soybean intercropping

Field Crops Research, 19 (1988) 227-239

227

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Effects of R e l a t i v e S o w i n g T i m e of Soybean on Growth and Yield of Cassava in C a s s a v a / S o y b e a n Intercropping J.S. TSAY ~, S. FUKAI2 and G.L. WILSON

Department of Agriculture, University of Queensland, St. Lucia, Qld. 4067 (Australia) {Accepted 15 August 1988)

ABSTRACT Tsay, J.S., Fukai, S. and Wilson, G.L., 1988. Effects of relative sowing time of soybean on growth and yield of cassava in cassava/soybean intercropping. Field Crops Res., 19: 227-239. The development of yield in cassava, either as a sole crop or intercropped with quick-maturing soybean sown 1, 5 or 9 weeks after cassava planting, or a succession of two soybean sowings 1 and 14 weeks after planting, was followed at a high latitude (27 °S ) where the cassava growing season is limited to 9 months by winter temperatures. Competition, at least largely for nitrogen, restricted the growth of cassava, but after soybean harvest, leaf-area index increased such that there was little difference in interception of radiation among crops, and consequently growth rates and thus amounts of assimilate potentially available for tuber growth were similar. Competition from earlier-sown soybean greatly reduced branching by cassava. The reduced number of branches were sufficient to provide adequate leaf-area index, but were a reduced sink for assimilates during the main period of tuber growth. The slightly reduced assimilate supply available in early intercropped cassava was offset by the increased partitioning to tubers. As a result, soybean intercropping did not reduce tuber yield, except slightly in the case of double-intercropped cassava, but provided an additional yield of grain. Land equivalent ratio was particularly high at about 1.6 when soybean was sown within 5 weeks of cassava planting. When soybean was sown 9 weeks after cassava planting, land equivalent ratio was reduced to about 1.3 as a result of lower soybean yield.

INTRODUCTION

Cassava (Manihot esculenta Crantz. ) is a major root crop in the wet tropics and subtropics. There is a widespread practice of cassava intercropping in these regions using many different species (Weber et al., 1979; Leihner, 1983 ). Grain legumes are among the crops most often used, this combination providing a better-balanced human diet in respect of protein and carbohydrate. Legumes ~Present address: The Asian Vegetable Research and Development Center, P.O. Box 42, Shanhua, Tainan, 74103, Taiwan. 2Author to whom correspondence should be addressed.

0378-4290/88/$03.50

© 1988 Elsevier Science Publishers B.V.

228 may also help to minimize the decline of soil fertility, which is a serious problem under continuous cropping with sole cassava (Sinthuprama, 1979). Our previous study (Tsay et al., 1987) showed that quick-maturing, short-statured soybean appears to be suitable as an intercrop in the subtropical regions where the growing season is limited to 9 months by low temperature in winter. When intercropped with soybean at 0.9 or 2.7-m row spacings of cassava, there was a large reduction in total biomass production, but tuber yield was not affected, because the harvest index increased. In intercropping, the relative times of planting of the component crops have both biological and practical implications because they change the relative competitive ability and hence the yield of component crops and the combined yield (Leihner, 1979, 1983; Willey, 1979a,b ). It is known that a small difference in relative planting times can cause a large difference in final economic yield, as for example shown by Ofori and Stern (1987) for a maize/cowpea intercrop. A study was made of the development of yield in cassava/soybean intercrops to examine physiological reasons for high harvest index of cassava in intercropping and to identify the optimum time of sowing of the soybean. This paper describes the performance of the cassava component, while Tsay et al. ( 1985 ) described that of the soybean component and of sole-soybean crops. MATERIALSAND METHODS General The experiment was carried out over the period October 1982-July 1983 at the Redland Bay Farm of the University of Queensland, Australia (at alt. 5 m, lat. 27 ° 37' S, long. 153 ° 19'E). The soil is a deep, fertile, well-drained red loam (krasnozem) with a mineral clay content of 60-80% throughout most of the profile. A detailed description of the climate of the site is given by Keating et al. (1982). There is a seasonal fluctuation in daily solar radiation (20-25 MJ m -2 day -1 in summer to 10 MJ m -2 day -1 in winter), in photoperiod (14.511.5 h ), and in monthly mean daily air temperature (25-15 ° C ). A basal fertilizer of 76 kg N, 105 kg P and 57 kg K h a - 1 was applied to the whole experimental area 1 day before cassava planting. Stem cuttings of cassava, each 10-15-cm long with one or two nodes, were planted horizontally on 5 October. The row direction was east-west. The area was irrigated immediately after planting and whenever necessary to minimize development of water stress in plants. Sprouting was almost 100%. Only one shoot per stem cutting was allowed to grow, excess shoots being removed by hand as they appeared. Soybean seedlings were thinned to the required spacing 2 weeks after sowing.

