have been studied in some d e t a i l at the I n t e r n a t i o n a l Crops Research I n s t i t u t e f o r the Semi Arid Tropics (ICRISAT), in India. Sorghum/pigeonpea India.
It
is
one of
the
commonest
intercropping
systems
in
i s t y p i c a l of many ' t e m p o r a l ' combinations throughout the world
in t h a t i t combines a rapid-growing, early-maturing crop (the sorghum) with a slow-growing,
later-maturing
one
(the
pigeonpea).
Tile
o b j e c t i v e with sorghum/pigeonpea i s to produce a reasonably
farmer's 'full'
the s t a p l e sorghum with some a d d i t i o n a l y i e l d of the pigeonpea. patterns
illustrated
main
y i e l d of The growth
in Fig. l a are f o r a p l a n t i n g p a t t e r n of 2 rows sorghum
to 1 row pigeonpea a t 45 cm row spacing. about 90 days d u r a t i o n
The sorghum was an e a r l y hybrid of
and the pigeonpea about
170-180 days.
Within-row
spacings were reduced in i n t e r c r o p p i n g so t h a t each component had the same p l an t population as i t s sole crop.
Sorghum was much the more competitive
of the two crops and its dry
matter accumulation in intercropping was little affected by the presence of pigeonpea; crop.
final sorghum grain yield in intercropping was 95% of the sole
In contrast,
the slow
initial
growth
of
the pigeonpea
was
even
further suppressed by competition from the sorghum and final dry matter yield of
the
pigeonpea
Interestingly, vegetative
in
because
intercropping sorghum
growth of pigeonpea,
was
only
competition
53%
of
sole
suppressed
only
seed yield was 72% of the sole crop. an
intercropping
the
early
the harvest index of intercropped pigeonpea
was increased to 30% compared with 22% in the sole crop. gave
pigeonpea.
advantage
of
Final pigeonpea
Combining these grain and seed yields 679
compared
with
growing
the
crops
separately (i.e. a Land Equivalent Ratio, or LER, of 1.67). The
effect
of
temporal
complementarity
in
this
illustrated by the light interception patterns in Fig. lb. sorghum pigeonpea
ensured
good
early
interception
some later interception
of light
of
light
and
combination the
presence
of
: total light interception was
higher than in sole crops, the increase being directly proportional higher yields produced.
is
The presence of
to the
Thus, a major feature of this combination was that
it achieved higher yields because of a fuller use of resources over time. This is a very simple principle but nonetheless an important one because it is undoubtedly the easiest and most assured way of achieving better resource use by intercropping.
Even for growing periods of only 5-6 months, temporal
differences between crops have commonly produced yield advantages of 509 or more (e.g. Andrews, 1972; Osiru and Willey, 1972; Krantz et al. 1976).
RESOURCEUSEININTERCROPPINGSYSTEMS a.
217
DRY MATTER ACCUMULATION
/ 10
Sole
///~Intercrop
S°rg7
~rghum
6 js
t/ha
,p
S
/" ~"
/
, "
~'Intercrop pigeonpea
/
_...I 0 100
i
b.
,
i
LIGHT INTERCEPTION Sorghum Harvest
.~. i//
80
Sole Sorghum/// 60 %
~ I"
~..""--'... -.
//Intercrop/" 40
i
,7
20
Sole -. pigeonpea
/ ~ ,
////
/
Jlntererop
#
I w~ J
20 Figure 1
40
60
i
!
80 100 120 Days after sowing
I
140
!
160
Dry matter accumulation and light interception in sorghum and pigeonpea sown as sole crops and as a 2-row sorghum : 1-row pigeonpea intererop (means of 3 years - after Willey et el, 1983).
218
R.W. WILLEY The
millet/groundnut
found
sorghum/pigeonpea
in
both
between t h e m a t u r i t y p e r i o d s of t h e c r o p s , and of c o u r s e i t cereal/low-canopy
legume
combinations
Farmers' yield
but the important groundnut cash
that
objectives
crop is usually
there are
is
and
It
many p a r t s of t h e w o r l d .
because
India
difference of
with
is
Africa. typical
contrasts
combination
the
of 1 row m i l l e t
same
within-row
'replacement' was
a
spacing
as
its
sole
so
plant
t h e growing p e r i o d
t h e growth of
little
than
crop yield
proportion;
the
75% s o l e
t h i s was o b v i o u s l y t h e r e s u l t
Fig.
for a planting Each c r o p had
populations
were
(25% : 75%).
For most of less
in
t h e major component.
crop
t o row p r o p o r t i o n s
is
so p r e v a l e n t
t o 3 rows g r o u n d n u t on 30 cm rows.
ones e q u i v a l e n t
much l e s s
seem t o v a r y a good d e a l
2a shows d r y m a t t e r a c c u m u l a t i o n a v e r a g e d o v e r 3 e x p e r i m e n t s , pattern
West
intercropped
'expected'
groundnut
from
its
sown
of c o m p e t i t i o n from t h e m i l l e t .
However, by t h e end of t h e s e a s o n g r o u n d n u t had produced a 74% biomass y i e l d , roughly
equivalent
accumulation
in
to
the
its
sown
more
proportion.
competitive
In
millet
was
contrast, more
dry
than
matter
twice
its
'expected' yield (25% of its sole crop) and final biomass was 62% of the sole crop.
Combining these yields gave an overall biomass advantage of 36% for
the intercropping
system
(LER = 1.36);
for seed yields the advantage was
rather less at 25%. Resource use in this combination but,
briefly,
there
was
no greater
is discussed
interception
in more detail
of
light
later
than would
be
achieved by growing separate sole crops (Figs. 2b), so yield increases were achieved not by intercepting more light but by making more efficient use of it.
In
contrast
combination,
to
the
temporal
differences
in
the
sorghum/pigeonpea
therefore, there must have been some 'spatial' complementarity
of light use because of canopy differences.
These spatial effects did not
produce such large yield advantages as occurred in the sorghum/pigeonpea but the avantages were still substantial and were very typical of those achieved by many 'spatial' combinations (e.g. Rao, 1986; Willey, 1979). 3.
RESOURCEUSE Distinguishing
between
temporal
way of t h i n k i n g a b o u t i n t e r c r o p p i n g crop d i f f e r e n c e s
that
can l e a d
(greater
resource
use.
can be a u s e f u l
some of t h e o b v i o u s Also,
it
indicates
u s e can be improved, namely by u t i l i s a t i o n
resource
g i v e n u n i t of r e s o u r c e ( g r e a t e r
systems
because it highlights
to better
two b a s i c ways i n which r e s o u r c e of more r e s o u r c e s
and s p a t i a l
capture)
or by more e f f i c i e n t
resource conversion efficiency).
u s e of a
RESOURCE USE IN INTERCROPPING SYSTEMS
a.
219
DRY MATTER ACCUMULATION
MILLET
GROUNDNUT
/
j.~
Sole crop
l I
6
l
Sole crop
l
t/ha "Expected"
i / ~ Actual / / / / / i n t e ....p
f~...-~m~--
4 intererop
~f
/ / ..o......Expected" / / ...-" intercrop
J
•
20
40
•
*
60
80
i
100
20
4o"
6o
8o
Days after sowing b. LIGHT INTERCEPTION 8O
60, ~% k k Sole millet
% 40
20,
"'%Sole groundnut Intercrop
J
I
,
20
40
i
|
60
80
i
100
Deys after so~,,ing
Figure 2. Dry matter accumulation and l i g h t i n t e r c e p t i o n in m i l l e t and groundnut as s o l e crops and as a 1-row m i l l e t : 3-rows groundnut i n t e r c r o p (means of 5 years - a f t e r Willey e t a l , 1985).
220
R.W. WILLEY
Light use Light is the most studied resource in intercropping systems so it is considered first to illustrate some of the approaches. light
(and with water)
A major proplem with
is that it is extremely difficult to determine how
much of the resource is being used by each of the component crops. therefore,
At best,
resource use can usually be examined only for the system as a
whole. Measurements efficiency
are
and calculations
illustrated
for
of light capture and light conversion
one
of
the
millet/groundnut
referred to earlier (Reddy and Willey, 1981 - Table 1). the intercropping
system was between
crops.
intercropping
However,
and
experiments
Light capture by
that of the two very different sole sole
cropping
cannot
be
compared
by
taking simple averages of light capture or conversion efficiency if there are differences in yields and conversion efficiencies of the two crops.
Thus,
Table 1 shows the 'expected' light capture by the intercropping system if it had produced no yield 'expected'
from
its
advantage sole
crop
intercropping was virtually
and if capture by each crop had been as capture.
identical
The
actual
light
to expected capture
capture
(I~ less,
by
or a
capture ratio of 0.99 - Marshall and Willey, 1983), indicating that despite its higher yields intercropping did not improve light capture compared with sole crops. Similarly, the light capture that would have been required to produce actual
intercropping
actual
light;
yields
it follows
at sole
crop efficiencies
was
30~ more
than
that light conversion efficiency must have been
improved by 501 in the intercropping system.
It also follows that the 28~
yield advantage in this particular intercropping system was attributable not to improved capture of light but to improved conversion efficiency. This millet/groundnut system was also the subject of a more detailed study that attempted to separate the light use of each component (Marshall and Willey, 1983).
Millet plants were found to have much greater biomass,
leaf area and light capture in intercropping similar
conversion
efficiency.
than in sole cropping but a
On the other hand,
the shaded groundnut
plants in intercropping were able to capture much less light than in sole cropping but their conversion efficiency was 461 higher and therefore their growth and yield were similar to sole crop plants.
This improved groundnut
efficiency was no doubt partly due to the greater efficiency of the C3 canopy at
lower
saturation
light of
intensities, upper
leaves
and
perhaps
: a
lower
partly energy
the cost
avoidance because
of of
N-fixation under shade may also have contributed (Nambiar et al, 1983).
light reduced
RESOURCEUSEININTERCROPPINGSYSTEMS
221
TABLE I Example of resource capture and resource conversition efficiency for light and water in millet/groundnut intercropping (after Reddy and Willey, 1981) Groundnut
Millet Sole crop biomass (kg D.M./ha) Intercrop biomass (kg D.M./ha) Component LERs Total LER .
