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cowpea intercrop as affected by maize plant density and cowpea cultivar
Field Crops Research, 35 (1993) 123-133
Field Crops Research
Radiation interception and growth of maize/cowpea intercrop as affected by maize plant density and cowpea cultivar J.M. Watiki a, S. Fukai a'*, J.A. Banda a, B.A. Keating b aDepartment of Agriculture, The Universityof Queensland, Brisbane, Queensland 4072, Australia bCSIRO Division of Crops and Pastures. St Lucia, Queensland 4067, Australia (Accepted 11 May 1993)
Abstract Sole and intercropped maize (Zea mays L.) and cowpea (Vigna unguiculata (L.) Walp.) were grown to examine how radiation interception and radiation-use efficiency (RUE) changed under intercropping. The work also examined whether intercropping advantage as measured by land equivalent ratio (LER) was determined by the yield of the dominated cowpea crop. In one experiment, maize plant density was varied and in another, it was held constant and 15 contrasting cowpea cultivars were used. Increased radiation interception by the intercrops prior to maize grain filling increased biomass production, particularly at the low maize density. There was, however, no intercropping advantage in RUE. Radiation-use efficiency of the combined intercrop was between that of maize and cowpea in sole cropping, the actual value being determined by the proportion of radiation intercepted by component crops. There was a rather small yield advantage of intercropping, with LER of around 1.1 at the two higher plant densities of maize. At the low maize density of 2.2 plants m - 2, however, LER was less than 1.0 despite an increase in partial LER of cowpea. Cowpea yield and partial LER varied greatly among 15 cowpea cultivars when intercropped. There were positive but weak correlations between yield and vegetative dry matter of cowpea, and between vegetative dry matter and radiation transmitted through the maize canopy to the cowpea. Grain yield and partial LER of maize, on the other hand, were similar when intercropped with different cowpea cultivars, and hence variation in total LER reflected mostly the variation in partial LER of cowpea. It is concluded that maize, except when planted at low densities, will dominate cowpea, and the performance of the dominated crop (cowpea) will have most influence on total LER. Key words: Cowpea; Intercropping; Maize; Radiation-useefficiency; Vigna; Zea
I. Introduction In a recent review of c e r e a l / l e g u m e intercropping, Ofori and Stern (1987b) concluded that the reduction in grain yield due to intercropping is less for the cereal than the legume and the land equivalent ratio ( L E R ) is commonly determined by the yields of legume under *Corresponding author.
various agronomic manipulations. This was the case in m a i z e / c o w p e a intercropping when relative sowing time and plant density of the two components were altered (Ofori and Stem, 1987a) and in millet (Pennisetum glaucum (L.) R . B r . ) / c o w p e a intercropping when different cultivars were used (Reddy et al., 1990). In m a i z e / c o w p e a intercropping, the two components grow vigorously at about the same time, and
0378-4290/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved SSD10378-4290(93) E0042-V
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competition for available growth resources such as solar radiation is high. Solar radiation is a major resource determining growth and yield of component crops in intercropping, particularly when other resources (e.g. water and N) are not severely limiting crop growth. As maize plants become increasingly,, taller than cowpea plants, radiation becomes less available to the cowpea. Some cowpea cultivars may mature before the adverse effect of associated cereal crop becomes severe, and often early-maturing cultivars are advantageous in intercropping with millet (Ntare, 1989). Availability of solar radiation to the cowpea may be improved by the use of maize cultivars with erect leaves in the upper canopy (Wahua et al., 1981 ). Less-determinate cowpea cultivars can climb maize stems, increasing their interception of radiation at the expense of that by the maize. This may have adverse effects on the overall performance of maize/cowpea intercropping. The experiments reported in this paper were aimed at quantifying the benefits of intercropping in terms of radiation capture and use. In one experiment radiation availability to cowpea was altered by using different plant densities of maize. In the other experiment, a range of cowpea cultivars were used to determine if differences in cowpea morphology influence the productivity of intercrops with maize. The contribution of component crops to LER was examined to test the hypothesis that LER is determined by the performance of the dominated crop, in this case, the shorter-statured cowpea.
2. Materials and Methods
Two experiments were carried out on The University of Queensland research farm at Redland Bay (latitude 27°31'S, longitude 153°19'E), S.E. Queensland, Australia. The soil is a deep, well-structured krasnozem with good aeration and drainage. The soil has a high clay content (60%) and a pH of 5.5-6.2. Both experiments commenced in mid-February 1991, when daily mean temperatures were about 25°C. Temperature decreased gradually to about 17°C in June when the experiments were completed.
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
2. I. Experiment I 2.1.1. Treatments In the main part of Experiment 1, there were nine treatments randomly allocated in each of three replications. There were three maize plant densities (2.2, 4.4 and 6.7 plants m -z) in both sole cropping and intercropping with two cowpea cultivars, Red Caloona and CPI 67233. The maize cultivar used was an earlymaturing hybrid DK 529. Row spacing of maize was 0.75 m, and intra-row spacing varied from 0.6 m to 0.2 m to produce the designated maize densities in both sole cropping and intercropping. In intercropping, one row of cowpea was planted between the maize rows with intra-row spacing of 0.1 m, giving a cowpea density of 13.3 plants m 2. Plot size was 7.5 m × 6.0 m and there were I0 maize rows in each plot. Sole-crop cowpea was grown separately beside the main experimental area to avoid shading from maize. There were four treatments (2 cultivars × 2 plant densities) of sole-crop cowpea and there were three replications. In addition to the density of 13.3 plants m -2 used in the intercropping, a higher density of 17.8 plants m - 2 was included as a check that cowpea sole-crop populations were optimal. 2.1.2. Cultural management Before planting, a commercial fertilizer at the rate of 60 kg ha- ~ each of N, P and K was broadcast and incorporated on all plots. Except in the sole-crop cowpea plots, a further 20 kg N ha-~ was banded in the form of ammonium nitrate along the maize rows at 24 days after maize sowing. Cowpea was sown on 16 February 1991 and maize was sown 4 days later when the cowpea emerged. Days after sowing (DAS) refer to sowing of the maize crop in both Experiment 1 and 2. Three seeds were planted per position for both crops and seedlings were thinned to one per position when they were 7-10 days old. The crops were maintained free of weeds by hand weeding in early stages of crop growth. Insecticides containing methomyl and perfekthion were applied at 22 and 47 DAS to control Heliothis spp. and other insect infestation on cowpea. Rainfall was evenly distributed, totalling 339 mm during crop growth and an additional 206 mm of irrigation was supplied to ensure water was non-limiting.
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J.M. Watiki et al. /Field Crops Research 35 (1993) 123-133 2.1.3. Measurements
Plant samples were taken at 20, 43 and 61 DAS and at maturity (107 days for cowpea and 127 days for maize). At each harvest, plants in 1.8 m 2 area for solecrop maize (2 rows of maize, each 1.2 m long) and intercropping plots (2 rows of maize and 2 rows of cowpea, each 1.2 m long) and in 1.2 m 2 area for solecrop cowpea plots were removed at ground level from each plot. A sub-sample of 3-4 plants was taken for each crop from each plot to determine leaf area, and dry weights of different plant parts. At the final harvest, both grain yield and plant biomass were determined. In cowpea, pods were harvested as they matured to avoid loss of seeds through splitting. Interception of photosynthetically active radiation (PAR) was measured by a l-m-long line quantum sensor (LI-COR, Lincoln, NE, USA) between noon and 2 pm at 7- to 10-day intervals until maximum interception was attained at 66 DAS. Horizontal flux density in intercropping was determined above maize canopy, immediately above the cowpea canopy and at ground level. The flux density determination was used to estimate, in addition to the total radiation intercepted by the combined canopy, the proportions of radiation intercepted by component crops. It was assumed that there was no radiation intercepted by maize in the lower layer. The presence of a small area of maize leaves together with stems in the lower layer meant that the proportion of maize interception was under-estimated and that of cowpea over-estimated. In sole crops, flux density was determined only above the canopy and at ground level. The line sensor was placed at an angle across the cowpea and maize rows so as to cover a width of 0.75 m between the rows. Plant height and canopy spread were determined on both cowpea and maize at weekly intervals. Tassels were included in the maize height measurements. Spread was measured as the average width of the maize or cowpea canopy along the rows. 2.1.4. Data analysis
Analysis of variance was used to analyze the data. Differences among treatment means were compared using least significant difference (LSD) at 0.05 probability. Means of measurements in sole-crop cowpea were analyzed separately from those of intercrop means since the two cropping systems were not randomized in the field.