229

Treatments There were nine treatments, randomized in each of four blocks. Treatments were: one sole cassava (cv. MAus 7) planting; four intercrops with soybean (cv. Fiskeby V, 85 days to maturity) sown 1, 5, 9 and 14 weeks after cassava planting; and four corresponding crops of sole soybean. Plot size was 10.8 m × 7.5 m for intercropping and sole cassava, and 3.6 m × 7.5 m for sole soybean. The intercropped soybean sown at week 14 occupied the area where the first intercropped soybean (sown at week 1) had just been harvested. The terms 'soybean (1) - soybean (1+14)' are used for the time-of-sowing treatments. Soybean was inoculated with Bradyrhizobium japonicum strain CB1809. Soybean row width was 30 cm, and in intercropping 2 (adjacent) out of 6 soybean rows were replaced by 1 row of cassava. Cassava row width was 180 cm, and there was therefore a 45-cm gap between a cassava row and an adjacent soybean row. Intra-row spacing was 45 cm (1.23 plants m -2) for cassava, and 10 cm (22.2 plants m -2) for soybean.

Measurements At 8-week intervals from 59 days after cassava planting (DACP) until 279 DACP, 3 consecutive plants from each of 2 adjacent cassava rows were harvested from each plot to record number of branches and to determine leaf area and dry weight of leaves, stems, swollen planting pieces and tubers. The transmission of shortwave radiation (SWR) was measured continuously during growth in one block as described by Tsay et al. (1985, 1987). The cassava material from dry-matter determination at each sampling was used for nitrogen analyses. In addition, 5 youngest fully expanded leaves from each of 5 plants in each plot were sampled on 86 and 143 DACP as index leaves, dried at 70 ° C, and analyzed for N. Dried and milled samples were analysed for total N using a modified micro-Kjeldahl method which included a salicylicacid/thiosulphate digestion to include nitrate and nitrite. Nitrogen concentration was determined by a colorimetric method (Henzell et al., 1968). At soybean maturity, seed yield was estimated from a harvest area per plot of 7.2 m 2 in sole-soybean, and 4.9 m 2 in intercropping. RESULTS

Cassava growth The total dry-matter (TDM) production was reduced by intercropped soybean, particularly by soybean (1) and (1+14) (Fig. l ( a ) ). Soybean (5) and (9) affected cassava growth at early stages, but TDM after 217 DACP was similar to that of the sole cassava. The highest crop growth rate (CGR) of sole-cassava

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was 16.7 g m -2 day -1 between 114 and 174 DACP while the intercropped cassava had the highest rates (14.6-19.8 g m -2 day-1 ) between 174 and 217 DACP. Patterns of stem dry-weight accumulation were similar to those of total dry weight, but the differences among treatments were more pronounced (Fig. 1 (b) ). Stem growth was severely reduced by intercropped soybean ( 1 ), ( 1 + 14) and (5).