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8085 4775 0.59
5617 3900 0.69 1.28
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LIGHT Light-capture (MJ/m2) Sole crop capture 'Expected' capture by intercropping if NO yield advantage but same ratio of crops (i.e., 46% millet + 54% groundnut)
612.5
936.2
282
506
Total
788 778
Actual light capture by intercropping CAPTORE RATIO (778/788) = 0.99 Light conversion e f f i c i e n c y (g D.M./MJ) Sole crop conversion e f f i c i e n c y 'Required' capture to produce i n t e r c r o p ) y i e l d s a t sole crop e f f i c i e n c i e s (MJ/m2
0.60
1.32 650
362
Total
1012
CONVERSION EFFICIENCY RATIO (1012/778) = 1.30 .
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WATLR Water capture (as evapotranspiration - cm) Sole crop capture 'Expected' intercrop capture (see light)
30.3 14.0 Total
36.8 19.8 33.8 40.8
Actual intercrop capture CAPTURE RATIO (40.8/33.8) = 1.21 Water conversion e f f i c i e n c y (kg/ha/cm) Sole crop conversion e f f i c i e n c i e s 'Required' capture to produce i n t e r c r o p y i e l d s at sole crop e f f i c i e n c i e s
266.8
152.6
17.9 Total
CONVERSION EFFICIENCY RATIO (43.5/40.8)
25.6 43.5
=
1.07
.
.
222
R.W. W[LLEY Summarising the l i g h t use f e a t u r e s in t h i s m i l l e t / g r o u n d n u t system,
capture
was not
index.
Thus the t w o - t i e r e d i n t e r c r o p p i n g canopy r e s u l t e d in the same amount
increased
despite
a substantially
higher
of l i g h t being d is p e r s e d over a l a r g e r area of l e a f .
total
leaf
area
The e r e c t canopy of
m i l l e t u t i l i s e d the higher l i g h t i n t e n s i t i e s a t i t s normal C4 s o l e crop efficiency,
and the
compact,
planophile,
C5 canopy of
the
particularly efficient
use of the lower l i g h t
intensities.
improvement in
use
considerable
light
that
occurred
has
groundnut made The very l a r g e implications
for
other i n t e r c r o p p i n g systems t h a t d i s p l a y any of these canopy c h a r a c t e r i s t i c s . Water use The concepts
of
capture
and conversion e f f i c i e n c y are
again u s e f u l
when thinking about water use, but f u r t h e r f a c t o r s have to be considered. The p o s s i b l e ways in which i n t e r c r o p p i n g could
improve water use compared
with sole crops can be l i s t e d : 1.
Increase p l a n t a v a i l a b l e water
2.
Increase the t o t a l amount of water withdrawn from the s o i l ( i . e . as e v a p o t r a n s p i r a t i o n )
5.
Increase the p r o p o r t i o n of e v a p o t r a n s p i r a t i o n w h i c h is transpiration
4.
Increase conversion e f f i c i e n c y
5.
Increase p a r t i t i o n i n g i n t o t h a t f r a c t i o n of the crop t h a t is r e q u i r e d as economic y i e l d ( i . e . ,
Strictly transpired
:
transpiration. thought
of
in
increase h a r v e s t index).
speaking, water capture should r e f e r to the amount of water similarly,
conversion
In p r a c t i c e , less
rigid
efficiency
however, both these
terms
and
may have
should
be
based
on
concepts may have to be to
be
based
on the
more
measurable e v a p o t r a n s p i r a t i o n . On the p o s s i b i l i t y of i n c r e a s i n g p l a n t a v a i l a b l e water, i n t e r c r o p p i n g may provide a g r e a t e r
canopy cover, p r o t e c t i n g the s o i l from the impact of
r a i n so improving i n f i l t r a t i o n
: greater
infiltration
has been r e p o r t e d
maize/cassava i n t e r c r o p p i n g compared with the s o l e crops (Lal, 1974).
in
These
e f f e c t s are p o t e n t i a l l y g r e a t e s t where a r a p i d l y e s t a b l i s h i n g crop is added to a slowly e s t a b l i s h i n g one which, i f grown on i t s own, would provide poor canopy cover.
There have a l s o been suggestions t h a t a r a p i d l y e s t a b l i s h i n g ,
shallow-rooted component may dry out the upper s o i l h o r i z o n s , slower-establishing, O'Brien,
1968;
IRRI,
deep-rooting 1972;
Fisher,
component 1977).
even
deeper
This e f f e c t
so f o r c i n g a
(Whittington
and
could undoubtedly
increase the amount of water p o t e n t i a l l y a v a i l a b l e to an i n t e r c r o p p i n g system
RESOURCEUSE IN INTERCROPPINGSYSTEMS
223
compared with sole crops, but there has been l i t t l e
r e a l evidence o£ such
deeper r o o t i n g a c t u a l l y o c c u r r i n g . On systems
water
have
capture
been
(as
shown
evapotranspiration),
to
withdraw more
water
several than
intercropping
their
sole
crops
(Lakhani, 1976 - sunflower/fodder r a d i s h ; Hazaheri, 1979 - maize/kale; Reddy and Willey, 1981 - m i l l e t / g r o u n d n u t ) . this
greater
withdrawal
is
that
The reason most commonly proposed for
the
different
crops
explore
different
horizons, g i v i n g a b e t t e r d i s p e r s i o n of roots throughout the whole p r o f i l e ; but g r e a t e r withdrawal could be due i n p a r t to g r e a t e r root c o n c e n t r a t i o n s . It
has
also
been suggested
that
simple
temporal
differences
in
rooting
p a t t e r n s may ensure g r e a t e r t o t a l withdrawal over the season (Baker, 1974). When
intercropping
increases
canopy
cover,
the
proportion
e v a p o t r a n s p i r a t i o n which is t r a n s p i r a t i o n i s l i k e l y to i n c r e a s e . thought
to
occur
in
both
the
systems r e f e r r e d to e a r l i e r 1980).
m i l l e t / g r o u n d n u t and the
(Reddy and Willey, 1981;
of
This was
sorghum/pigeonpea
Natarajan and Willey,
This e f f e c t i s l i k e l y to be p a r t i c u l a r l y b e n e f i c i a l under wet s o i l
c o n d i t i o n s when evaporation losses are p o t e n t i a l l y high. (1987)
has
proportion
argued of
temperature. increasing
that
greater
transpiration A
canopy
under
particularly
cover
drier
important
could
conditions
aspect
the proportion of transpiration
of
However, Rogers
still by
increase
decreasing
the soil
improving water use by
is that increased growth may be
possible without increasing total water demands (Rogers, 1987). Considering
conversion
efficiency
based
on
transpiration,
Rogers
(1987) has pointed out that this is a relatively inflexible character for any given species. on another turbulent reach
Nevertheless, he argues that a sheltering effect of one crop
could increase conversion efficiency by reducing wind speed or air movement.
light
saturation
He also points out that with C3 species, which at lower
intensities
than C4
species,
some
light
reduction at high intensities may increase conversion efficiency by reducing transpiration without a commensurate reduction in photosynthesis. Some reference
of
these
to
evapotranspiration) Calculations
aspects
of water
millet/groundnut
showed
use
(Table
21%
more
capture
than
Thus most of the intercropping
attributed
to greater water uptake.
profile
root
illustrated Water
with
further
capture
(as
was higher in intercropping than in either sole crop.
capture. total
are i).
concentrations
and
ratio of 1.07 suggested
from
sole
crop
This was probably because of higher
better
(Gregory and Reddy, 1982).
'expected'
yield advantage of 28% could be
dispersion
of
roots
throughout
the
However, a water conversion efficiency
that there might be a small increase in conversion
224
R.W. WILLEY
efficiency as well.
But this ratio was based on evapotranspiration. When an
attempt was made to bare it on transpiration (estimated from the proportion of
incoming
suggesting because
energy
intercepted
by
the
crop),
that at least some of the apparent
intercropping
the
ratio
increase
was
reduced,
in efficiency was
increased that proportion of evapotranspiration which
was transpiration. It was seen earlier that the competitive effects experienced by pigeonpea
in
intercropping
sorghum/groundnut 'line-source'
study,
could
which
technique,
increase
examined
harvest
different
showed that harvest
water
index.
regimes
A
using
a
index could also be markedly
affected by the interaction of intercropping and water regime (Natarajan and Willey, crops
1986). but
Increasing
much
less
so
although the relative
water
in
stress
decreased
intercropping
than
harvest
in sole
index in both
cropping.
Thus
intercropping advantages for biomass were relatively
constant, those for reproductive yields increased very markedly from 14~ with least water stress to a very considerable 93~ at the severest stress. reasons for these effects were not at all clear.
The
The simple explanation
would be that there was some complementarity of water use by the two crops (e.g. their roots being concentrated in different horizons), which resulted in less inter-species competition in intercropping than in sole cropping. But this does not not explain why the resultant crop effects were wholly on harvest
indices and not on biomass.
reduced
competition
groundnut,
shading
reproductive
favoured may
have
partitioning
It is possible
reproductive reduced
(Harris
growth.
plant
and
that the timing of
Or,
at least for the
temperatures
Natarajan,
and
1988).
so
favoured
Whatever
the
reasons, the results indicate that compared with sole cropping, intercropping may be able to increase harvest index and so improve the water conversion efficiency for reproductive yield.
Equally important the results indicate
that such changes in harvest index may be very dependent on water regime. It must
be
emphasized,
however
referred to was a 'replacement'
that
the
sorghum/groundnut
system
one in which each crop was sown at only a
proportion of its sole crop population and total population was equivalent to sole
crop
optima.
replacement
systems,
Given their
some
reduced
complementarity populations
experiencing less competition than in sole cropping. intercropping
systems,
where
total
populations
between
can are
result
the
crops
in each
in crop
However, in 'additive' higher
than
in sole
cropping, crops may well experience greater competition in intercropping even though some complementarity occurs.
Such high populations could at least
partly explain why some workers have found poorer intercropping advantages
RESOURCEUSE IN INTERCROPPINGSYSTEMS
225
with higher water s t r e s s ( e . g . F i s h e r , 1976). Nutrients For n u t r i e n t s , resource capture can be very e a s i l y measured as nutrient
uptake,
intercropping
and
has
many
produced
studies a
yield
have
shown
advantage.
greater As with
uptake water,
where greater
n u t r i e n t uptake i s u s u a l l y presumed to be p o s s i b l e because of g r e a t e r root c o n c e n t r a t i o n s or some complementary e x p l o r a t i o n of the p r o f i l e .