2.2. Experiment 2
This experiment was conducted at the same time as Experiment 1, in the same field. Site preparation and other crop management were the same as for Experiment I. The experiment was also separated into two parts; the main part consisting of sole-crop maize and maize/ cowpea intercropping, and the other, sole-crop cowpea. Both parts used a randomized complete block design with three replications. In the main part the treatments consisted of sole-crop maize (cultivar DK 529), and intercrop where DK 529 maize was intercropped with 15 different cowpea cultivars which ranged in flowering time from 40-70 days after planting. The 15 cultivars used were: CPI 57317, CPI 72790, CPI 78213, Banjo, Red Caloona, Lawes, Arafura, Chino, California Blackeye 5, Poona, Narayen, SSD 90, CPI 71276, CP196963, CPI 67233. The plot size of each treatment was 3.0 m X 3.0 m for both intercrops and sole-crop maize. Each plot of intercrop had four rows of maize each 3.0 m long and 0.75 m between rows and a plant spacing of 0.3 m, giving a maize population of 4.4 plants m - 2. Spatial arrangement of intercropped cowpea was the same as described for Experiment 1 ( 13.3 plants m - 2). The sole-crop maize had also 0.75-m row spacing, but the plant spacing was 0.2 m to give a plant density of 6.6 plants m -2. The same 15 cowpea cultivars were planted in sole cropping adjacent to the main experiment. The plot size was 2.0 m × 2.0 m. The row spacing was 0.375 m and the plant spacing was 0.15 m, giving a plant population of 17.7 plants m -2. Plants were harvested at maturity of respective crops from an area of 2.7 m 2 per plot in intercropping and sole-crop maize, and of 0.79 m 2 in sole-crop cowpea. Harvesting procedures and methods of determination of vegetative biomass and yield were as described for Experiment 1. Plant height, canopy width, radiation interception by the whole canopy, and radiation above the intercropped cowpea canopy were determined weekly. The measurement methods were the same as for Experiment 1.
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J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133 3.0
3. Results
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3.1. Experiment 1 The two cowpea cultivars, Red Caloona and CPI 67233, behaved similarly in both sole cropping and intercropping, and therefore only results involving one cowpea cultivar (Red Caloona) are presented. The performance of maize in the medium plant density was between that of high and low densities for plant height, canopy spread, leaf area index (LAI) and radiation interception on most measurement occasions, and the results of the medium density are omitted from most figures for the sake of simplicity. Similarly, in solecrop cowpea, the results of the lower density (13.3 plants m -2) are used as this is the density used in intercropping and also the yield was slightly higher.
3.1.1. Flowering and maturity Flowering in cowpeas started about 45 days after planting and continued for the next 4 to 5 weeks. Tassel emergence in maize started at about 48 DAS. By 58 DAS, both tasselling and silking were complete. By final harvest at 127 DAS, maize in all the treatments had reached 50% black layer development in grains. Physiological maturity of cowpea pods started at about 83 DAS and continued up to 107 DAS.
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3.1.2. Plant height and canopy spread Effects of maize density on height and canopy spread of both maize and cowpea were small in intercropping (Fig. 1). Both maize and cowpea plants were taller in high-density maize treatments. Maize was always taller than cowpea, and the difference increased rapidly after 43 DAS when cowpea reached maximum height. Intercropping had no significant effect on maize height (data not shown), but cowpea elongated in intercropping, particularly when high maize density was used and maximum height was about 0.3 m more than in sole cropping. Both maize and cowpea canopies in intercropping exhibited a rapid spread which was faster for maize than for cowpea. In all maize densities, the maize and cowpea canopies met (i.e. total spread exceeding 0.75 m) by 25 DAS and maize canopies of adjacent rows met by 30 DAS. There were two layers of canopy (i.e. maize in top and cowpea in bottom) covering the whole area by 43 DAS. Maize canopy spread continued
60
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- 3"~
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30
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Days after sowing Fig. 1. Changes in plant height (a) and canopy spread (b) of intercropped maize (continuous lines) and cowpea (dotted lines) in intercropping under high (D) and low ( O ) maize plant density, Experiment 1. In (a), height of sole-crop cowpea ( ~ ) is also shown. The horizontal line in (b) indicates maize row spacing of 0.75 m. Vertical bars represent LSD at P = 0.05.
beyond 0.75 m in all treatments indicating that maize leaves from adjacent rows completely overlapped the cowpeas underneath.
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
127
3.1.3. L e a f area index and radiation interception
ping affected maize LAI more than it did cowpea LAI. In sole cropping, radiation interception by cowpea was similar to that of the maize in the high density (Fig. 3). There was a decrease in radiation interception with decrease in maize density in both intercropping and sole cropping, but the effect of density was greater in sole crop. Total interception was higher in intercrop-
Increase in LAI up to 20 DAS was small in all crops, but a rapid increase occurred between 20 and 43 DAS (Fig. 2). While intercropping reduced the LAI of both cowpea and maize in general, the effect depended on plant density of maize. At the high maize density, the effect of intercropping was small in maize, but cowpea LAI was almost halved by the associated maize crop. At the low maize density, on the other hand, intercrop-
(a) Sole crop (a) Maize
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Days after sowing Fig. 2. Change in leaf area index (LAI) of maize (a) and cowpea (b), Experiment 1. Dotted lines are for sole cropping (A cowpea) and continuouslines for intercroppingunder high ( [] ) and low (O) maize plant density. Vertical bars represent LSD at P = 0.05.
20
40
i
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Days after sowing Fig. 3. Radiation interceptionin sole cropping (a) and intercropping (b), Experiment I (N high maize density, O low maize density, A sole-crop cowpca). Solid symbols in (b) are for radiation interccplion by intcrcropped maize, determined by measuring radiation flux density at the top of the corresponding cowpea canopy. Verticalbars represent L S D at P = 0.05.
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
128
ping than in sole-crop maize of the corresponding density. Radiation (shortwave solar radiation) interception for sole-crop maize and the combined intercrop for 21-61 DAS was 371 and 557 MJ m -2 at the low maize density and 494 and 596 MJ m - 2 at the high density. Thus, the increase in radiation interception due to intercropping was higher in the low density (50% increase) than in the high density (21% increase). At 24 DAS, radiation intercepted by the maize leaves located above the cowpea canopy was only a small fraction of the radiation intercepted by the whole intercrop canopy (Fig. 3), because the height of maize plants was only slightly more than that of cowpea in intercropping at the time. Some maize leaves located in the lower cowpea layer would have also intercepted radiation as there were no overlaps of maize and cowpea canopy at this stage, and hence interception by the maize component is underestimated and that by the
1600
cowpea overestimated when the plants were small. However, as maize LAI increased rapidly and the leaves were mostly located above the cowpea canopy after 24 DAS, interception by the intercrop maize increased rapidly, particularly in the high maize density plots, and interception by the whole intercrop was due mostly to the maize canopy in the high density by 52 DAS. In the low maize density, on the other hand, the maize canopy intercepted only about 50% in 52-67 DAS because of its low LAI, thus allowing the associated cowpea to intercept about 50% of the incident radiation.