231 Tuber dry-matter production was significantly reduced by intercropped soybean during the early growth stages (Fig. 1 (c)). However, between 174 and 217 DACP, the rate of weight increment was much higher in the intercropped t h a n the sole-cassava, and yields were similar at the final harvest, ranging from 12.8 (in sole-cropping) to 10.3 t h a - ' (in intercropping with soybean ( 1 + 14 ) ). Therefore, partial land equivalent ratio (LEa) of cassava in intercropping was over 0.8 in all treatments, and was close to 1.0 when soybean was sown only once (Table 1). Total LER ranged from 1.30 to 1.65, higher values being obtained in intercropping with soybean (1) and (5). The seed yield of sole soybean decreased as sowing was delayed from 1 to 9 weeks after cassava planting. Tuber yields are plotted against total dry weights in Fig. 2, the slopes of lines indicating the proportion of total dry-matter production going to the tubers (partitioning index). This overestimates the index during the later stages because the dry weights of shed leaves were not included in total dry weights. TABLE1 Land equivalent ratio (LER) of cassava, soybeanand total in cassava/soybeanintercroppingand seed yield of sole soybean (t ha- ' ) with different times of soybeansowing (TSS)

TSS

LER

Sole-soybeanyield'

(week) 1 1+ 14 5 9

Cassava

Soybean'

Total

0.93 0.81 0.96 0.99

0.70 0.49 0.69 0.34

1.63 1.30 1.65 1.33

2.8 2.8+ 1.8 2.1 1.7

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intercroppingwith soybean. Symbols as Fig. 1.

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Fig. 3. Changes in number of branches of cassava in sole cropping and intereropping with soybean. Symbols as Fig. I.

Cassava intercropped with soybean ( 1 ) - including soybean ( 1 + 14) - and to a lesser extent soybean (5), had higher partitioning indices than that of sole cassava. The partitioning index during the period of maximum CGR of cassava intercropped with soybean ( 174 to 217 DACP ) tended to decrease with the delay in soybean sowing, the values being 0.65, 0.56 and 0.46 for cassava intercropped with soybean (1), (5) and (9), respectively. With soybean (1+14), the response was dominated by the early sowing, the plants of the second sowing being far less competitive, and the index was highest, 0.70. During this same period it was 0.50 in the sole-cropping. The brances of cassava were developed mainly during the early growth stages (Fig. 3). The total number of stems is branch n u m b e r + 1 (main stem). These were branches formed well below the stem apex. At a late stage of crop development there was a small amount of what is known as 'forking', branching immediately below the apex and associated with terminal flower development. The extent of this was negligible, and such branching was not recorded. Branching was suppressed by intercropping, the effect being more pronounced with earlier sowing. At the final harvest, cassava intercropped with soybean (9) had a similar branch number to that of sole cassava.

Leaf areas, radiation interception and efficiency of conversion Leaf area indices (LAI) w e r e significantly reduced by intercropping during early growth stages, the more so the earlier the soybean was sown (Fig. 4). After removal of soybean, L A I values converged so that differences were small. Maximum LAI was reached on 217 DACP in all treatments, and varied from 6.7

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Fig. 4. Changes in leaf area index of cassava in sole-croppingand intercropping with soybean. Symbols and horizontal lines as Fig. 1. in cassava intercropped with soybean (9) to 5.4 with the double crop of soybean ( 1 + 1 4 ) . At the last harvest (in July), LAX had typically fallen for this location as the result of leaf-shedding caused by low temperatures. The particular interest in this experiment is the enhancement of tuber growth in intercropped cassava, relative to that of sole cassava, during the second half of the crop duration. It is only from 174 DACP that the measurements of radiation interception do not include the interference of soybean in any of the crops, except where soybean (14) was present, but this crop of soybean was so small as to have little effect on measured interception. Total SWR interception for each canopy was estimated from daily transmission through the canopy and corresponding incident SWR. Table 2 shows percentage interception at 174, 217 and 279 DACP. O n the second of these occasions, interception was the same (about 90% ) for all, with LAI values ranging from 5.4 to 6.7. On the first and last occasions, LAI values were substantially lower (3.4 to 4.6 in the former, and 4.0 to 5.4 in the latter) but interception was not much less. The efficiencies of conversion of SWR during the whole of the last 2 periods are also shown in Table 2. These lack precision because they are based on a single replicate, and these errors are combined in the calculation of efficiencies with the sampling errors of dry-weight increments between harvests. Efficiencies were similar for all cassava crops except perhaps that the value for cassava intercropped with soybean (9) was higher. Nitrogen content of cassava The youngest fully expanded leaves of cassava intercropped with soybean ( 1 ) and with soybean ( 1 + 14) had significantly lower N concentrations (4.2 % ) than that of the sole cassava (5.3%) on 86 DACP (Table 3). The values for