In some
i n s t a n c e s g r e a t e r uptake might be due to the use of n u t r i e n t s u n a v a i l a b l e to sole
crops
general
(e.g.
the
where
greater
i n t e r c r o p p i n g promotes deeper
uptake
probably
represents
resource t h a t is a v a i l a b l e to sole crops.
rooting).
fuller
use
of
But i n the
same
The i m p l i c a t i o n of t h i s i s t h a t
i n t e r c r o p p i n g probably makes g r e a t e r demands on the s o i l and thus i n the long term
intercropping yield
fertilizer
advantages may have to be paid for with higher
inputs.
The concept of conversion e f f i c i e n c y , i n terms of y i e l d produced per u n i t of n u t r i e n t taken up, i s not r e a l l y a p p l i c a b l e to n u t r i e n t use, though several
s t u d i e s have shown changes i n crop n u t r i e n t c o n c e n t r a t i o n s due to
competitive 1980).
effects
in
intercropping
(Hall,
1974;
Natarajan and Willey,
These changes are u s u a l l y small, and sometimes t r a n s i t o r y , but they
have p o s s i b l e i m p l i c a t i o n s where n u t r i e n t content i s a determinant of y i e l d quality. A rather
special
case
of
nutrient
use
in
intercropping
p o s s i b i l i t y of a legume crop t r a n s f e r r i n g fixed n i t r o g e n to non-legume. sole
crops
is
the
an i n t e r p l a n t e d
The p o s s i b l e n e t r e t u r n of n i t r o g e n to the s o i l by legumes as or
i n pastures
is
of course well-known.
But i t
is
easy to
overestimate t h e i r c o n t r i b u t i o n i n i n t e r c r o p p i n g systems where they are o f t e n grown as ' p a r t i a l '
crops and are s u b j e c t to competition from other crops.
In p a r t i c u l a r , Nambiar e t a l (1984) have shown t h a t the N - f i x a t i o n process, a t l e a s t i n groundnuts, may be p a r t i c u l a r l y s u s c e p t i b l e to shading. In the vast m a j o r i t y of experiments there has been no evidence of a legume t r a n s f e r r i n g any appreciable amount of n i t r o g e n during i t s period of a c t i v e growth.
However, such t r a n s f e r has been i n d i c a t e d i n some i n s t a n c e s
(Singh, 1977; Wein and Nangju, 1976); and lack of evidence may be p a r t l y due to
the
difficulties
competitive e f f e c t s .
in
separating
nitrogen
transfer
effects
from other,
A more probable t r a n s f e r e f f e c t seems to be t h a t a f t e r
senescence, or h a r v e s t ,
the legume may leave some r e s i d u a l n i t r o g e n for a
l a t e r - m a t u r i n g component, or for a following crop (Singh, 1977; Agboola Fayemi, 1972).
226 4.
R.w. WILLEY SOME RESEARCHANDMANAGEMENTIMPLICATIONS FOR SOGARCANE INTERCROPPING As with
any
intercropping
system,
the
physiological
objectives
of
sugarcane i n t e r c r o p p i n g must be to maximise complementary e f f e c t s between the component crops resources.
to ensure
the f u l l e s t
and most e f f i c i e n t
use of a v a i l a b l e
But these p h y s i o l o g i c a l o b j e c t i v e s must be a l l i e d
proportions
of
the
different
crops
are
required
by the
to whatever
grower.
And of
course t h ere may be crop management c o n s t r a i n t s t h a t determine what kind of i n t e r c r o p p i n g system i s p r a c t i c a l l y f e a s i b l e . Given the
slow e s ta b l is h m e n t
of
sugar
cane
during
the
first
3-4
months, much the g r e a t e s t scope f o r complementary e f f e c t s l i e s with temporal systems
in which annual
intercrops
are
added between cane rows to improve
resource use in the e a r l y part of the growing p er i o d .
These systems f i t
in
very well with growers' y i e l d requirements,
which are u s u a l l y to produce a
'full'
yields
yield
experiments
of
cane
have
with
'additional'
testified
producing
useful
yields.
Temporal d i f f e r e n c e s
varieties, before
yields
to
of
the
potential
intercrops
of
without
can be best
of
intercrops.
such
temporal
seriously
Many systems,
jeopardising
e x p l o i t e d by using
cane
species,
or
of i n t e r c r o p s t h a t are s u f f i c i e n t l y e a r l y - m a t u r i n g to be harvested
they
selecting
compete with
sugarcane
the
varieties
cane.
competition,
or which are b e t t e r
such as
suppression
the
differences
of
There may a l s o
which
are
able
tillering.
not
to
so
be
some scope
susceptible
to
for
intercrop
recover from competitive e f f e c t s In theory,
could be achieved by sowing i n t e r c r o p s
even g r e a t e r earlier
than
temporal the cane;
but e a r l i e r sowing of i n t e r c r o p s means t h a t f o r a given stage of cane growth the i n t e r c r o p s are more advanced and t h e r e f o r e p o t e n t i a l l y more c o m p e t i t i v e , so the y i e l d outcome is d i f f i c u l t As with
most
to p r e d i c t .
intercropping
systems,
arrangement are l i k e l y to be c r u c i a l f a c t o r s Maintaining f u l l as high as
p l an t
population
and
spatial
in sugarcane i n t e r c r o p p i n g .
cane y i e l d i s l i k e l y to r e q u i r e a cane population at l e a s t
a sole
cane
crop;
this
helps
to maintain c a n e ' s
co m p et i t i v e
a b i l i t y and i t ensures f u l l use of resources a f t e r removal of i n t e r c r o p s . (With pigeonpea, which i s a l s o the l a t e r - m a t u r i n g crop in temporal systems, even higher populations than the s o l e crop optimum are r e q u i r e d to compensate f o r reduced e a r l y branching.) particularly 'partial'
Yields of i n t e r c r o p s in cane are l i k e l y to be
dependent on t h e i r
crops;
their
'optimum'
own p o p u l a t i o n s , populations
given t h a t
would u s u a l l y
they are only be the h i g h e s t
t h a t do not compete with cane. Again thinking in terms of maintaining cane y i e l d , s p a t i a l arrangement must not be unfavourable to the cane because t h i s may reduce the resources
RESOURCEUSE IN INTERCROPPINGSYSTEMS available
to
the
crop
and
227
its
competitive
i n t e r c r o p p i n g s t u d i e s have manipulated s p a t i a l to maintain resources)
its
full
for a
A
population while allowing more space
second crop.
distances
number
of
(and thus more
This has been very s u c c e s s f u l with c e r e a l
i n t e r c r o p p i n g in I n d i a , p a r t i c u l a r l y 'inter-pair'
ability.
arrangement of one component
(Freyman
' p a i r i n g ' of c e r e a l rows to l eav e l a r g e r
and Venkateswalu,
1977).
But
sole
crop
c e r e a l s in India have c l o s e i n t e r - r o w spacings (45-60 cm) and t h e r e i s l i t t l e
scope to intercrop within these without some row modification. large
inter-row
manipulation
distances
in sugar
cane,
it seems
Given the
unlikely
that
similar
of cane rows would allow much increase in the space available
for intercrops, particularly
if a pairing arrangement excluded intercropping
from the within-pair spaces.
It also seems unlikely that pairing Cane rows
would directly benefit yield of cane, though it has been shown that pairing cereal
rows
in dry areas
within-species other
hand
spacings'
it
is
claimed
facilitates
intercrops,
can increase
cereal yields
because
the earlier
competition can increase rooting depth (Blum, 1967). that
pairing
mechanical
of cane
operations
rows
in
to give
the
On the 'pineapple
crop.
For
the
their spatial arrangement should give as uniform a distribution
as possible throughout the cane inter-row spaces, but without being so near to the cane
that
they
produce
too much
competitionbetween
the
component
crops. Briefly
considering
resources
use
aspects,
emphasis
on
temporal
complementarity
in cane intercropping means that improved light use must be
mainly brought
about by additional
There may be a l i t t l e
capture of light by the intercrops.
scope f o r improved l i g h t conversion e f f i c i e n c y i f the
i n t e r c r o p s remain long enough to provide a combined canopy with the cane, but obviously the longer the growing period of the i n t e r c r o p s the more they are likely
to
compete.
achieved with thus
Improved conversion e f f i c i e n c y
low-growing, C3 i n t e r c r o p s ,
providing
millet/groundnut
a
two-tier
one
referred
C4/C3 to
most l i k e l y
such as groundnuts
canopy
earlier.
is
like If
the
the
or p o t a t o e s ,
very
cane
component of the canopy, as i t can be with a maize i n t e r c r o p ,
to be
is
efficient the
lower
there is much
l e s s l i k e i h o o d of improved l i g h t conversion, and of course the cane is much more l i k e l y to be suppressed. Improved water use in cane i n t e r c r o p p i n g must a l s o be dependent mainly on
temporal
complementarity.
i n t e r c r o p s may u t i l i s e
The
cane
stand,
the
effect
is
likely
to
be
that
some of the water t h a t would otherwise have been l o s t
as e v a p o r a t i o n , or p o s s i b l y d r a i n a g e . full
major
intercropping
But i f i n t e r c r o p s are a d d i t i o n a l to a system
will
almost
certainly
produce
228
R.W. WILLEY
greater
d e p l e t i o n from the p r o f i l e
high r a i n f a l l intercrop
areas where the p r o f i l e
maturity,
all
the
depletion
requirement
if
by
full
water.
the
can be regarded
represents
an
additional
cane y i e l d s are to be maintained.
intercropping
as
But i f i r r i g a t i o n i s normally p r a c t i s e d , the
intercropping
some r ed u ct i o n in e v a p o r a t io n or drainage l o s s e s , mean t h a t
In
i s f u l l y r ep l en i sh ed by the time of
water used by i n t e r c r o p s
b e n e f i c i a l a d d i t i o n a l capture. greater
than would occur under sole cane.
irrigation
At the same time,
as suggested above, could
system g i v e s more economic use
I f water s t r e s s occurs, there may be p o s s i b i l i t i e s
of
irrigation
of improved water
conversion e f f i c i e n c y because of shading or s h e l t e r i n g e f f e c t s , or because of higher h a r v e s t i n d ic e s in r e p r o d u c t i v e i n t e r c r o p s (see e a r l i e r ) . are
'spatial'
enough to
effects
interact
that
with
again depend on the
the
cane;
in
these
intercrops
But these
remaining
circumstances
the
long
additional
i n t e r c r o p population w i l l almost c e r t a i n l y cause some increased water s t r e s s on the
cane,
even
if
some complementarity
occurs.