3.1.4. Total dry matter and radiation-use efficiency In sole-crop maize, total dry matter (TDM) production was promoted by high density during early stages of growth, and the difference in TDM was maintained to maturity, though the differences were not significant at the final harvest (Fig. 4). The effect of intercropping b) I n t e r e r o p p e d m a i z e
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J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133 2.0
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Table 1 Radiation-use efficiency (g MJ - j shortwave solar radiation) of maize and cowpea in sole cropping and maize/cowpea intercrop under three maize plant densities in two periods (A: 21--43, B: 4 4 61 days after sowing), Experiment 1 Maize density (plants m -2)
-3 1.o
Sole crop
Intercrop
Maize
Cowpea
A
B
A
1.51 1.59 1.65
1.51 1.49 1.53
B
A
B
1.03 1.35 1.47
1.16 1.30 1.60
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0.59
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Maize i n t e r c e p t i o n Fig. 5. Relationship between radiation-use efficiency (RUE, shortwave solarradiation) and the proportionof radiationinterceptedby cowpea, Experiment I (sole-crop m a i z e , [ ] high maize density, V medium maize density, © low maize density; A sole-crop cowpea; solid symbols are for intercropping). Dotted line indicatesexpected RUE in intercropping, estimated from RUE in sole cropping and proportion of radiation interceptedby each componentcrop.
on maize TDM production was small at high and medium density of maize, but TDM production was affected greatly by intercropping at low maize density. Total dry matter production of cowpea was severely affected by associated maize, particularly when the high maize density was used in intercropping. There was no TDM increase in intercropped cowpea after 43 DAS in the high maize density treatment whereas there was some increase in TDM to maturity when the low maize density was used. Combined TDM (data not shown) up to 61 DAS, when radiation measurements were made, was higher than that of sole-crop maize at any maize plant density. Radiation-use efficiency (based on shortwave radiation) was calculated for 21-43 DAS and 43-61 DAS separately. In sole-crop maize, RUE was similar ( 1.51.7 g MJ- ') in all densities for the two periods (Table 1 ). In sole-crop cowpea, RUE was lower than that of sole-crop maize and it decreased during the two periods. In the combined intercrop, RUE did not differ much between the two periods, but it was positively related to maize plant density. At the high maize den-
sity, RUE was similar in the two cropping systems, but at the low maize density it was lower in intercropping. Since RUE did not differ much during the two periods in all crops, RUE was estimated for the whole 40 days. The variation in RUE of the sole crops and intercrops was related to the proportion of radiation intercepted by each component crop (Fig. 5). The RUE of combined intercrop did not differ much from what could be expected from RUE of the sole crops and the proportion of radiation intercepted by each component crop, when maize density was low. However, the RUE of intercrop appears to be slightly higher than expected at high maize density (RUE of 1.5 g MJ- l compared with the expected value of 1.3 g MJ- ').
3.1.5. Grain yield and land equivalent ratio In both sole cropping and intercropping, maize grain yield increased with increase in maize plant density from 2.2 to 4.4 plants m - 2 but further increase in plant Table 2 Grain yield of maize and cowpea (g m z) in sole cropping (S) and intercropping (I) under three maize plant densities, Experiment 1 Maize density (plants m -2)
2.2 4.4 6.7
Maize yield
Cowpea yield
S
I
I
527 636 683
331 591 621
29.1 16.6 10.1
S
62.1 LSD
161
NS
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
130 Table 3 Total land equivalent ratio (LER) and partial LER of maize and cowpea grown under three maize plant densities, Experiment 1 Maize density (plants m 2)
2.2 4.4 6.7
Total LER
Partial LER
0.95 1.13 1.07
Maize
Cowpea
0.48 0.86 0.91
0.47 0.27 0.16
Table 4 Ranges of some attributes of 15 cowpea cultivars in sole cropping (S) and in intercropping (I), and corresponding values for cultivar Red Caloona, Experiment 2 Range Grain yield (g m -2) TDM (g m -2) Harvest index Grain size (mg) Flowering time (DAS) Height at 56 DAS (m) Canopy spread at 28 DAS (m) Solar radiation (%) above intercropped cowpea at 28 DAS
S
32-152
I
12-28
S I S I S I S I S I S I
322-528 77-160 0.06--0.34 0.074).21 49-245 67-178 43--66 47--66 0.50-0.88 0.85-1.57 0.26--0.38 0.26--0.42 45-50
I
Red Caloona 33 17 408 117 0.08 0.14 62 68 49 51 0.77
density resulted in only a small increase in yield (Table 2). The effect of intercropping on maize yield was small (i.e. reduction of less than 10%) at the medium and high maize density, but intercropping reduced maize yield by about 40% at the low density. Associated maize reduced the cowpea yield greatly, even at the lowest maize density, and it was reduced further with the increase in maize density. The effect of maize density on cowpea yield was not significant for the cultivar (Red Caloona) shown in the table, but it was significant when two cowpea cultivars were used in analysis. The land equivalent ratio (LER) was calculated using the highest yield in sole crops of maize (which was obtained in the high maize plant density) and cowpea (which was obtained in the low cowpea plant density). The total LER was lowest in the low maize density as partial LER of maize was low (Table 3). With increase in maize density, there was an increase in partial LER of maize but there was a decrease in partial LER of cowpea. Total LER at the medium and high maize density was similar at around 1.1.
3.2. Experiment 2 There was significant variation among 15 cultivars in all characters examined in both sole cropping and intercropping (Table 4). Grain yield and TDM at maturity were greatly reduced by associated maize in all cultivars. Red Caloona, which was used in Experiment 1, was one of the low-yielding cultivars with
1.37
0.27 0.26 49
Table 5 Correlation coefficients between grain yield and some attributes (vegetative dry matter at maturity, canopy spread at 28 days after sowing, and plant height at 56 days after sowing ) in sole cropping (S) and intercropping (I), and correlation coefficients between radiation interception by sole crop cowpea (S) or radiation above intercropped cowpea (I) and these attributes, Experiment 2 Vegetative DM
Grain yield
S
Radiation above cowpea *Significant at P = 0.05.
S I
Height
S
I
S
I
- 0.38*
0.30* 0.33*
0.45* 0.57* -
0.30* -0.05
I Radiation interception
Canopy spread
0.18 -
S
I 0.14 0.06
-0.47* -0.10
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
rather low TDM also. Harvest index was affected less by intercropping than were grain yield and TDM. Grain size and flowering time were little affected by intercropping. In most cultivars (e.g. Red Caloona), plants were taller under intercropping. There was a tendency for climbing types, which were taller in intercropping, to be shorter in sole cropping with resultant negative correlation ( r = - 0.43, P < 0.05) between the heights of cultivars at 56 DAS in the two cropping systems. Canopy spread in intercropping had the largest range at 28 DAS, but the effect of intercropping was small. Solar radiation reaching the intercropped cowpea canopy varied little among cowpea cultivars (Table 4). While intercropped cowpea yield varied 2.5 fold, yields of maize intercropped with different cowpea cultivars ranged only from 512 to 640 g m -2, and there was no relationship between cowpea yield and corresponding maize yield in intercropping (Fig. 6a). Similarly there was no relationship between partial LER of cowpea and that of maize (Fig. 6b). Since the range in maize partial LER was relatively small, the difference in total LER reflected mostly the difference in partial LER of cowpea. Thus four cultivars with partial cowpea LER of 0.5-0.7 produced total LER of 1.2-1.4, whereas five cultivars with partial cowpea LER of less than 0.2 resulted in total LER of less than 1.0. Three high-yield cultivars in intercropping did not have high partial cowpea LER, and hence total LER, as they also produced high yield in sole cropping. Similarly, five low-yielding cultivars in intercropping were not necessarily low in partial LER, as they were low yielders also in sole cropping, except CP172790 which produced the highest yield among all cultivars in sole cropping. There was no significant correlation for cowpea yields of cultivars between sole cropping and intercropping, because there w~re two cultivars which behaved quite differently under different cropping systems; CPI 72790 as mentioned above and CPI 96963, which had the lowest yield among all cultivars in sole cropping but which produced intermediate yield in intercropping. The cultivar CPI 72790 matured late with rather small TDM, and its growth may have been affected greatly by associated maize. On the other hand, CP196963 was early maturing with high TDM, and the effect of maize may have been small. Cowpea yield in intercropping was not correlated strongly with all characters determined, i.e. canopy
131
spread, height and radiation above the cowpea canopy (Table 5). It was, however, positively correlated with vegetative DM and canopy width, indicating that cowpea cultivars which spread fast and produced high biomass tended to produce high yield in intercropping. Grain yield in sole cropping was also positively correlated with canopy spread, but it was negatively cot(a) 700
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Cowpea yield (g m -~)
(b)
Total LER 1.2
1.0
1.4
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cd ~
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Cowpea LER Fig. 6. Relationshipsbetween maize yieldand cowpca yield (a) and between partialland equivalentratio (LER) of maize and that of cowpea (b) with 15 cowpea cultivarsin maize/cowpca intcrcropping, Experiment 2. Cowpea cultivarswith high (o), medium ( • ) and low (ll) yield.In (a) bars representLSD at P=0.05. In (b) diagonal linesindicatetotalLER.
132
related with vegetative DM. Therefore cultivars with vigorous growth may produce low grain yield in sole cropping because of the low partitioning of assimilates to grain, while in intercropping vigorous growth may be required to compete against associated maize. Canopy spread was positively related to radiation interception in sole crop (r=0.57, P < 0 . 0 5 ) , but it was not related to radiation available to cowpea in intercropping. Radiation above the cowpea canopy in intercropping was positively associated with vegetative DM production in intercropping.