234 TABLE 2 Interception of shortwave radiation (%), corresponding values (in square brackets) of leaf area index at the last three sampling occasions, and efficiency of conversion of intercepted radiation to dry matter (g M J - ~) during the period, of sole and intercropped cassava Crop 1

Sole (1) (1+14) (5) (9)

Interception at

Efficiency

DACP 2

174

217

279

91 73 78 81 80

90 86 87 89 88

81 82 83 81 82

[4.6] [4.0] [3.6] [3.4] [4.1]

[6.6] [6.0] [5.4] [6.0] [6.7]

[4.7] [4.3] [4.0] [4.7] [5.4]

1.48 1.43 1.53 1.39 1.75

1Numbers in brackets are weeks after cassava planting to sowing of intercropped soybean. 2Days after cassava planting.

TABLE 3 Mean nitrogen concentrations (%) in youngest fully expanded leaves of cassava, at 86 and 143 DACP, in sole cropping and intercropping with soybean sown at different times Crop 1

Concentration 862

1432

Sole (1) (1+14) (5) (9)

5.31 4.15 4.18 4.87 5.39

4.39 4.12 3.93 4.14 4.23

LSD 5%

0.93

NS

1'2Footnotes as in Table 2.

soybean (5) and (9) suggest that the later the soybean was sown, the higher the leaf N content of cassava. There were no differences on 143 DACP when all values had fallen to about those of (1) and (1 + 14) at the earlier date. Accumulation of total N was affected by intercropped soybean sown up to 5 weeks after cassava planting (Fig. 5). The uptake between 59 and 114 DACP was reduced, particularly in cassava intercropped with soybean (1), while it appeared to be similar among treatments between 114 and 174 DACP. Only a small amount of N was taken up between 174 and 217 DACP in the cassava intercropped with soybean (1) or soybean (1 + 14 ). After 217 DACP, almost no N was taken up by cassava in any t r e a t m e n t except with soybean (9).

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DISCUSSION Results have shown that ( 1 ) the growth of cassava was significantly reduced by the presence of soybean sown up to 5 weeks after cassava planting, (2) drymatter production by cassava after soybean harvest was similar to that of the sole crop, and (3) during this recovery period, a higher proportion of the current assimilates was allocated to tubers of cassava which had been intercropped so that its final tuber yield was not significantly different from that of sole cassava. These observations are now discussed.

Immediate effect of soybean on cassava growth The leaf-area development and dry-matter production of cassava during the early growth stages (before 114 DACP ) were significantly reduced by associated soybeans, particularly those sown early. Ramanujam (1982) reported that low availability of soil N resulted in a decrease in LAI, and consequently CGR, in cassava. Nitrogen deficiency in cassava is often indicated by low leaf N, particularly in the index leaf (Fox et al., 1975; Howeler, 1978). In the present study, the N uptake (Fig. 5 ) and concentrations in the youngest fully expanded leaves (Table 3) were reduced by intercropped soybean, and the effect was mitigated by the delay in soybean sowing. Leaf N concentrations (about 4.2 % ) in cassava intercropped with soybean ( 1 ) and ( 1 + 14) were below the critical concentrations (5.1-5.7%) reported by Fox et al. (1975) and Howeler (1978). It appears that, although soybeans fixed atmospheric nitrogen, they competed with cassava for mineral N. Similar results were reported by Simpson (1965) in legume/grass combinations. There are no data which show the extent of shading of cassava caused by the intercropped soybean, but a photograph of the soybean (1) intercrop - where the effects would be greatest - at 53 DACP