Thus
improved water
conversion e f f i c i e n c y might be achievable only at the expense of lower cane yields. As f o r n u t r i e n t s ,
i n t e r c r o p s may well improve t o t a l n u t r i e n t capture
by taking up some n u t r i e n t s
t h a t are u n a v a i l a b l e to the cane, or t h a t might
otherwise be leached.
But i t seems probable t h a t the bulk of the n u t r i e n t s
utilised
would have been e q u a l l y a c c e s s i b l e
by i n t e r c r o p s
to cane.
Thus
maintenance of f u l l cane y i e l d in i n t e r c r o p p i n g w i l l almost c e r t a i n l y r e q u i r e higher f e r t i l i s e r
inputs than in s o l e cane.
The obvious ex cep t i o n to t h i s
could be the n i t r o g e n requirement with legume i n t e r c r o p s
: and in f a c t the
temporal nature of cane i n t e r c r o p p i n g systems provides the ideal s i t u a t i o n in which some r e s i d u a l n i t r o g e n could be passed on to the cane a f t e r i n t e r c r o p harvest. But study.
all
these
suggested
effects
require
much f u r t h e r
Measurements on the micro-environment, or on p l a n t f a c t o r s
temperature
and water
interactions,
but
status,
ideally
Techniques of s e p a r a t i n g explored.
resource-use can i n d i c a t e
resource
such as
the occurrence of resource-use
use needs
to be measured d i r e c t l y .
the l i g h t and water use component a l s o need to be
Separate l i g h t use may be measurable when two crop canopies are
at d i s t i n c t l y
different heights,
but t h i s
whole of the i n t e r c r o p p i n g p e r i o d .
i s seldom the case throughout the
I t may be p o s s i b l e to separate l i g h t use
in i n t e r s p e r s e d canopies by taking measurements immediately before and a f t e r the removal of one of the components. by porometry,
and the
Water use can in theory be separated
s i l i c o n accumulation technique described by Trenbath
(1986) seems worth e x p l o r i n g f u r t h e r .
RESOURCEUSE IN INTERCROPPINGSYSTEMS As
a
evaluation
final
of
comment,
intercropping
229
careful
thought
systems.
always
And t h i s
needs
to
be
given to
e v a l u a t i o n must take
into
account the kind of o b j e c t i v e s t h a t were d iscu ssed a t the beginning of t h i s section.
The Land Equivalent Ratio
sugarcane
systems.
While
this
has
(LER) has been q u i t e widely used an important
function,
it
is
in
only a
l i m i t e d one.
An LER based on s i n g l e s o l e crops of each component ( i g n o r i n g
the f a c t
i n t e r c r o p s probably occupy the land f o r much l e s s
that
time than
cane) i n d i c a t e s i f t h e r e are any b e n e f i c i a l i n t e r a c t i o n s between the crops. For example,
thinking
only in terms of resource use,
an LER g r e a t e r
than
u n i t y i n d i c a t e s t h a t t h e r e has been some improvement in resource use due to complementary e f f e c t s of the kind discussed e a r l i e r . is
extremely important because
it
indicates
if
This b a s i c information
t h er e
are
any i n t e r a c t i o n s
t h a t might be e x p l o i t e d t o provide y i e l d advantages in p r a c t i c a l systems, or t h a t might m e r i t f u r t h e r study and development in a However, i f y i e l d o b j e c t i v e s r e q u i r e f u l l
r e s e a r c h programme. cane y i e l d plus a d d i t i o n a l
i n t e r c r o p s , the LER i s not u s u a l l y s u i t a b l e f o r f u r t h e r e v a l u a t i o n .
In
p r a c t i c a l terms the a l t e r n a t i v e to i n t e r c r o p p i n g i s a s o l e crop of cane and it
is therefore this
s o l e crop t h a t must form the b a s i s of comparison.
If
the f u l l cane y i e l d is achieved, the economic e v a l u a t i o n i s simply the value of
the
additional
(including
any
intercrops
extra
maintain f u l l cane y i e l d ) . acceptable l i m i t s ,
less
irrigation
or
the
additional
fertilizer
co st s
that
of
might
growing be
them
needed
to
I f t h e r e is some l o s s in cane y i e l d , but w i t h i n
then t h i s l o s s must a l s o be s e t a g a i n s t the value of the
intercrops. A d i f f e r e n t s i t u a t i o n may occur on small farms where crops o t h e r than cane have to be grown f o r s u b s i s t e n c e purposes.
Given an LER of g r e a t e r
than u n i t y , a t l e a s t some of the y i e l d requirements of the o t h er crops should be met by i n t e r c r o p p i n g full are
in cane.
If
an i n t e r c r o p p i n g system maintaining
cane y i e l d does not allow s u f f i c i e n t two a l t e r n a t i v e s .
more i n t e r c r o p y i e l d
One i s at
production of o t h e r crops,
to modify the
the expense of cane
t h er e
i n t e r c r o p p i n g system t o give : the o t h e r
minimum area of cane land to growing the ot h er crops.
i s to d i v e r t the I t should be noted
t h a t both these a l t e r n a t i v e s s t i l l
e x p l o i t as much as they can the advantages
to be gained from i n t e r c r o p p i n g .
For the second a l t e r n a t i v e , c a l c u l a t i o n of
how a
given land area should be d i v i d e d between cane i n t e r c r o p p i n g and o t h er
crops
to
meet p a r t i c u l a r
(Mead and W i l l e y ,
1980).
yield In
requirements practice,
has
this
been d i scu ssed
calculation
could
elsewhere be
more
complex than simply determining what p r o p o r t i o n of a given crop should be produced as an i n t e r c r o p in cane and what p r o p o r t i o n of t h a t same crop should
230
R.W.WILLEY
be produced separately.
The non-cane area may provide scope for a sequence
of crops that is not feasible in cane. quite
a
complex
one
where
cane
Thus the overall system could be
intercropping
satisfies
certain
crop
requirements and the non-cane area satisfies some different ones.
REFERENCES Agboola, A.A. and Fayemi, A.A. 1972. F i x a t i o n and e x c r e t i o n of n i t r o g e n by t r o p i c a l legumes. Agronomy J o u r n a l , 64 : 409-412. Andrews, D . J . , 1972. Intercropping with sorghum i n Nigeria. Experimental A g r i c u l t u r e , l0 : 57-63 Baker, E . F . I . , 1974. Research on mixed cropping with c e r e a l s i n Nigerian farming systems - a system for improvement. In : I n t e r n a t i o n a l Workshop on Farming Systems. Proceedings of a Symposium, 18-21 November, Hyderabad, India. pp. 287-301. Blum, A., 1967. Effects of row width and seedling spacing on y i e l d and i t s components in grain sorghum under dryland conditions. Agronomy Journal, 59 : 400-403. Fisher, N.M., 1977. Studies in mixed cropping. 1. Seasonal differences in relative productivity of crop mixtures and pure stands in the Kenya Highlands. Experimental Agriculture, 15 : 177-184. Freyman, S. and Venkateswalu, J. 1977. Intercropping on rainfed red soils of the Decaan Plateau, India. Canadian Journal of Plant Science, 57 : 697-705. Gregory, P . J . and Reddy, M.S. 1982. Root growth i n an i n t e r c r o p of pearl millet/groundnut. F i e l d Crops Research, 5 : 241-252. Hall, R.L., 1974. Analysis of the nature of i n t e r f e r e n c e between p l a n t s of d i f f e r e n t species. I I . N u t r i e n t r e l a t i o n s i n a Nandi S e t a r i a and a Greenleaf Desmodium association with particular reference to potassium. Australian Journal of Agricultural Research, 25 : 749-756. H a r r i s , D. and Natarajan, M., 1988. Physiological basis for y i e l d advantage in a sorghum/groundnut i n t e r c r o p exposed to drought. 2. Plant temperature, water s t a t u s and components of y i e l d . Field Crops Research ( I n p r e s s ) . IRRI ( I n t e r n a t i o n a l Rice Research I n s t i t u t e ) , 1972. Multiple cropping. IRRI Annual Report for 1972, pp. 21-34. Krantz, B.A., Virmani, S.M., Sardar Singh and Rao, M.R., 1976. Intercropping for increased and more s t a b l e a g r i c u l t u r a l production i n the semi-arid tropics. In : Munyo, J . H . , Ker, A.D.R. and Campbell, M., (Eds) Intercropping in Semi-arid areas. Proceedings of a Symposium , 10-12 May, a t Morogoro, Tanzania. p. 15. Lakhani, D.A., 1976. A crop p h y s i o l o g i c a l study of mixtures of sunflower and fodder r a d i s h . P h . D . Thesis. Reading U n i v e r s i t y , England. 171 pp. Lal, R., 1974. Soil temperature, s o i l moisture and maize y i e l d from mulched and unmulched t r o p i c a l s o i l s . P l a n t and S o i l , 40 : 129-143. Marshall, B. and Willey, R.W., 1985. Radiation i n t e r c e p t i o n and growth i n an i n t e r c r o p of pearl m i l l e t / g r o u n d n u t . F i e l d crops Research, 7 : 141-160. Mazaheri, D., 1979. Intercropping s t u d i e s with maize and kale. P h . D . Thesis. Reading U n i v e r s i t y , England, 228 pp. Mead, R. and Willey, R.W., 1980. Yield advantages i n intercropping and the Land Equivalent Ratio concept. Experimental A g r i c u l t u r e , 16 : 117-125.