4. Discussion
4.1. Radiation interception and radiation-use efficiency The results of Experiment I show that total interception of radiation by intercrops was higher than that of sole crops by 20-50% during the period up to the commencement of grain filling in maize. This is because the experiment employed an 'additive' series in which another crop (cowpea) was added to planting patterns used in the main crop (maize) under sole cropping. Similar results were obtained in cassava (Manihot esculenta Crantz)/soybean (Glycine max (L.) Merrill) intercropping by Tsay et al. (1988). The higher radiation interception in intercropping is the reason for the increase in combined TDM over the TDM of solecrop maize at 61 DAS, as RUE of combined intercrop never exceeded that of sole-crop maize. The RUE of the intercrop was between that of the maize and cowpea sole crops, the actual value being most strongly dependent on the proportion of radiation intercepted by the component crops (Fig. 5). The departure from a simple 1 : 1 mixing relationship seen in intercrops with medium or high maize densities, indicated that these canopies were operating 10 to 20% more efficiently than would be expected from their performance as sole-crop canopies alone. It should be pointed out however, that the proportion of radiation intercepted by maize was slightly underestimated for the reason given earlier, and thus the 10--20% would be an overestimation. In addition, the extrapolation from discrete measurements of radiation interception to cumulative interception does introduce further uncertainty. It is therefore likely that RUE of an inter-
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
crop follows closely to that expected from RUEs of sole crops and the proportion of radiation intercepted by the component crops. The literature is equivocal on the impact of intercropping on RUE. Marshall and Willey (1983) showed that before grain filling, RUE of a combined millet/ groundnut (Arachis hypogaea L.) intercrop (4.3 g MJ- ~ of PAR) was slightly higher than that of solecrop millet (4.1 g MJ- 1) although this difference was non-significant. As shown in the current work, the proportion of C4 species in a mixture has a dominant impact on the RUE of the mixture. Natarajan and Willey (1980) showed this effect when high plant densities of sorghum (Sorghum bicolor Moench) intercropped with pigeonpea (Cajanus cajan L.) produced higher RUE of combined intercrop than low densities, presumably because at the high densities more radiation was intercepted by the C4 sorghum.
4.2. Dry matter production, yields and LER Examination of the yield data show that maize dominated cowpea in both experiments at the medium and high densities (Experiments 1 and 2, Tables 2, 3 and 4). Maize yields were generally maintained in the intercrops at near sole-crop levels, except when maize was present as a low-density component. In contrast, the cowpea component was clearly dominated by maize and yields were reduced accordingly. The reason for the depression in growth of low-density maize when intercropped is unclear. Others have shown no effect of low cowpea densities on maize grown at low densities (Wahua et al., 1981; Fawusi et al., 1982), except in dry environments where competition for water means that even low legume densities can restrict maize growth (Allen and Obura, 1983). In Experiment 2, partial LER of cowpea varied greatly when different cultivars were used and this caused a large variation in total LER. This agrees with the general conclusion of Ofori and Stern (1987a) that total LER is determined by LER of the legume, the dominated component, in cereal/legume intercropping. Similarly in Experiment 1, the increase in maize plant density from medium (4.4 plants m -2) to high (6.7 plants m-2) reduced partial LER of cowpea and total LER as the increase in partial LER of maize was only marginal. However, the result of increase in plant density from low (2.2 plants m -2) to medium (4.4 plants m -z) was different; the increase in maize LER
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J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
was large and more than compensated for the decrease in cowpea LER with resultant increase in total LER. There were two reasons for the response of maize LER to density at the low-to-medium density range. One is reduced yield potential at the low density, as can be seen in the decline in maize yield with the decrease in density in sole cropping. The other is that at the low maize density there was a larger adverse effect of associated cowpea, and hence grain yield of the maize in intercropping was reduced compared to that in sole cropping. This result suggests that, while maize is generally a dominant crop in maize/cowpea intercropping, the competitiveness may be affected by agronomic manipulations, and when the maize growth is affected by associated cowpea, the change in total LER does not follow the change in LER of cowpea. The advantage of intercropping as judged by LER was rather small (i.e. LER of around 1.1 ) in the experiments. This small advantage was at least partly related to the very low biomass production of cowpea after flowering in intercropping, despite 20-50% radiation transmitted through the maize canopy. Similar problems in intercropped legume during pod filling was observed in soybean in cassava/soybean intercropping (Tsay et al., 1988) and groundnut in millet/groundnut intercropping (Marshall and Willey, 1983). It would be worth examining whether this problem is related to poor adaptation of the legumes to low-radiation environments or to other reasons such as increased disease occurrence in the understorey crops.
4.3. Implications for cultivar selection In situations where maize dominated cowpea (i.e. medium to high maize densities), there were no differences among cowpea cultivars in their effects on maize yields (Experiment 2, Fig. 6a) and the productivity of the intercrop was maximized by using high-yielding cowpea cultivars. Selection of cowpea cultivars for such high-density maize intercropping systems could proceed based on performance within the intercrop of the dominated cowpea component alone. At the low maize density, cowpea had a major detrimental effect on maize yield (Experiment 1, Tables 2 and 3). Under such circumstances, no single crop was clearly dominant. While not examined in this experiment, it is likely that selection of cowpea cultivars for intercropping in such an arrangement with maize would
need to examine yields of both cowpea and maize. This suggests that selection of cultivars for intercropping depends on whether the component crop is dominated, or competitive or dominant. For the dominant crop, its adverse effect on the performance of the associated crop (Cenpukdee and Fukai, 1992) needs to be considered, when selecting cultivars.
Acknowledgements We wish to thank Dr B.C. Imrie for providing cowpea seed.
References Allen, J.R. and Obura, R.K., 1983. Yield of corh, cowpea, and soybean under different intercropping systems. Agron. J., 75: 10051009. Cenpukdee, U. and Fukai, S., 1992. Cassava/legume intercropping with contrasting cassava cultivars. 2. Selection criteria for cassava genotypes in intercropping with two contrasting legume crops. Field Crops Res., 29: 135-149. Fawusi, M.O.A., Wanki, S.B.C. and Nangju, D., 1982. Plant density effects on growth, yield, leaf area index and light transmission on intercropped maize and Vigna unguiculata (L.) Walp. in Nigeria. J. Agric. Sci., 99: 19-23. Marshall, B. and Willey, R.W., 1983. Radiation interception and growth in an intercrop of pearl millet/groundnut. Field Crops Res., 7: 141-160. Natarajan, M. and Willey, R.W., 1980. Sorghum-pigeonpea intercropping and the effects of plant population density. 2. Resource use. J. Agric. Sci., Camb., 95: 59-65. Ntare, B.R., 1989. Evaluation of cowpea cultivars for intercropping with pearl millet in the Sahelian zone of West Africa. Field Crops Res., 20: 31-40. Ofori, F. and Stem, W.R., 1987a. Relative sowing time and density of component crops in a maize/cowpea intercrop system. Exp. Agric., 23: 41-52. Ofori, F. and Stem, W.R., 1987b. Cereal-legume intercropping systems. Adv. Agron., 41: 41-90. Reddy, K.C., Van der Ploeg, J. and Maga, I., 1990. Genotype effects in millet/cowpea intercropping in the semi-arid tropics of Niger. Exp. Agric., 26: 387-396. Tsay, J.S., Fukai, S. and Wilson, G.L., 1988. Intercropping cassava with soybean cultivars of varying maturities. Field Crops Res., 19:211-225. Wahua, T.A.T., Babalola, O. and Akenova, M.E., 1981. lntercropping morphologically different types of maize with cowpeas: LER and growth attributes of associated cowpeas. Exp. Agric., 17: 407-413.