236 (Tsay et al., 1985 ), shows that shading is unlikely to have been of any importance in retarding the cassava. Cassava intercropped with soybean (5) and (9) were always taller than the corresponding soybean plants (Tsay et al., 1985), but the cassava growth was affected by the intercrop. The branches of cassava developed mainly during the early growth stages, before 114 DACP (Fig. 3 ). They form under conditions of good illumination and soil fertility (Hunt et al., 1977), but whether these have direct effects on branching, or act by influencing growth rates, is not known. As with rates of d r y - m a t t e r (DM) accumulation, it seems unlikely that direct shading effects could have been involved.

Cassava growth after soybean removal The pattern of LAI development (Fig. 4) shows that the early sowing and hence early harvesting of quick-maturing soybean allowed sufficient time for the cassava canopy to recover. The maximum LAI's (6.0) of cassava intercropped with soybean (1) and soybean (5) were not significantly different from that of the sole cassava, and were higher than the estimated optimum LAI of 3.5-4.0 for tuber growth in Colombia (Cock et al., 1979), but similar to that of 4.7 to 6.9 in Tanzania (Enyi, 1973). Thus during the second half of TDM and tuber growth there was adequate LAI to give similar interception of SWR in all cassava crops. The failure of variation in LAI (between 3.4 and 6.7) to have any clear effect on interception probably arises largely from the structure of cassava canopies, in which there is poor light penetration (Zamora et al., 1984). These similar amounts of intercepted SWR were converted to DM with about the same efficiency in all cassava crops (except perhaps soybean (9)), resulting in similar DM increases; and thus the relatively large TDM differences at earlier stages were much reduced by the final harvest. Leaf N concentrations were not measured after 143 DACP but there is a suggestion from Fig. 5 that the cassava in the soybean (9) intercrop had some advantage in N supply at later stages; if so, this could account for its seemingly higher efficiency of photosynthetic conversion. However, whatever effects the particular soybean intercrops may have had on TDM production by cassava, they are of little importance compared with those on DM partitioning to tubers, and hence on yield. Except perhaps for cassava intercropped with late-sown soybean (1 + 14), stem growth was affected much more than tuber growth by the presence of soybean (Fig. 1 ). The distribution of DM to tubers (Fig. 2 ) was improved with earlier soybean sowing, and this was associated with reduction of branching (Fig. 3). Stem DM at the final harvest was significantly correlated {r=0.98) with the total stem number {main stem plus branches). Cock et al. (1979) reported that tops have priority over storage organ growth in assimilate partitioning in cassava. Similar effects of reduction in the size of the shoot sink

237 and subsequent partitioning to tubers were observed by Tan and Cock (1979) who experimentally controlled branch production, and Connor et al. (1981) in studying effects of water stress. The conclusions, at least in this experiment, appear to be that at the cassava population used, the reduced number of stems associated with soybean intercropping were sufficient to carry an adequate leaf canopy for effective radiation interception during the main period of tuber growth, and that an increased number of stems represented a commitment to unnecessary allocation of DM to stem growth at the expense of tuber growth. The result was that the early advantage in cassava growth in the absence of soybeans was not maintained to any significant extent by maturity.