RESOURCE USE IN INTERCROPPING SYSTEMS
231
Nambiar, P.T.C., Rao, M.R., Reddy, M.S., Floyd, C.N., Dart, P.J. and Willey, R.W., 1983. Effect of intercropping on nodulation and N-fixation by groundnut. Experimental Agriculture, 19 : 79-86. Natarajan, M. and Willey, R.W., 1980. Sorghum-pigeonpea intercropping and the effects of plant population density. 2. Resource Use. Journal of Agricultural Science, Cambridge, 95 : 59-65. Natarajan, M. and Willey, R.W., 1986. The effects of water stress on yield, advantages of intercropping systems. Field Crops Research, 13 : ll7-131. Osiru, D.S.O. and Willey, R.W., 1976. Studies on mixtures of dwarf sorghum and beans (Phaseolus ~ ) with particular reference to plant population. Journal of Agricultural Science, Cambridge, 79 : 531-540. Rao, M.R. 1986. Cereals in multiple cropping. In : Francis, C.A. (Editor), Y~ltiple Cropping Systems. Macmillan, New York. pp. 96-132. Reddy, M.S. and Willey, R.W. 1981. Growth and resource-use studies in an intercrop of pearl millet/groundnut. Field Crops Research, 4 : 13-24. Rogers, S. 1987. Water use efficiency in intercropping systems in the semiarid tropics. M.Sc. Thesis. Reading University,, England. 84 pp. Singh, S.P. 1977. Intercropping and double-cropping'studies in grain sorghum In : International Sorghum Workshop, 6-12 March, at Hyderabad, India (Unpublished). Trenbath, B.R., 1986. Resource use by intercrops. In : Francis, C.A. (Editor), Multiple Cropping Systems. Macmillan, New York. pp. 57-81. Whitington, W.J. and O'Brien, T.A., 1968. A comparison of yields from plots sown with a single species or mixture of grass species. J. of Applied Ecol.., S : 209-213. Wien, H.C. and Nangju, D. 1976. The cow pea as an intercrop under cereals. In : Fmnyo, J.H., Ker, A.D.R. and Campbell, (Eds.), Intercropping in Semi-Arid Areas. Proceedings of a Symposium on Intercropping in Semi-arid Areas, 10-12 May at Morogoro, Tanzania. Willey, R.W., 1979. Intercropping - Its importance and research needs. Part i. Competition and yield advantages. Field Crop Anstracts, 32(1) : i-i0. Willey, R.W., Natarajan, M. Reddy, M.S., Rao, M.R., Nambiar, P.T.C., Kanmaiyan, J. and Bhatmagar, V.S., 1983. Intercropping studies with annual crops. In 'Better Crops for Food', London (Ciba, Foundation Symposium 97) pp. 83-100.
215
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
RESOURCE USE IN INTERCROPPING SYSTt~ R.W. WILLEY School of Development S t u d i e s , U n i v e r s i t y of East Anglia, Norwich, England ABSTRACT Willey, R.W.,1990. R esource useinintercroppingsystems. Agric. Wa~rManage.,17:215-231.
Sorghum/pigeonpea and millet/groundnut intercropping systems are described to illustrate temporal and spatial complementarity and the magnitude of yield advantages that can be achieved in intercropping compared with sole cropping. The concept of considering environmental resource use in terms of 'resource capture' and 'resource conversion efficiency' is outlined and is then used to examine the resources of light, water and nutrients. Some implications for sugarcane intercropping are considered with particular emphasis on complementarity, resource use and the need for clear evaluation of objectives. i.
INTRODUCTION Intercropping
more
crops
on
is usually defined as the growing together of two or
the same
area of
land at
the
same time.
It is a very
widespread practice in the developing tropics and much recent research has shown that it can often produce substantially higher yields than sole crop systems (pure stand systems).
One of the great attractions of intercropping
is that this yield
can usually be achieved simply and cheaply,
advantage
namely by growing crops together rather than separately. Of the several mechanisms
that can bring about these higher yields
(Willey, 1979), the most widely-applicable one is that intercropping systems can make better use of environmental resources : it is this mechanism that is the subject of this paper. 2.
POTENTIAL YIELD ADVANTAGES The underlying principle
of better resource use in intercropping is
that if crops differ in the way they utilise environmental resources then, when
grown
together,
they
can
'complement'
each
other
and
make
combined use of resources than when they are grown separately. this complementarity
can be regarded
their major demands
on resources
as
'temporal',
at different
times,
where or
© 1990 Elsevier Science Publishers B.V.
Broadly,
the crops make 'spatial',
differences in resource use arise from differences in canopy or root
0378-3774/90/$03.50
better
where
216
R.W. WILLEY
dispersion.
These
illustrated
by
simple
two
annual
principles
of
complementary
intercropping
systems,
resource
use
are
sorghum/pigeonpea
and
m i l l e t / g r o u n d n u t , which have been studied in some d e t a i l at the I n t e r n a t i o n a l Crops Research I n s t i t u t e f o r the Semi Arid Tropics (ICRISAT), in India. Sorghum/pigeonpea India.
It
is
one of
the
commonest
intercropping
systems
in
i s t y p i c a l of many ' t e m p o r a l ' combinations throughout the world
in t h a t i t combines a rapid-growing, early-maturing crop (the sorghum) with a slow-growing,
later-maturing
one
(the
pigeonpea).
Tile
o b j e c t i v e with sorghum/pigeonpea i s to produce a reasonably
farmer's 'full'
the s t a p l e sorghum with some a d d i t i o n a l y i e l d of the pigeonpea. patterns
illustrated
main
y i e l d of The growth
in Fig. l a are f o r a p l a n t i n g p a t t e r n of 2 rows sorghum
to 1 row pigeonpea a t 45 cm row spacing. about 90 days d u r a t i o n
The sorghum was an e a r l y hybrid of
and the pigeonpea about
170-180 days.
Within-row
spacings were reduced in i n t e r c r o p p i n g so t h a t each component had the same p l an t population as i t s sole crop.
Sorghum was much the more competitive
of the two crops and its dry
matter accumulation in intercropping was little affected by the presence of pigeonpea; crop.
final sorghum grain yield in intercropping was 95% of the sole
In contrast,
the slow
initial
growth
of
the pigeonpea
was
even
further suppressed by competition from the sorghum and final dry matter yield of
the
pigeonpea
Interestingly, vegetative
in
because
intercropping sorghum
growth of pigeonpea,
was
only
competition
53%
of
sole
suppressed
only
seed yield was 72% of the sole crop. an
intercropping
the
early
the harvest index of intercropped pigeonpea
was increased to 30% compared with 22% in the sole crop. gave
pigeonpea.
advantage
of
Final pigeonpea
Combining these grain and seed yields 679
compared
with
growing
the
crops
separately (i.e. a Land Equivalent Ratio, or LER, of 1.67). The
effect
of
temporal
complementarity
in
this
illustrated by the light interception patterns in Fig. lb. sorghum pigeonpea
ensured
good
early
interception
some later interception
of light
of
light
and
combination the
presence
of
: total light interception was
higher than in sole crops, the increase being directly proportional higher yields produced.
is
The presence of
to the
Thus, a major feature of this combination was that
it achieved higher yields because of a fuller use of resources over time. This is a very simple principle but nonetheless an important one because it is undoubtedly the easiest and most assured way of achieving better resource use by intercropping.
Even for growing periods of only 5-6 months, temporal
differences between crops have commonly produced yield advantages of 509 or more (e.g. Andrews, 1972; Osiru and Willey, 1972; Krantz et al. 1976).
RESOURCEUSEININTERCROPPINGSYSTEMS a.
217
DRY MATTER ACCUMULATION
/ 10
Sole
///~Intercrop
S°rg7
~rghum
6 js
t/ha
,p
S
/" ~"
/
, "
~'Intercrop pigeonpea
/
_...I 0 100
i
b.
,
i
LIGHT INTERCEPTION Sorghum Harvest
.~. i//
80
Sole Sorghum/// 60 %
~ I"
~..""--'... -.
//Intercrop/" 40
i
,7
20
Sole -. pigeonpea
/ ~ ,
////
/
Jlntererop
#
I w~ J
20 Figure 1
40
60
i
!
80 100 120 Days after sowing
I
140
!
160
Dry matter accumulation and light interception in sorghum and pigeonpea sown as sole crops and as a 2-row sorghum : 1-row pigeonpea intererop (means of 3 years - after Willey et el, 1983).
218
R.W. WILLEY The
millet/groundnut
found
sorghum/pigeonpea
in
both
between t h e m a t u r i t y p e r i o d s of t h e c r o p s , and of c o u r s e i t cereal/low-canopy
legume
combinations
Farmers' yield
but the important groundnut cash
that
objectives
crop is usually
there are
is
and
It
many p a r t s of t h e w o r l d .
because
India
difference of
with
is
Africa. typical
contrasts
combination
the
of 1 row m i l l e t
same
within-row
'replacement' was
a
spacing
as
its
sole
so
plant
t h e growing p e r i o d
t h e growth of
little
than
crop yield
proportion;
the
75% s o l e
t h i s was o b v i o u s l y t h e r e s u l t
Fig.
for a planting Each c r o p had
populations
were
(25% : 75%).
For most of less
in
t h e major component.
crop
t o row p r o p o r t i o n s
is
so p r e v a l e n t
t o 3 rows g r o u n d n u t on 30 cm rows.
ones e q u i v a l e n t
much l e s s
seem t o v a r y a good d e a l
2a shows d r y m a t t e r a c c u m u l a t i o n a v e r a g e d o v e r 3 e x p e r i m e n t s , pattern
West
intercropped
'expected'
groundnut
from
its
sown
of c o m p e t i t i o n from t h e m i l l e t .
However, by t h e end of t h e s e a s o n g r o u n d n u t had produced a 74% biomass y i e l d , roughly
equivalent
accumulation
in
to
the
its
sown
more
proportion.
competitive
In
millet
was
contrast, more
dry
than
matter
twice
its
'expected' yield (25% of its sole crop) and final biomass was 62% of the sole crop.
Combining these yields gave an overall biomass advantage of 36% for
the intercropping
system
(LER = 1.36);
for seed yields the advantage was
rather less at 25%. Resource use in this combination but,
briefly,
there
was
no greater
is discussed
interception
in more detail
of
light
later
than would
be
achieved by growing separate sole crops (Figs. 2b), so yield increases were achieved not by intercepting more light but by making more efficient use of it.
In
contrast
combination,
to
the
temporal
differences
in
the
sorghum/pigeonpea
therefore, there must have been some 'spatial' complementarity
of light use because of canopy differences.
These spatial effects did not
produce such large yield advantages as occurred in the sorghum/pigeonpea but the avantages were still substantial and were very typical of those achieved by many 'spatial' combinations (e.g. Rao, 1986; Willey, 1979). 3.