Field Crops Research
Radiation interception and growth of maize/cowpea intercrop as affected by maize plant density and cowpea cultivar J.M. Watiki a, S. Fukai a'*, J.A. Banda a, B.A. Keating b aDepartment of Agriculture, The Universityof Queensland, Brisbane, Queensland 4072, Australia bCSIRO Division of Crops and Pastures. St Lucia, Queensland 4067, Australia (Accepted 11 May 1993)
Abstract Sole and intercropped maize (Zea mays L.) and cowpea (Vigna unguiculata (L.) Walp.) were grown to examine how radiation interception and radiation-use efficiency (RUE) changed under intercropping. The work also examined whether intercropping advantage as measured by land equivalent ratio (LER) was determined by the yield of the dominated cowpea crop. In one experiment, maize plant density was varied and in another, it was held constant and 15 contrasting cowpea cultivars were used. Increased radiation interception by the intercrops prior to maize grain filling increased biomass production, particularly at the low maize density. There was, however, no intercropping advantage in RUE. Radiation-use efficiency of the combined intercrop was between that of maize and cowpea in sole cropping, the actual value being determined by the proportion of radiation intercepted by component crops. There was a rather small yield advantage of intercropping, with LER of around 1.1 at the two higher plant densities of maize. At the low maize density of 2.2 plants m - 2, however, LER was less than 1.0 despite an increase in partial LER of cowpea. Cowpea yield and partial LER varied greatly among 15 cowpea cultivars when intercropped. There were positive but weak correlations between yield and vegetative dry matter of cowpea, and between vegetative dry matter and radiation transmitted through the maize canopy to the cowpea. Grain yield and partial LER of maize, on the other hand, were similar when intercropped with different cowpea cultivars, and hence variation in total LER reflected mostly the variation in partial LER of cowpea. It is concluded that maize, except when planted at low densities, will dominate cowpea, and the performance of the dominated crop (cowpea) will have most influence on total LER. Key words: Cowpea; Intercropping; Maize; Radiation-useefficiency; Vigna; Zea
I. Introduction In a recent review of c e r e a l / l e g u m e intercropping, Ofori and Stern (1987b) concluded that the reduction in grain yield due to intercropping is less for the cereal than the legume and the land equivalent ratio ( L E R ) is commonly determined by the yields of legume under *Corresponding author.
various agronomic manipulations. This was the case in m a i z e / c o w p e a intercropping when relative sowing time and plant density of the two components were altered (Ofori and Stem, 1987a) and in millet (Pennisetum glaucum (L.) R . B r . ) / c o w p e a intercropping when different cultivars were used (Reddy et al., 1990). In m a i z e / c o w p e a intercropping, the two components grow vigorously at about the same time, and
0378-4290/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved SSD10378-4290(93) E0042-V
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competition for available growth resources such as solar radiation is high. Solar radiation is a major resource determining growth and yield of component crops in intercropping, particularly when other resources (e.g. water and N) are not severely limiting crop growth. As maize plants become increasingly,, taller than cowpea plants, radiation becomes less available to the cowpea. Some cowpea cultivars may mature before the adverse effect of associated cereal crop becomes severe, and often early-maturing cultivars are advantageous in intercropping with millet (Ntare, 1989). Availability of solar radiation to the cowpea may be improved by the use of maize cultivars with erect leaves in the upper canopy (Wahua et al., 1981 ). Less-determinate cowpea cultivars can climb maize stems, increasing their interception of radiation at the expense of that by the maize. This may have adverse effects on the overall performance of maize/cowpea intercropping. The experiments reported in this paper were aimed at quantifying the benefits of intercropping in terms of radiation capture and use. In one experiment radiation availability to cowpea was altered by using different plant densities of maize. In the other experiment, a range of cowpea cultivars were used to determine if differences in cowpea morphology influence the productivity of intercrops with maize. The contribution of component crops to LER was examined to test the hypothesis that LER is determined by the performance of the dominated crop, in this case, the shorter-statured cowpea.
2. Materials and Methods
Two experiments were carried out on The University of Queensland research farm at Redland Bay (latitude 27°31'S, longitude 153°19'E), S.E. Queensland, Australia. The soil is a deep, well-structured krasnozem with good aeration and drainage. The soil has a high clay content (60%) and a pH of 5.5-6.2. Both experiments commenced in mid-February 1991, when daily mean temperatures were about 25°C. Temperature decreased gradually to about 17°C in June when the experiments were completed.
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
2. I. Experiment I 2.1.1. Treatments In the main part of Experiment 1, there were nine treatments randomly allocated in each of three replications. There were three maize plant densities (2.2, 4.4 and 6.7 plants m -z) in both sole cropping and intercropping with two cowpea cultivars, Red Caloona and CPI 67233. The maize cultivar used was an earlymaturing hybrid DK 529. Row spacing of maize was 0.75 m, and intra-row spacing varied from 0.6 m to 0.2 m to produce the designated maize densities in both sole cropping and intercropping. In intercropping, one row of cowpea was planted between the maize rows with intra-row spacing of 0.1 m, giving a cowpea density of 13.3 plants m 2. Plot size was 7.5 m × 6.0 m and there were I0 maize rows in each plot. Sole-crop cowpea was grown separately beside the main experimental area to avoid shading from maize. There were four treatments (2 cultivars × 2 plant densities) of sole-crop cowpea and there were three replications. In addition to the density of 13.3 plants m -2 used in the intercropping, a higher density of 17.8 plants m - 2 was included as a check that cowpea sole-crop populations were optimal. 2.1.2. Cultural management Before planting, a commercial fertilizer at the rate of 60 kg ha- ~ each of N, P and K was broadcast and incorporated on all plots. Except in the sole-crop cowpea plots, a further 20 kg N ha-~ was banded in the form of ammonium nitrate along the maize rows at 24 days after maize sowing. Cowpea was sown on 16 February 1991 and maize was sown 4 days later when the cowpea emerged. Days after sowing (DAS) refer to sowing of the maize crop in both Experiment 1 and 2. Three seeds were planted per position for both crops and seedlings were thinned to one per position when they were 7-10 days old. The crops were maintained free of weeds by hand weeding in early stages of crop growth. Insecticides containing methomyl and perfekthion were applied at 22 and 47 DAS to control Heliothis spp. and other insect infestation on cowpea. Rainfall was evenly distributed, totalling 339 mm during crop growth and an additional 206 mm of irrigation was supplied to ensure water was non-limiting.
125
J.M. Watiki et al. /Field Crops Research 35 (1993) 123-133 2.1.3. Measurements
Plant samples were taken at 20, 43 and 61 DAS and at maturity (107 days for cowpea and 127 days for maize). At each harvest, plants in 1.8 m 2 area for solecrop maize (2 rows of maize, each 1.2 m long) and intercropping plots (2 rows of maize and 2 rows of cowpea, each 1.2 m long) and in 1.2 m 2 area for solecrop cowpea plots were removed at ground level from each plot. A sub-sample of 3-4 plants was taken for each crop from each plot to determine leaf area, and dry weights of different plant parts. At the final harvest, both grain yield and plant biomass were determined. In cowpea, pods were harvested as they matured to avoid loss of seeds through splitting. Interception of photosynthetically active radiation (PAR) was measured by a l-m-long line quantum sensor (LI-COR, Lincoln, NE, USA) between noon and 2 pm at 7- to 10-day intervals until maximum interception was attained at 66 DAS. Horizontal flux density in intercropping was determined above maize canopy, immediately above the cowpea canopy and at ground level. The flux density determination was used to estimate, in addition to the total radiation intercepted by the combined canopy, the proportions of radiation intercepted by component crops. It was assumed that there was no radiation intercepted by maize in the lower layer. The presence of a small area of maize leaves together with stems in the lower layer meant that the proportion of maize interception was under-estimated and that of cowpea over-estimated. In sole crops, flux density was determined only above the canopy and at ground level. The line sensor was placed at an angle across the cowpea and maize rows so as to cover a width of 0.75 m between the rows. Plant height and canopy spread were determined on both cowpea and maize at weekly intervals. Tassels were included in the maize height measurements. Spread was measured as the average width of the maize or cowpea canopy along the rows. 2.1.4. Data analysis
Analysis of variance was used to analyze the data. Differences among treatment means were compared using least significant difference (LSD) at 0.05 probability. Means of measurements in sole-crop cowpea were analyzed separately from those of intercrop means since the two cropping systems were not randomized in the field.
