The productivity of the maize/soybean intercropping system in Nepal has been
declining compared to the past, as reported by many farmers. To understand the
constraints and to overcome this problem, field experiments and a survey of farmers
were conducted at Deorali VDC (mid hills), in Nepal during 2001 and 2002. One pot
experiment at Henfaes Research Centre, University of Wales, Bangor was conducted in
2003.
Two field experiments consisting of a combination of three densities each of maize
(26.5, 40 and 53 x 103
) and soybean (100, 150 and 200 103 ha·1 in 2001 and 50, 100 and
150 x 103 ha·1 in 2002) along with their sole crops were studied for two seasons to
determine optimum populations for the component crops. In the same seasons, another
field experiment was conducted to detennine the effect of time of maize thinning at
different maize densities on LAI and yield attributes of maize and soybean. A survey of
farmers was conducted during 2001 to understand the causes of low productivity of
intercrops and existing farming practices. A pot experiment was conducted to compare
photosynthetic rates of soybean in open, under artificial shade and intercropped with
maize.
In neither season was maize yield affected by presence of soybean but grain yield of
soybean was reduced in mixture by 59 % and 53 % during 2001 and 2002, respectively.
The interception of PAR by the maize canopy increased with increasing maize density
and was greatest at highest maize density of 53 x 103 plants ha·1 due to greater LAI and
dry weight of maize at recommended maize density. LAI and dry weight of
intercropped soybean increased as maize density was reduced from 53 to 26.5 x 103
plants ha·1
. Biomass and grain yield of maize were greatest at 53 x 103 plants ha·1 and
least at lowest maize density, whilst conversely biomass and grain yield of soybean
increased. The numbers of cobs/plant and grains/cob were highest at low maize density
and reduced as maize density increased ( except for cobs/plant in 2001) but these did not
compensate fully for reduced grain yield due to low density. In soybean, a greater
number of pods/plant at low maize density contributed to higher grain yield of
intercropped soybean, compared to when grown with recommended maize density. In
2001 , grain yield of soybean was not affected by soybean density but in 2002 soybean
density of 50 x 103 produced a lower biomass and grain yield than 100 and 150 x 103
plants ha·1 but the differences between these densities were not significant. In both
seasons, land equivalent ration (LER) of all treatments was greater than unity indicating
higher efficiency of intercropping compared to sole crops.
In the time of thinning experiment, leaf area index (LAI) of maize increased with
increasing plant population but the difference between 53 and 66 x 103 plants ha·1
was
not sigrrificant. The recommended maize density produced the highest grain yield and
declined to lowest at highest maize density of 66 x 103 plants ha·1 but it did not differ
from maize density of 38 x 103 plant ha·1
. Thus, the population density response was
parabolic. Grain yield did not differ sigrrificantly between recommended maize density
and 38 x 103 plants ha·1 during the second season. The greater number of cobs/plant and
grains/cob at low maize density compensated for the reduced grain yield due to decrease
in plant population. However, biomass, yield and pods/plant of intercropped soybean
were greatest under low maize density and were reduced as maize density increased. In
V
the second season, wide maize rows at 50 x 103 plants ha-1
a produced lower grain yield
of maize but increased grain yield and pods/plant of intercropped soybean compared to
the recommended maize density. In both seasons, thinning of maize beyond 30 DAS
reduced biomass, grain yield and grains/cob of maize significantly while biomass, grain
yield and harvest index of soybean increased in the second season only. LER of all
treatments in both experiments were higher in the second season which had less rainfall
than the first season, favouring higher grain yield of soybean which contributed to the
higher LER. In both seasons, delayed thinning reduced LER but this was more
pronounced in first season, and reduced the LER to one, indicating no advantage of
intercropping over sole crops if thinning was delayed.
A survey of farmers showed that intercropping is predominant ( 41 % of maize area) in
this area due to limited mean size of landholding (0.75 ha). Mixed cropping of maize
and soybean was advantageous over growing them separately because maize yield was
not affected by soybean, and this provided additional output as a bonus crop. In
addition, it provided security against crop failure; helped in maintaining soil fertility and
gave an income to farmers due to high market price. About 95 % respondents claimed
that productivity of intercropped soybean has been declining from the past. The reasons
given by them were the introduction of high yielding competitve maize varieties and
application of urea as a topdressing to maize. Crop sampling results indicated that low
plant populations of both crops at harvest contributed to low yields. Continuous
thinning of maize for security against drought, and insect/pest damage and livestock
fodder was the main cause of poor plant population at harvest. Other constraints leading
to low productivity as indicated by farmers, were poor crop management, drought
during germination, sub-optimal ratios and densities of component crops, untimely
weeding, excessive rainfall and wild rabbits.
The net rate of photosynthesis in maize (C4 carbon pathway) was double that of soybean
(C3 pathway) at the same levels of incident PAR. Net photosynthesis rate in soybean
increased with increasing PAR up to 500 µmol m-2
s-1 but leveled off thereafter. Net
photosynthetic rate in soybean was significantly greater in the open than under artificial
shade and when intercropped. Interactions between slopes of regressions for the three
treatments was non significant indicating that soybean does not adapt to shade, although
specific leaf area (SLA) of soybean was greater under shade than in open and when
intercropped.
It is suggested that soybean could be grown successfully as an intercrop with maize by
increasing row spacing of maize from 75 to 100 cm with reduced populations of around
40 x 103 plants ha·1 which would provide better light penetration to the soybean canopy
resulting in increased grain yield. This would compensate for reduced grain yield due to
decrease in maize density.