RESOURCEUSE Distinguishing
between
temporal
way of t h i n k i n g a b o u t i n t e r c r o p p i n g crop d i f f e r e n c e s
that
can l e a d
(greater
resource
use.
can be a u s e f u l
some of t h e o b v i o u s Also,
it
indicates
u s e can be improved, namely by u t i l i s a t i o n
resource
g i v e n u n i t of r e s o u r c e ( g r e a t e r
systems
because it highlights
to better
two b a s i c ways i n which r e s o u r c e of more r e s o u r c e s
and s p a t i a l
capture)
or by more e f f i c i e n t
resource conversion efficiency).
u s e of a
RESOURCE USE IN INTERCROPPING SYSTEMS
a.
219
DRY MATTER ACCUMULATION
MILLET
GROUNDNUT
/
j.~
Sole crop
l I
6
l
Sole crop
l
t/ha "Expected"
i / ~ Actual / / / / / i n t e ....p
f~...-~m~--
4 intererop
~f
/ / ..o......Expected" / / ...-" intercrop
J
•
20
40
•
*
60
80
i
100
20
4o"
6o
8o
Days after sowing b. LIGHT INTERCEPTION 8O
60, ~% k k Sole millet
% 40
20,
"'%Sole groundnut Intercrop
J
I
,
20
40
i
|
60
80
i
100
Deys after so~,,ing
Figure 2. Dry matter accumulation and l i g h t i n t e r c e p t i o n in m i l l e t and groundnut as s o l e crops and as a 1-row m i l l e t : 3-rows groundnut i n t e r c r o p (means of 5 years - a f t e r Willey e t a l , 1985).
220
R.W. WILLEY
Light use Light is the most studied resource in intercropping systems so it is considered first to illustrate some of the approaches. light
(and with water)
A major proplem with
is that it is extremely difficult to determine how
much of the resource is being used by each of the component crops. therefore,
At best,
resource use can usually be examined only for the system as a
whole. Measurements efficiency
are
and calculations
illustrated
for
of light capture and light conversion
one
of
the
millet/groundnut
referred to earlier (Reddy and Willey, 1981 - Table 1). the intercropping
system was between
crops.
intercropping
However,
and
experiments
Light capture by
that of the two very different sole sole
cropping
cannot
be
compared
by
taking simple averages of light capture or conversion efficiency if there are differences in yields and conversion efficiencies of the two crops.
Thus,
Table 1 shows the 'expected' light capture by the intercropping system if it had produced no yield 'expected'
from
its
advantage sole
crop
intercropping was virtually
and if capture by each crop had been as capture.
identical
The
actual
light
to expected capture
capture
(I~ less,
by
or a
capture ratio of 0.99 - Marshall and Willey, 1983), indicating that despite its higher yields intercropping did not improve light capture compared with sole crops. Similarly, the light capture that would have been required to produce actual
intercropping
actual
light;
yields
it follows
at sole
crop efficiencies
was
30~ more
than
that light conversion efficiency must have been
improved by 501 in the intercropping system.
It also follows that the 28~
yield advantage in this particular intercropping system was attributable not to improved capture of light but to improved conversion efficiency. This millet/groundnut system was also the subject of a more detailed study that attempted to separate the light use of each component (Marshall and Willey, 1983).
Millet plants were found to have much greater biomass,
leaf area and light capture in intercropping similar
conversion
efficiency.
than in sole cropping but a
On the other hand,
the shaded groundnut
plants in intercropping were able to capture much less light than in sole cropping but their conversion efficiency was 461 higher and therefore their growth and yield were similar to sole crop plants.
This improved groundnut
efficiency was no doubt partly due to the greater efficiency of the C3 canopy at
lower
saturation
light of
intensities, upper
leaves
and
perhaps
: a
lower
partly energy
the cost
avoidance because
of of
N-fixation under shade may also have contributed (Nambiar et al, 1983).
light reduced
RESOURCEUSEININTERCROPPINGSYSTEMS
221
TABLE I Example of resource capture and resource conversition efficiency for light and water in millet/groundnut intercropping (after Reddy and Willey, 1981) Groundnut
Millet Sole crop biomass (kg D.M./ha) Intercrop biomass (kg D.M./ha) Component LERs Total LER .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
8085 4775 0.59
5617 3900 0.69 1.28
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
LIGHT Light-capture (MJ/m2) Sole crop capture 'Expected' capture by intercropping if NO yield advantage but same ratio of crops (i.e., 46% millet + 54% groundnut)
612.5
936.2
282
506
Total
788 778
Actual light capture by intercropping CAPTORE RATIO (778/788) = 0.99 Light conversion e f f i c i e n c y (g D.M./MJ) Sole crop conversion e f f i c i e n c y 'Required' capture to produce i n t e r c r o p ) y i e l d s a t sole crop e f f i c i e n c i e s (MJ/m2
0.60
1.32 650
362
Total
1012
CONVERSION EFFICIENCY RATIO (1012/778) = 1.30 .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
WATLR Water capture (as evapotranspiration - cm) Sole crop capture 'Expected' intercrop capture (see light)
30.3 14.0 Total
36.8 19.8 33.8 40.8
Actual intercrop capture CAPTURE RATIO (40.8/33.8) = 1.21 Water conversion e f f i c i e n c y (kg/ha/cm) Sole crop conversion e f f i c i e n c i e s 'Required' capture to produce i n t e r c r o p y i e l d s at sole crop e f f i c i e n c i e s
266.8
152.6
17.9 Total
CONVERSION EFFICIENCY RATIO (43.5/40.8)
25.6 43.5
=
1.07
.
.
222
R.W. W[LLEY Summarising the l i g h t use f e a t u r e s in t h i s m i l l e t / g r o u n d n u t system,
capture
was not
index.
Thus the t w o - t i e r e d i n t e r c r o p p i n g canopy r e s u l t e d in the same amount
increased
despite
a substantially
higher
of l i g h t being d is p e r s e d over a l a r g e r area of l e a f .
total
leaf
area
The e r e c t canopy of
m i l l e t u t i l i s e d the higher l i g h t i n t e n s i t i e s a t i t s normal C4 s o l e crop efficiency,
and the
compact,
planophile,
C5 canopy of
the
particularly efficient
use of the lower l i g h t
intensities.
improvement in
use
considerable
light
that
occurred
has
groundnut made The very l a r g e implications
for
other i n t e r c r o p p i n g systems t h a t d i s p l a y any of these canopy c h a r a c t e r i s t i c s . Water use The concepts
of
capture
and conversion e f f i c i e n c y are
again u s e f u l
when thinking about water use, but f u r t h e r f a c t o r s have to be considered. The p o s s i b l e ways in which i n t e r c r o p p i n g could
improve water use compared
with sole crops can be l i s t e d : 1.
Increase p l a n t a v a i l a b l e water
2.
Increase the t o t a l amount of water withdrawn from the s o i l ( i . e . as e v a p o t r a n s p i r a t i o n )
5.
Increase the p r o p o r t i o n of e v a p o t r a n s p i r a t i o n w h i c h is transpiration
4.
Increase conversion e f f i c i e n c y
5.
Increase p a r t i t i o n i n g i n t o t h a t f r a c t i o n of the crop t h a t is r e q u i r e d as economic y i e l d ( i . e . ,
Strictly transpired
:
transpiration. thought
of
in
increase h a r v e s t index).
speaking, water capture should r e f e r to the amount of water similarly,
conversion
In p r a c t i c e , less
rigid
efficiency
however, both these
terms
and
may have
should
be
based
on
concepts may have to be to
be
based
on the
more
measurable e v a p o t r a n s p i r a t i o n . On the p o s s i b i l i t y of i n c r e a s i n g p l a n t a v a i l a b l e water, i n t e r c r o p p i n g may provide a g r e a t e r
canopy cover, p r o t e c t i n g the s o i l from the impact of
r a i n so improving i n f i l t r a t i o n
: greater
infiltration
has been r e p o r t e d
maize/cassava i n t e r c r o p p i n g compared with the s o l e crops (Lal, 1974).
in
These
e f f e c t s are p o t e n t i a l l y g r e a t e s t where a r a p i d l y e s t a b l i s h i n g crop is added to a slowly e s t a b l i s h i n g one which, i f grown on i t s own, would provide poor canopy cover.
There have a l s o been suggestions t h a t a r a p i d l y e s t a b l i s h i n g ,
shallow-rooted component may dry out the upper s o i l h o r i z o n s , slower-establishing, O'Brien,
1968;
IRRI,
deep-rooting 1972;
Fisher,
component 1977).
even
deeper
This e f f e c t
so f o r c i n g a
(Whittington
and
could undoubtedly
increase the amount of water p o t e n t i a l l y a v a i l a b l e to an i n t e r c r o p p i n g system
RESOURCEUSE IN INTERCROPPINGSYSTEMS
223
compared with sole crops, but there has been l i t t l e
r e a l evidence o£ such
deeper r o o t i n g a c t u a l l y o c c u r r i n g . On systems
water
have
capture
been
(as
shown
evapotranspiration),
to
withdraw more
water
several than
intercropping
their
sole
crops
(Lakhani, 1976 - sunflower/fodder r a d i s h ; Hazaheri, 1979 - maize/kale; Reddy and Willey, 1981 - m i l l e t / g r o u n d n u t ) . this
greater
withdrawal
is
that
The reason most commonly proposed for
the
different
crops
explore
different
horizons, g i v i n g a b e t t e r d i s p e r s i o n of roots throughout the whole p r o f i l e ; but g r e a t e r withdrawal could be due i n p a r t to g r e a t e r root c o n c e n t r a t i o n s . It
has
also
been suggested
that
simple
temporal
differences
in
rooting
p a t t e r n s may ensure g r e a t e r t o t a l withdrawal over the season (Baker, 1974). When
intercropping
increases
canopy
cover,
the
proportion
e v a p o t r a n s p i r a t i o n which is t r a n s p i r a t i o n i s l i k e l y to i n c r e a s e . thought
to
occur
in
both
the
systems r e f e r r e d to e a r l i e r 1980).
m i l l e t / g r o u n d n u t and the
(Reddy and Willey, 1981;
of
This was
sorghum/pigeonpea
Natarajan and Willey,
This e f f e c t i s l i k e l y to be p a r t i c u l a r l y b e n e f i c i a l under wet s o i l
c o n d i t i o n s when evaporation losses are p o t e n t i a l l y high. (1987)
has
proportion
argued of
temperature. increasing
that
greater
transpiration A
canopy
under
particularly
cover
drier
important
could
conditions
aspect
the proportion of transpiration
of
However, Rogers
still by
increase
decreasing
the soil
improving water use by
is that increased growth may be
possible without increasing total water demands (Rogers, 1987). Considering
conversion
efficiency
based
on
transpiration,
Rogers
(1987) has pointed out that this is a relatively inflexible character for any given species. on another turbulent reach
Nevertheless, he argues that a sheltering effect of one crop
could increase conversion efficiency by reducing wind speed or air movement.
light
saturation
He also points out that with C3 species, which at lower
intensities
than C4
species,
some
light
reduction at high intensities may increase conversion efficiency by reducing transpiration without a commensurate reduction in photosynthesis. Some reference
of
these
to
evapotranspiration) Calculations
aspects
of water
millet/groundnut
showed
use
(Table
21%
more
capture
than
Thus most of the intercropping
attributed
to greater water uptake.
profile
root
illustrated Water
with
further
capture
(as
was higher in intercropping than in either sole crop.
capture. total
are i).
concentrations
and
ratio of 1.07 suggested
from
sole
crop
This was probably because of higher
better
(Gregory and Reddy, 1982).