2.2. Experiment 2
This experiment was conducted at the same time as Experiment 1, in the same field. Site preparation and other crop management were the same as for Experiment I. The experiment was also separated into two parts; the main part consisting of sole-crop maize and maize/ cowpea intercropping, and the other, sole-crop cowpea. Both parts used a randomized complete block design with three replications. In the main part the treatments consisted of sole-crop maize (cultivar DK 529), and intercrop where DK 529 maize was intercropped with 15 different cowpea cultivars which ranged in flowering time from 40-70 days after planting. The 15 cultivars used were: CPI 57317, CPI 72790, CPI 78213, Banjo, Red Caloona, Lawes, Arafura, Chino, California Blackeye 5, Poona, Narayen, SSD 90, CPI 71276, CP196963, CPI 67233. The plot size of each treatment was 3.0 m X 3.0 m for both intercrops and sole-crop maize. Each plot of intercrop had four rows of maize each 3.0 m long and 0.75 m between rows and a plant spacing of 0.3 m, giving a maize population of 4.4 plants m - 2. Spatial arrangement of intercropped cowpea was the same as described for Experiment 1 ( 13.3 plants m - 2). The sole-crop maize had also 0.75-m row spacing, but the plant spacing was 0.2 m to give a plant density of 6.6 plants m -2. The same 15 cowpea cultivars were planted in sole cropping adjacent to the main experiment. The plot size was 2.0 m × 2.0 m. The row spacing was 0.375 m and the plant spacing was 0.15 m, giving a plant population of 17.7 plants m -2. Plants were harvested at maturity of respective crops from an area of 2.7 m 2 per plot in intercropping and sole-crop maize, and of 0.79 m 2 in sole-crop cowpea. Harvesting procedures and methods of determination of vegetative biomass and yield were as described for Experiment 1. Plant height, canopy width, radiation interception by the whole canopy, and radiation above the intercropped cowpea canopy were determined weekly. The measurement methods were the same as for Experiment 1.
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J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133 3.0
3. Results
I
I
I
I
I
(a)
3.1. Experiment 1 The two cowpea cultivars, Red Caloona and CPI 67233, behaved similarly in both sole cropping and intercropping, and therefore only results involving one cowpea cultivar (Red Caloona) are presented. The performance of maize in the medium plant density was between that of high and low densities for plant height, canopy spread, leaf area index (LAI) and radiation interception on most measurement occasions, and the results of the medium density are omitted from most figures for the sake of simplicity. Similarly, in solecrop cowpea, the results of the lower density (13.3 plants m -2) are used as this is the density used in intercropping and also the yield was slightly higher.
3.1.1. Flowering and maturity Flowering in cowpeas started about 45 days after planting and continued for the next 4 to 5 weeks. Tassel emergence in maize started at about 48 DAS. By 58 DAS, both tasselling and silking were complete. By final harvest at 127 DAS, maize in all the treatments had reached 50% black layer development in grains. Physiological maturity of cowpea pods started at about 83 DAS and continued up to 107 DAS.
2,5
Z
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E 1.5
1.0
0.5
I
0.0
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0
1.5
I
20 30 40 Days after sowing
I
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I
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(b) 1.2
E
0.9 .
3.1.2. Plant height and canopy spread Effects of maize density on height and canopy spread of both maize and cowpea were small in intercropping (Fig. 1). Both maize and cowpea plants were taller in high-density maize treatments. Maize was always taller than cowpea, and the difference increased rapidly after 43 DAS when cowpea reached maximum height. Intercropping had no significant effect on maize height (data not shown), but cowpea elongated in intercropping, particularly when high maize density was used and maximum height was about 0.3 m more than in sole cropping. Both maize and cowpea canopies in intercropping exhibited a rapid spread which was faster for maize than for cowpea. In all maize densities, the maize and cowpea canopies met (i.e. total spread exceeding 0.75 m) by 25 DAS and maize canopies of adjacent rows met by 30 DAS. There were two layers of canopy (i.e. maize in top and cowpea in bottom) covering the whole area by 43 DAS. Maize canopy spread continued
60
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OLo
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0.6
.
.
.
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.
.
- 3"~
0.3
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30
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50
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Days after sowing Fig. 1. Changes in plant height (a) and canopy spread (b) of intercropped maize (continuous lines) and cowpea (dotted lines) in intercropping under high (D) and low ( O ) maize plant density, Experiment 1. In (a), height of sole-crop cowpea ( ~ ) is also shown. The horizontal line in (b) indicates maize row spacing of 0.75 m. Vertical bars represent LSD at P = 0.05.
beyond 0.75 m in all treatments indicating that maize leaves from adjacent rows completely overlapped the cowpeas underneath.
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
127
3.1.3. L e a f area index and radiation interception
ping affected maize LAI more than it did cowpea LAI. In sole cropping, radiation interception by cowpea was similar to that of the maize in the high density (Fig. 3). There was a decrease in radiation interception with decrease in maize density in both intercropping and sole cropping, but the effect of density was greater in sole crop. Total interception was higher in intercrop-
Increase in LAI up to 20 DAS was small in all crops, but a rapid increase occurred between 20 and 43 DAS (Fig. 2). While intercropping reduced the LAI of both cowpea and maize in general, the effect depended on plant density of maize. At the high maize density, the effect of intercropping was small in maize, but cowpea LAI was almost halved by the associated maize crop. At the low maize density, on the other hand, intercrop-
(a) Sole crop (a) Maize
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o
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,
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20
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Days after sowing Fig. 2. Change in leaf area index (LAI) of maize (a) and cowpea (b), Experiment 1. Dotted lines are for sole cropping (A cowpea) and continuouslines for intercroppingunder high ( [] ) and low (O) maize plant density. Vertical bars represent LSD at P = 0.05.
20
40
i
60
Days after sowing Fig. 3. Radiation interceptionin sole cropping (a) and intercropping (b), Experiment I (N high maize density, O low maize density, A sole-crop cowpca). Solid symbols in (b) are for radiation interccplion by intcrcropped maize, determined by measuring radiation flux density at the top of the corresponding cowpea canopy. Verticalbars represent L S D at P = 0.05.
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
128
ping than in sole-crop maize of the corresponding density. Radiation (shortwave solar radiation) interception for sole-crop maize and the combined intercrop for 21-61 DAS was 371 and 557 MJ m -2 at the low maize density and 494 and 596 MJ m - 2 at the high density. Thus, the increase in radiation interception due to intercropping was higher in the low density (50% increase) than in the high density (21% increase). At 24 DAS, radiation intercepted by the maize leaves located above the cowpea canopy was only a small fraction of the radiation intercepted by the whole intercrop canopy (Fig. 3), because the height of maize plants was only slightly more than that of cowpea in intercropping at the time. Some maize leaves located in the lower cowpea layer would have also intercepted radiation as there were no overlaps of maize and cowpea canopy at this stage, and hence interception by the maize component is underestimated and that by the
1600
cowpea overestimated when the plants were small. However, as maize LAI increased rapidly and the leaves were mostly located above the cowpea canopy after 24 DAS, interception by the intercrop maize increased rapidly, particularly in the high maize density plots, and interception by the whole intercrop was due mostly to the maize canopy in the high density by 52 DAS. In the low maize density, on the other hand, the maize canopy intercepted only about 50% in 52-67 DAS because of its low LAI, thus allowing the associated cowpea to intercept about 50% of the incident radiation.
3.1.4. Total dry matter and radiation-use efficiency In sole-crop maize, total dry matter (TDM) production was promoted by high density during early stages of growth, and the difference in TDM was maintained to maturity, though the differences were not significant at the final harvest (Fig. 4). The effect of intercropping b) I n t e r e r o p p e d m a i z e
a) Sole c r o p m a i z e I
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Days a f t e r sowing Fig. 4. Change in total dry matter for sole-crop maize (a), intercropped maize (b) and cowpea (c), Experiment 1 ([3 high maize density, ~7 medium maize density, O low maize density, A sole-crop cowpea). Vertical bars represent LSD at P = 0.05.
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133 2.0
,
,
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Table 1 Radiation-use efficiency (g MJ - j shortwave solar radiation) of maize and cowpea in sole cropping and maize/cowpea intercrop under three maize plant densities in two periods (A: 21--43, B: 4 4 61 days after sowing), Experiment 1 Maize density (plants m -2)
-3 1.o
Sole crop
Intercrop
Maize
Cowpea
A
B
A
1.51 1.59 1.65
1.51 1.49 1.53
B
A
B
1.03 1.35 1.47
1.16 1.30 1.60
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2.2 4.4 6.7 0.0
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t
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0.2
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1.0
0.88
0.59
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0.8
0.6
0.4
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0.0
Maize i n t e r c e p t i o n Fig. 5. Relationship between radiation-use efficiency (RUE, shortwave solarradiation) and the proportionof radiationinterceptedby cowpea, Experiment I (sole-crop m a i z e , [ ] high maize density, V medium maize density, © low maize density; A sole-crop cowpea; solid symbols are for intercropping). Dotted line indicatesexpected RUE in intercropping, estimated from RUE in sole cropping and proportion of radiation interceptedby each componentcrop.
on maize TDM production was small at high and medium density of maize, but TDM production was affected greatly by intercropping at low maize density. Total dry matter production of cowpea was severely affected by associated maize, particularly when the high maize density was used in intercropping. There was no TDM increase in intercropped cowpea after 43 DAS in the high maize density treatment whereas there was some increase in TDM to maturity when the low maize density was used. Combined TDM (data not shown) up to 61 DAS, when radiation measurements were made, was higher than that of sole-crop maize at any maize plant density. Radiation-use efficiency (based on shortwave radiation) was calculated for 21-43 DAS and 43-61 DAS separately. In sole-crop maize, RUE was similar ( 1.51.7 g MJ- ') in all densities for the two periods (Table 1 ). In sole-crop cowpea, RUE was lower than that of sole-crop maize and it decreased during the two periods. In the combined intercrop, RUE did not differ much between the two periods, but it was positively related to maize plant density. At the high maize den-
sity, RUE was similar in the two cropping systems, but at the low maize density it was lower in intercropping. Since RUE did not differ much during the two periods in all crops, RUE was estimated for the whole 40 days. The variation in RUE of the sole crops and intercrops was related to the proportion of radiation intercepted by each component crop (Fig. 5). The RUE of combined intercrop did not differ much from what could be expected from RUE of the sole crops and the proportion of radiation intercepted by each component crop, when maize density was low. However, the RUE of intercrop appears to be slightly higher than expected at high maize density (RUE of 1.5 g MJ- l compared with the expected value of 1.3 g MJ- ').
3.1.5. Grain yield and land equivalent ratio In both sole cropping and intercropping, maize grain yield increased with increase in maize plant density from 2.2 to 4.4 plants m - 2 but further increase in plant Table 2 Grain yield of maize and cowpea (g m z) in sole cropping (S) and intercropping (I) under three maize plant densities, Experiment 1 Maize density (plants m -2)
2.2 4.4 6.7
Maize yield
Cowpea yield
S
I
I
527 636 683
331 591 621
29.1 16.6 10.1
S
62.1 LSD
161
NS
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
130 Table 3 Total land equivalent ratio (LER) and partial LER of maize and cowpea grown under three maize plant densities, Experiment 1 Maize density (plants m 2)
2.2 4.4 6.7
Total LER
Partial LER
0.95 1.13 1.07
Maize
Cowpea
0.48 0.86 0.91
0.47 0.27 0.16
Table 4 Ranges of some attributes of 15 cowpea cultivars in sole cropping (S) and in intercropping (I), and corresponding values for cultivar Red Caloona, Experiment 2 Range Grain yield (g m -2) TDM (g m -2) Harvest index Grain size (mg) Flowering time (DAS) Height at 56 DAS (m) Canopy spread at 28 DAS (m) Solar radiation (%) above intercropped cowpea at 28 DAS
S
32-152
I
12-28
S I S I S I S I S I S I
322-528 77-160 0.06--0.34 0.074).21 49-245 67-178 43--66 47--66 0.50-0.88 0.85-1.57 0.26--0.38 0.26--0.42 45-50
I
Red Caloona 33 17 408 117 0.08 0.14 62 68 49 51 0.77
density resulted in only a small increase in yield (Table 2). The effect of intercropping on maize yield was small (i.e. reduction of less than 10%) at the medium and high maize density, but intercropping reduced maize yield by about 40% at the low density. Associated maize reduced the cowpea yield greatly, even at the lowest maize density, and it was reduced further with the increase in maize density. The effect of maize density on cowpea yield was not significant for the cultivar (Red Caloona) shown in the table, but it was significant when two cowpea cultivars were used in analysis. The land equivalent ratio (LER) was calculated using the highest yield in sole crops of maize (which was obtained in the high maize plant density) and cowpea (which was obtained in the low cowpea plant density). The total LER was lowest in the low maize density as partial LER of maize was low (Table 3). With increase in maize density, there was an increase in partial LER of maize but there was a decrease in partial LER of cowpea. Total LER at the medium and high maize density was similar at around 1.1.
3.2. Experiment 2 There was significant variation among 15 cultivars in all characters examined in both sole cropping and intercropping (Table 4). Grain yield and TDM at maturity were greatly reduced by associated maize in all cultivars. Red Caloona, which was used in Experiment 1, was one of the low-yielding cultivars with
1.37
0.27 0.26 49
Table 5 Correlation coefficients between grain yield and some attributes (vegetative dry matter at maturity, canopy spread at 28 days after sowing, and plant height at 56 days after sowing ) in sole cropping (S) and intercropping (I), and correlation coefficients between radiation interception by sole crop cowpea (S) or radiation above intercropped cowpea (I) and these attributes, Experiment 2 Vegetative DM
Grain yield
S
Radiation above cowpea *Significant at P = 0.05.
S I
Height
S
I
S
I
- 0.38*
0.30* 0.33*
0.45* 0.57* -
0.30* -0.05
I Radiation interception
Canopy spread
0.18 -
S
I 0.14 0.06
-0.47* -0.10
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
rather low TDM also. Harvest index was affected less by intercropping than were grain yield and TDM. Grain size and flowering time were little affected by intercropping. In most cultivars (e.g. Red Caloona), plants were taller under intercropping. There was a tendency for climbing types, which were taller in intercropping, to be shorter in sole cropping with resultant negative correlation ( r = - 0.43, P < 0.05) between the heights of cultivars at 56 DAS in the two cropping systems. Canopy spread in intercropping had the largest range at 28 DAS, but the effect of intercropping was small. Solar radiation reaching the intercropped cowpea canopy varied little among cowpea cultivars (Table 4). While intercropped cowpea yield varied 2.5 fold, yields of maize intercropped with different cowpea cultivars ranged only from 512 to 640 g m -2, and there was no relationship between cowpea yield and corresponding maize yield in intercropping (Fig. 6a). Similarly there was no relationship between partial LER of cowpea and that of maize (Fig. 6b). Since the range in maize partial LER was relatively small, the difference in total LER reflected mostly the difference in partial LER of cowpea. Thus four cultivars with partial cowpea LER of 0.5-0.7 produced total LER of 1.2-1.4, whereas five cultivars with partial cowpea LER of less than 0.2 resulted in total LER of less than 1.0. Three high-yield cultivars in intercropping did not have high partial cowpea LER, and hence total LER, as they also produced high yield in sole cropping. Similarly, five low-yielding cultivars in intercropping were not necessarily low in partial LER, as they were low yielders also in sole cropping, except CP172790 which produced the highest yield among all cultivars in sole cropping. There was no significant correlation for cowpea yields of cultivars between sole cropping and intercropping, because there w~re two cultivars which behaved quite differently under different cropping systems; CPI 72790 as mentioned above and CPI 96963, which had the lowest yield among all cultivars in sole cropping but which produced intermediate yield in intercropping. The cultivar CPI 72790 matured late with rather small TDM, and its growth may have been affected greatly by associated maize. On the other hand, CP196963 was early maturing with high TDM, and the effect of maize may have been small. Cowpea yield in intercropping was not correlated strongly with all characters determined, i.e. canopy
131
spread, height and radiation above the cowpea canopy (Table 5). It was, however, positively correlated with vegetative DM and canopy width, indicating that cowpea cultivars which spread fast and produced high biomass tended to produce high yield in intercropping. Grain yield in sole cropping was also positively correlated with canopy spread, but it was negatively cot(a) 700
I
I
1
I
4
650 I
600 "0
mmm
550 500 4-50
4-O0 5
I
i
I
I
10
15
20
25
30
Cowpea yield (g m -~)
(b)
Total LER 1.2
1.0
1.4
1.6
cd ~
0.75
:~
0.70
•
m
0,65
0.60 0.0
i 0.1
'~ 0.2
J 0.3
'l 0.4
I 0.5
~ 0.6
i 0.7
0.8
Cowpea LER Fig. 6. Relationshipsbetween maize yieldand cowpca yield (a) and between partialland equivalentratio (LER) of maize and that of cowpea (b) with 15 cowpea cultivarsin maize/cowpca intcrcropping, Experiment 2. Cowpea cultivarswith high (o), medium ( • ) and low (ll) yield.In (a) bars representLSD at P=0.05. In (b) diagonal linesindicatetotalLER.