'expected'
yield advantage of 28% could be
dispersion
of
roots
throughout
the
However, a water conversion efficiency
that there might be a small increase in conversion
224
R.W. WILLEY
efficiency as well.
But this ratio was based on evapotranspiration. When an
attempt was made to bare it on transpiration (estimated from the proportion of
incoming
suggesting because
energy
intercepted
by
the
crop),
that at least some of the apparent
intercropping
the
ratio
increase
was
reduced,
in efficiency was
increased that proportion of evapotranspiration which
was transpiration. It was seen earlier that the competitive effects experienced by pigeonpea
in
intercropping
sorghum/groundnut 'line-source'
study,
could
which
technique,
increase
examined
harvest
different
showed that harvest
water
index.
regimes
A
using
a
index could also be markedly
affected by the interaction of intercropping and water regime (Natarajan and Willey, crops
1986). but
Increasing
much
less
so
although the relative
water
in
stress
decreased
intercropping
than
harvest
in sole
index in both
cropping.
Thus
intercropping advantages for biomass were relatively
constant, those for reproductive yields increased very markedly from 14~ with least water stress to a very considerable 93~ at the severest stress. reasons for these effects were not at all clear.
The
The simple explanation
would be that there was some complementarity of water use by the two crops (e.g. their roots being concentrated in different horizons), which resulted in less inter-species competition in intercropping than in sole cropping. But this does not not explain why the resultant crop effects were wholly on harvest
indices and not on biomass.
reduced
competition
groundnut,
shading
reproductive
favoured may
have
partitioning
It is possible
reproductive reduced
(Harris
growth.
plant
and
that the timing of
Or,
at least for the
temperatures
Natarajan,
and
1988).
so
favoured
Whatever
the
reasons, the results indicate that compared with sole cropping, intercropping may be able to increase harvest index and so improve the water conversion efficiency for reproductive yield.
Equally important the results indicate
that such changes in harvest index may be very dependent on water regime. It must
be
emphasized,
however
referred to was a 'replacement'
that
the
sorghum/groundnut
system
one in which each crop was sown at only a
proportion of its sole crop population and total population was equivalent to sole
crop
optima.
replacement
systems,
Given their
some
reduced
complementarity populations
experiencing less competition than in sole cropping. intercropping
systems,
where
total
populations
between
can are
result
the
crops
in each
in crop
However, in 'additive' higher
than
in sole
cropping, crops may well experience greater competition in intercropping even though some complementarity occurs.
Such high populations could at least
partly explain why some workers have found poorer intercropping advantages
RESOURCEUSE IN INTERCROPPINGSYSTEMS
225
with higher water s t r e s s ( e . g . F i s h e r , 1976). Nutrients For n u t r i e n t s , resource capture can be very e a s i l y measured as nutrient
uptake,
intercropping
and
has
many
produced
studies a
yield
have
shown
advantage.
greater As with
uptake water,
where greater
n u t r i e n t uptake i s u s u a l l y presumed to be p o s s i b l e because of g r e a t e r root c o n c e n t r a t i o n s or some complementary e x p l o r a t i o n of the p r o f i l e .
In some
i n s t a n c e s g r e a t e r uptake might be due to the use of n u t r i e n t s u n a v a i l a b l e to sole
crops
general
(e.g.
the
where
greater
i n t e r c r o p p i n g promotes deeper
uptake
probably
represents
resource t h a t is a v a i l a b l e to sole crops.
rooting).
fuller
use
of
But i n the
same
The i m p l i c a t i o n of t h i s i s t h a t
i n t e r c r o p p i n g probably makes g r e a t e r demands on the s o i l and thus i n the long term
intercropping yield
fertilizer
advantages may have to be paid for with higher
inputs.
The concept of conversion e f f i c i e n c y , i n terms of y i e l d produced per u n i t of n u t r i e n t taken up, i s not r e a l l y a p p l i c a b l e to n u t r i e n t use, though several
s t u d i e s have shown changes i n crop n u t r i e n t c o n c e n t r a t i o n s due to
competitive 1980).
effects
in
intercropping
(Hall,
1974;
Natarajan and Willey,
These changes are u s u a l l y small, and sometimes t r a n s i t o r y , but they
have p o s s i b l e i m p l i c a t i o n s where n u t r i e n t content i s a determinant of y i e l d quality. A rather
special
case
of
nutrient
use
in
intercropping
p o s s i b i l i t y of a legume crop t r a n s f e r r i n g fixed n i t r o g e n to non-legume. sole
crops
is
the
an i n t e r p l a n t e d
The p o s s i b l e n e t r e t u r n of n i t r o g e n to the s o i l by legumes as or
i n pastures
is
of course well-known.
But i t
is
easy to
overestimate t h e i r c o n t r i b u t i o n i n i n t e r c r o p p i n g systems where they are o f t e n grown as ' p a r t i a l '
crops and are s u b j e c t to competition from other crops.
In p a r t i c u l a r , Nambiar e t a l (1984) have shown t h a t the N - f i x a t i o n process, a t l e a s t i n groundnuts, may be p a r t i c u l a r l y s u s c e p t i b l e to shading. In the vast m a j o r i t y of experiments there has been no evidence of a legume t r a n s f e r r i n g any appreciable amount of n i t r o g e n during i t s period of a c t i v e growth.
However, such t r a n s f e r has been i n d i c a t e d i n some i n s t a n c e s
(Singh, 1977; Wein and Nangju, 1976); and lack of evidence may be p a r t l y due to
the
difficulties
competitive e f f e c t s .
in
separating
nitrogen
transfer
effects
from other,
A more probable t r a n s f e r e f f e c t seems to be t h a t a f t e r
senescence, or h a r v e s t ,
the legume may leave some r e s i d u a l n i t r o g e n for a
l a t e r - m a t u r i n g component, or for a following crop (Singh, 1977; Agboola Fayemi, 1972).
226 4.
R.w. WILLEY SOME RESEARCHANDMANAGEMENTIMPLICATIONS FOR SOGARCANE INTERCROPPING As with
any
intercropping
system,
the
physiological
objectives
of
sugarcane i n t e r c r o p p i n g must be to maximise complementary e f f e c t s between the component crops resources.
to ensure
the f u l l e s t
and most e f f i c i e n t
use of a v a i l a b l e
But these p h y s i o l o g i c a l o b j e c t i v e s must be a l l i e d
proportions
of
the
different
crops
are
required
by the
to whatever
grower.
And of
course t h ere may be crop management c o n s t r a i n t s t h a t determine what kind of i n t e r c r o p p i n g system i s p r a c t i c a l l y f e a s i b l e . Given the
slow e s ta b l is h m e n t
of
sugar
cane
during
the
first
3-4
months, much the g r e a t e s t scope f o r complementary e f f e c t s l i e s with temporal systems
in which annual
intercrops
are
added between cane rows to improve
resource use in the e a r l y part of the growing p er i o d .
These systems f i t
in
very well with growers' y i e l d requirements,
which are u s u a l l y to produce a
'full'
yields
yield
experiments
of
cane
have
with
'additional'
testified
producing
useful
yields.
Temporal d i f f e r e n c e s
varieties, before
yields
to
of
the
potential
intercrops
of
without
can be best
of
intercrops.
such
temporal
seriously
Many systems,
jeopardising
e x p l o i t e d by using
cane
species,
or
of i n t e r c r o p s t h a t are s u f f i c i e n t l y e a r l y - m a t u r i n g to be harvested
they
selecting
compete with
sugarcane
the
varieties
cane.
competition,
or which are b e t t e r
such as
suppression
the
differences
of
There may a l s o
which
are
able
tillering.
not
to
so
be
some scope
susceptible
to
for
intercrop
recover from competitive e f f e c t s In theory,
could be achieved by sowing i n t e r c r o p s
even g r e a t e r earlier
than
temporal the cane;
but e a r l i e r sowing of i n t e r c r o p s means t h a t f o r a given stage of cane growth the i n t e r c r o p s are more advanced and t h e r e f o r e p o t e n t i a l l y more c o m p e t i t i v e , so the y i e l d outcome is d i f f i c u l t As with
most
to p r e d i c t .
intercropping
systems,
arrangement are l i k e l y to be c r u c i a l f a c t o r s Maintaining f u l l as high as
p l an t
population
and
spatial
in sugarcane i n t e r c r o p p i n g .
cane y i e l d i s l i k e l y to r e q u i r e a cane population at l e a s t
a sole
cane
crop;
this
helps
to maintain c a n e ' s
co m p et i t i v e
a b i l i t y and i t ensures f u l l use of resources a f t e r removal of i n t e r c r o p s . (With pigeonpea, which i s a l s o the l a t e r - m a t u r i n g crop in temporal systems, even higher populations than the s o l e crop optimum are r e q u i r e d to compensate f o r reduced e a r l y branching.) particularly 'partial'
Yields of i n t e r c r o p s in cane are l i k e l y to be
dependent on t h e i r
crops;
their
'optimum'
own p o p u l a t i o n s , populations
given t h a t
would u s u a l l y
they are only be the h i g h e s t
t h a t do not compete with cane. Again thinking in terms of maintaining cane y i e l d , s p a t i a l arrangement must not be unfavourable to the cane because t h i s may reduce the resources
RESOURCEUSE IN INTERCROPPINGSYSTEMS available
to
the
crop
and
227
its
competitive
i n t e r c r o p p i n g s t u d i e s have manipulated s p a t i a l to maintain resources)
its
full
for a
A
population while allowing more space
second crop.
distances
number
of
(and thus more
This has been very s u c c e s s f u l with c e r e a l
i n t e r c r o p p i n g in I n d i a , p a r t i c u l a r l y 'inter-pair'
ability.
arrangement of one component
(Freyman
' p a i r i n g ' of c e r e a l rows to l eav e l a r g e r
and Venkateswalu,
1977).