132
related with vegetative DM. Therefore cultivars with vigorous growth may produce low grain yield in sole cropping because of the low partitioning of assimilates to grain, while in intercropping vigorous growth may be required to compete against associated maize. Canopy spread was positively related to radiation interception in sole crop (r=0.57, P < 0 . 0 5 ) , but it was not related to radiation available to cowpea in intercropping. Radiation above the cowpea canopy in intercropping was positively associated with vegetative DM production in intercropping.
4. Discussion
4.1. Radiation interception and radiation-use efficiency The results of Experiment I show that total interception of radiation by intercrops was higher than that of sole crops by 20-50% during the period up to the commencement of grain filling in maize. This is because the experiment employed an 'additive' series in which another crop (cowpea) was added to planting patterns used in the main crop (maize) under sole cropping. Similar results were obtained in cassava (Manihot esculenta Crantz)/soybean (Glycine max (L.) Merrill) intercropping by Tsay et al. (1988). The higher radiation interception in intercropping is the reason for the increase in combined TDM over the TDM of solecrop maize at 61 DAS, as RUE of combined intercrop never exceeded that of sole-crop maize. The RUE of the intercrop was between that of the maize and cowpea sole crops, the actual value being most strongly dependent on the proportion of radiation intercepted by the component crops (Fig. 5). The departure from a simple 1 : 1 mixing relationship seen in intercrops with medium or high maize densities, indicated that these canopies were operating 10 to 20% more efficiently than would be expected from their performance as sole-crop canopies alone. It should be pointed out however, that the proportion of radiation intercepted by maize was slightly underestimated for the reason given earlier, and thus the 10--20% would be an overestimation. In addition, the extrapolation from discrete measurements of radiation interception to cumulative interception does introduce further uncertainty. It is therefore likely that RUE of an inter-
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
crop follows closely to that expected from RUEs of sole crops and the proportion of radiation intercepted by the component crops. The literature is equivocal on the impact of intercropping on RUE. Marshall and Willey (1983) showed that before grain filling, RUE of a combined millet/ groundnut (Arachis hypogaea L.) intercrop (4.3 g MJ- ~ of PAR) was slightly higher than that of solecrop millet (4.1 g MJ- 1) although this difference was non-significant. As shown in the current work, the proportion of C4 species in a mixture has a dominant impact on the RUE of the mixture. Natarajan and Willey (1980) showed this effect when high plant densities of sorghum (Sorghum bicolor Moench) intercropped with pigeonpea (Cajanus cajan L.) produced higher RUE of combined intercrop than low densities, presumably because at the high densities more radiation was intercepted by the C4 sorghum.
4.2. Dry matter production, yields and LER Examination of the yield data show that maize dominated cowpea in both experiments at the medium and high densities (Experiments 1 and 2, Tables 2, 3 and 4). Maize yields were generally maintained in the intercrops at near sole-crop levels, except when maize was present as a low-density component. In contrast, the cowpea component was clearly dominated by maize and yields were reduced accordingly. The reason for the depression in growth of low-density maize when intercropped is unclear. Others have shown no effect of low cowpea densities on maize grown at low densities (Wahua et al., 1981; Fawusi et al., 1982), except in dry environments where competition for water means that even low legume densities can restrict maize growth (Allen and Obura, 1983). In Experiment 2, partial LER of cowpea varied greatly when different cultivars were used and this caused a large variation in total LER. This agrees with the general conclusion of Ofori and Stern (1987a) that total LER is determined by LER of the legume, the dominated component, in cereal/legume intercropping. Similarly in Experiment 1, the increase in maize plant density from medium (4.4 plants m -2) to high (6.7 plants m-2) reduced partial LER of cowpea and total LER as the increase in partial LER of maize was only marginal. However, the result of increase in plant density from low (2.2 plants m -2) to medium (4.4 plants m -z) was different; the increase in maize LER
133
J.M. Watiki et al. / Field Crops Research 35 (1993) 123-133
was large and more than compensated for the decrease in cowpea LER with resultant increase in total LER. There were two reasons for the response of maize LER to density at the low-to-medium density range. One is reduced yield potential at the low density, as can be seen in the decline in maize yield with the decrease in density in sole cropping. The other is that at the low maize density there was a larger adverse effect of associated cowpea, and hence grain yield of the maize in intercropping was reduced compared to that in sole cropping. This result suggests that, while maize is generally a dominant crop in maize/cowpea intercropping, the competitiveness may be affected by agronomic manipulations, and when the maize growth is affected by associated cowpea, the change in total LER does not follow the change in LER of cowpea. The advantage of intercropping as judged by LER was rather small (i.e. LER of around 1.1 ) in the experiments. This small advantage was at least partly related to the very low biomass production of cowpea after flowering in intercropping, despite 20-50% radiation transmitted through the maize canopy. Similar problems in intercropped legume during pod filling was observed in soybean in cassava/soybean intercropping (Tsay et al., 1988) and groundnut in millet/groundnut intercropping (Marshall and Willey, 1983). It would be worth examining whether this problem is related to poor adaptation of the legumes to low-radiation environments or to other reasons such as increased disease occurrence in the understorey crops.
4.3. Implications for cultivar selection In situations where maize dominated cowpea (i.e. medium to high maize densities), there were no differences among cowpea cultivars in their effects on maize yields (Experiment 2, Fig. 6a) and the productivity of the intercrop was maximized by using high-yielding cowpea cultivars. Selection of cowpea cultivars for such high-density maize intercropping systems could proceed based on performance within the intercrop of the dominated cowpea component alone. At the low maize density, cowpea had a major detrimental effect on maize yield (Experiment 1, Tables 2 and 3). Under such circumstances, no single crop was clearly dominant. While not examined in this experiment, it is likely that selection of cowpea cultivars for intercropping in such an arrangement with maize would
need to examine yields of both cowpea and maize. This suggests that selection of cultivars for intercropping depends on whether the component crop is dominated, or competitive or dominant. For the dominant crop, its adverse effect on the performance of the associated crop (Cenpukdee and Fukai, 1992) needs to be considered, when selecting cultivars.
Acknowledgements We wish to thank Dr B.C. Imrie for providing cowpea seed.
References Allen, J.R. and Obura, R.K., 1983. Yield of corh, cowpea, and soybean under different intercropping systems. Agron. J., 75: 10051009. Cenpukdee, U. and Fukai, S., 1992. Cassava/legume intercropping with contrasting cassava cultivars. 2. Selection criteria for cassava genotypes in intercropping with two contrasting legume crops. Field Crops Res., 29: 135-149. Fawusi, M.O.A., Wanki, S.B.C. and Nangju, D., 1982. Plant density effects on growth, yield, leaf area index and light transmission on intercropped maize and Vigna unguiculata (L.) Walp. in Nigeria. J. Agric. Sci., 99: 19-23. Marshall, B. and Willey, R.W., 1983. Radiation interception and growth in an intercrop of pearl millet/groundnut. Field Crops Res., 7: 141-160. Natarajan, M. and Willey, R.W., 1980. Sorghum-pigeonpea intercropping and the effects of plant population density. 2. Resource use. J. Agric. Sci., Camb., 95: 59-65. Ntare, B.R., 1989. Evaluation of cowpea cultivars for intercropping with pearl millet in the Sahelian zone of West Africa. Field Crops Res., 20: 31-40. Ofori, F. and Stem, W.R., 1987a. Relative sowing time and density of component crops in a maize/cowpea intercrop system. Exp. Agric., 23: 41-52. Ofori, F. and Stem, W.R., 1987b. Cereal-legume intercropping systems. Adv. Agron., 41: 41-90. Reddy, K.C., Van der Ploeg, J. and Maga, I., 1990. Genotype effects in millet/cowpea intercropping in the semi-arid tropics of Niger. Exp. Agric., 26: 387-396. Tsay, J.S., Fukai, S. and Wilson, G.L., 1988. Intercropping cassava with soybean cultivars of varying maturities. Field Crops Res., 19:211-225. Wahua, T.A.T., Babalola, O. and Akenova, M.E., 1981. lntercropping morphologically different types of maize with cowpeas: LER and growth attributes of associated cowpeas. Exp. Agric., 17: 407-413.