But
sole
crop
c e r e a l s in India have c l o s e i n t e r - r o w spacings (45-60 cm) and t h e r e i s l i t t l e
scope to intercrop within these without some row modification. large
inter-row
manipulation
distances
in sugar
cane,
it seems
Given the
unlikely
that
similar
of cane rows would allow much increase in the space available
for intercrops, particularly
if a pairing arrangement excluded intercropping
from the within-pair spaces.
It also seems unlikely that pairing Cane rows
would directly benefit yield of cane, though it has been shown that pairing cereal
rows
in dry areas
within-species other
hand
spacings'
it
is
claimed
facilitates
intercrops,
can increase
cereal yields
because
the earlier
competition can increase rooting depth (Blum, 1967). that
pairing
mechanical
of cane
operations
rows
in
to give
the
On the 'pineapple
crop.
For
the
their spatial arrangement should give as uniform a distribution
as possible throughout the cane inter-row spaces, but without being so near to the cane
that
they
produce
too much
competitionbetween
the
component
crops. Briefly
considering
resources
use
aspects,
emphasis
on
temporal
complementarity
in cane intercropping means that improved light use must be
mainly brought
about by additional
There may be a l i t t l e
capture of light by the intercrops.
scope f o r improved l i g h t conversion e f f i c i e n c y i f the
i n t e r c r o p s remain long enough to provide a combined canopy with the cane, but obviously the longer the growing period of the i n t e r c r o p s the more they are likely
to
compete.
achieved with thus
Improved conversion e f f i c i e n c y
low-growing, C3 i n t e r c r o p s ,
providing
millet/groundnut
a
two-tier
one
referred
C4/C3 to
most l i k e l y
such as groundnuts
canopy
earlier.
is
like If
the
the
or p o t a t o e s ,
very
cane
component of the canopy, as i t can be with a maize i n t e r c r o p ,
to be
is
efficient the
lower
there is much
l e s s l i k e i h o o d of improved l i g h t conversion, and of course the cane is much more l i k e l y to be suppressed. Improved water use in cane i n t e r c r o p p i n g must a l s o be dependent mainly on
temporal
complementarity.
i n t e r c r o p s may u t i l i s e
The
cane
stand,
the
effect
is
likely
to
be
that
some of the water t h a t would otherwise have been l o s t
as e v a p o r a t i o n , or p o s s i b l y d r a i n a g e . full
major
intercropping
But i f i n t e r c r o p s are a d d i t i o n a l to a system
will
almost
certainly
produce
228
R.W. WILLEY
greater
d e p l e t i o n from the p r o f i l e
high r a i n f a l l intercrop
areas where the p r o f i l e
maturity,
all
the
depletion
requirement
if
by
full
water.
the
can be regarded
represents
an
additional
cane y i e l d s are to be maintained.
intercropping
as
But i f i r r i g a t i o n i s normally p r a c t i s e d , the
intercropping
some r ed u ct i o n in e v a p o r a t io n or drainage l o s s e s , mean t h a t
In
i s f u l l y r ep l en i sh ed by the time of
water used by i n t e r c r o p s
b e n e f i c i a l a d d i t i o n a l capture. greater
than would occur under sole cane.
irrigation
At the same time,
as suggested above, could
system g i v e s more economic use
I f water s t r e s s occurs, there may be p o s s i b i l i t i e s
of
irrigation
of improved water
conversion e f f i c i e n c y because of shading or s h e l t e r i n g e f f e c t s , or because of higher h a r v e s t i n d ic e s in r e p r o d u c t i v e i n t e r c r o p s (see e a r l i e r ) . are
'spatial'
enough to
effects
interact
that
with
again depend on the
the
cane;
in
these
intercrops
But these
remaining
circumstances
the
long
additional
i n t e r c r o p population w i l l almost c e r t a i n l y cause some increased water s t r e s s on the
cane,
even
if
some complementarity
occurs.
Thus
improved water
conversion e f f i c i e n c y might be achievable only at the expense of lower cane yields. As f o r n u t r i e n t s ,
i n t e r c r o p s may well improve t o t a l n u t r i e n t capture
by taking up some n u t r i e n t s
t h a t are u n a v a i l a b l e to the cane, or t h a t might
otherwise be leached.
But i t seems probable t h a t the bulk of the n u t r i e n t s
utilised
would have been e q u a l l y a c c e s s i b l e
by i n t e r c r o p s
to cane.
Thus
maintenance of f u l l cane y i e l d in i n t e r c r o p p i n g w i l l almost c e r t a i n l y r e q u i r e higher f e r t i l i s e r
inputs than in s o l e cane.
The obvious ex cep t i o n to t h i s
could be the n i t r o g e n requirement with legume i n t e r c r o p s
: and in f a c t the
temporal nature of cane i n t e r c r o p p i n g systems provides the ideal s i t u a t i o n in which some r e s i d u a l n i t r o g e n could be passed on to the cane a f t e r i n t e r c r o p harvest. But study.
all
these
suggested
effects
require
much f u r t h e r
Measurements on the micro-environment, or on p l a n t f a c t o r s
temperature
and water
interactions,
but
status,
ideally
Techniques of s e p a r a t i n g explored.
resource-use can i n d i c a t e
resource
such as
the occurrence of resource-use
use needs
to be measured d i r e c t l y .
the l i g h t and water use component a l s o need to be
Separate l i g h t use may be measurable when two crop canopies are
at d i s t i n c t l y
different heights,
but t h i s
whole of the i n t e r c r o p p i n g p e r i o d .
i s seldom the case throughout the
I t may be p o s s i b l e to separate l i g h t use
in i n t e r s p e r s e d canopies by taking measurements immediately before and a f t e r the removal of one of the components. by porometry,
and the
Water use can in theory be separated
s i l i c o n accumulation technique described by Trenbath
(1986) seems worth e x p l o r i n g f u r t h e r .
RESOURCEUSE IN INTERCROPPINGSYSTEMS As
a
evaluation
final
of
comment,
intercropping
229
careful
thought
systems.
always
And t h i s
needs
to
be
given to
e v a l u a t i o n must take
into
account the kind of o b j e c t i v e s t h a t were d iscu ssed a t the beginning of t h i s section.
The Land Equivalent Ratio
sugarcane
systems.
While
this
has
(LER) has been q u i t e widely used an important
function,
it
is
in
only a
l i m i t e d one.
An LER based on s i n g l e s o l e crops of each component ( i g n o r i n g
the f a c t
i n t e r c r o p s probably occupy the land f o r much l e s s
that
time than
cane) i n d i c a t e s i f t h e r e are any b e n e f i c i a l i n t e r a c t i o n s between the crops. For example,
thinking
only in terms of resource use,
an LER g r e a t e r
than
u n i t y i n d i c a t e s t h a t t h e r e has been some improvement in resource use due to complementary e f f e c t s of the kind discussed e a r l i e r . is
extremely important because
it
indicates
if
This b a s i c information
t h er e
are
any i n t e r a c t i o n s
t h a t might be e x p l o i t e d t o provide y i e l d advantages in p r a c t i c a l systems, or t h a t might m e r i t f u r t h e r study and development in a However, i f y i e l d o b j e c t i v e s r e q u i r e f u l l
r e s e a r c h programme. cane y i e l d plus a d d i t i o n a l
i n t e r c r o p s , the LER i s not u s u a l l y s u i t a b l e f o r f u r t h e r e v a l u a t i o n .
In
p r a c t i c a l terms the a l t e r n a t i v e to i n t e r c r o p p i n g i s a s o l e crop of cane and it
is therefore this
s o l e crop t h a t must form the b a s i s of comparison.
If
the f u l l cane y i e l d is achieved, the economic e v a l u a t i o n i s simply the value of
the
additional
(including
any
intercrops
extra
maintain f u l l cane y i e l d ) . acceptable l i m i t s ,
less
irrigation
or
the
additional
fertilizer
co st s
that
of
might
growing be
them
needed
to
I f t h e r e is some l o s s in cane y i e l d , but w i t h i n
then t h i s l o s s must a l s o be s e t a g a i n s t the value of the
intercrops. A d i f f e r e n t s i t u a t i o n may occur on small farms where crops o t h e r than cane have to be grown f o r s u b s i s t e n c e purposes.
Given an LER of g r e a t e r
than u n i t y , a t l e a s t some of the y i e l d requirements of the o t h er crops should be met by i n t e r c r o p p i n g full are
in cane.
If
an i n t e r c r o p p i n g system maintaining
cane y i e l d does not allow s u f f i c i e n t two a l t e r n a t i v e s .
more i n t e r c r o p y i e l d
One i s at
production of o t h e r crops,
to modify the
the expense of cane
t h er e
i n t e r c r o p p i n g system t o give : the o t h e r
minimum area of cane land to growing the ot h er crops.
i s to d i v e r t the I t should be noted
t h a t both these a l t e r n a t i v e s s t i l l
e x p l o i t as much as they can the advantages
to be gained from i n t e r c r o p p i n g .
For the second a l t e r n a t i v e , c a l c u l a t i o n of
how a
given land area should be d i v i d e d between cane i n t e r c r o p p i n g and o t h er
crops
to
meet p a r t i c u l a r
(Mead and W i l l e y ,
1980).
yield In
requirements practice,
has
this
been d i scu ssed
calculation
could
elsewhere be
more
complex than simply determining what p r o p o r t i o n of a given crop should be produced as an i n t e r c r o p in cane and what p r o p o r t i o n of t h a t same crop should
230
R.W.WILLEY
be produced separately.
The non-cane area may provide scope for a sequence
of crops that is not feasible in cane. quite
a
complex
one
where
cane
Thus the overall system could be
intercropping
satisfies
certain
crop
requirements and the non-cane area satisfies some different ones.
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