Optimum time of soybean sowing The LER's (Table 1 ) show that the best advantage of intercropping was obtained when soybean was sown early (1-5 weeks after cassava planting). Although cassava intercropped with soybean (9) was not affected by the intercrop, the soybean competed very poorly with the well-established cassava (Tsay et al., 1985 ) and the advantage of intercropping was relatively small in this combination. The optimum time of sowing is, however, likely to depend on environmental conditions and cultural practices. The duration of the period available for acceptable growth of an intercrop obviously depends on the rate of growth of the cassava and hence the earliness with which cassava becomes a severe competitor. The relatively long period within which sowing of soybean gave optimum productivity arose largely from the slow growth of cassava planted in spring at this latitude. Summer plantings, with lower final tuber yield than in spring plantings, result in more rapid early growth (Fukai et al., 1984), which would reduce the opportunity for soybean growth. Thung and Cock (1979) found the yield of intercropped soybean sown 1 month after cassava planting to be severely reduced. Their work was carried out at latitude 3 ° N with a nearly constant mean temperature of 24 oC. It is also likely that the use of the wide row spacing ( 180 cm) in this experiment provided a much better opportunity than would exist with the more conventional 90-cm rows. However, our studies (Tsay et al., 1987) showed that the wide spacing does not result in loss of tuber yield. Under those conditions in which competition between the two crops starts earlier, the optimum period during which soybean is sown may be shorter than that shown in this experiment. The results of this experiment were obtained with a short-statured, quickmaturing soybean cultivar. Later-maturing taller soybean cultivars with higher yield potentials under sole-cropping may compete more strongly against intercropped cassava. Cassava dry-matter production during soybean growth and

238 after soybean harvest, and assimilate partitioning, may depend on soybean genotypes. This aspect will be reported later.

REFERENCES Cock, J.H., Franklin, D., Sandoval, G. and Juri, P., 1979. The ideal cassava plant for maximum yield. Crop Sci., 19: 271-279. Connor, D.J., Cock, J.H. and Parra, G.E., 1981. Response of cassava to water shortage. I. Growth and yield. Field Crops Res., 4: 181-200. Enyi, B.A.C., 1973. Growth rates of three cassava varieties (Manihot esculenta Crantz) under varying population densities. J. Agric. Sci., 81: 15-28. Fox, R.H., Talleyrand, R.H. and Scott, T.W., 1975. Effect of nitrogen fertilization on yields and nitrogen content of cassava, Llanera cultivar. J. Agric. Univ. P. R., 59:112-124. Fukai, S., Alcoy, A.B., Llamelo, A.B. and Patterson, R.D., 1984. Effects of solar radiation on growth of cassava (Manihot esculenta Crantz). I. Canopy development and dry matter growth. Field Crops Res., 9: 347-360. Henzell, E.F., Vallis, I. and Lindquist, J.E., 1968. Automatic colorimetric methods for the determination of nitrogen in digests and extracts of soils. Trans. 9th Int. Congr. Soil Sci., 3: 513519. Howeler, R.H., 1978. The mineral nutrition and fertilization of cassava. In: Cassava Production Course. CIAT, Cali, Colombia, pp. 247-292. Hunt, L.A., Wholey, D.W. and Cock, J.H., 1977. Growth physiology of cassava. Field Crop Abstr., 30: 77-91. Keating, B.A., Evenson, J.P. and Fukai, S., 1982. Environmental effects on growth and development of cassava (Manihot esculenta Crantz). I. Crop development. Field Crops Res., 5: 271281. Leihner, D.E., 1979. Agronomic implications of cassava-legume intercropping systems. In: E. Weber, B. Nestel and M. Campbell (Editors), Intercropping with Cassava. Proc. Int. Workshop, Trivandrum, India, 27 November-1 December, 1978. IDRC, Ottawa, pp. 103-112. Leihner, D.E., 1983. Management and evaluation of intercropping systems with cassava. CIAT, Cali, Colombia, 70 pp. Ofori, F. and Stern, W.R., 1987. The combined effects of nitrogen fertilizer and density of the legume component on production efficiency in a maize/cowpea intercrop system. Field Crops Res., 16: 43-52. Ramanujam, T., 1982. Influence of nitrogen on leaf area index, crop growth rate, net assimilation rate and yield of cassava. J. Root Crops, 8: 27-33. Simpson, J.R., 1965. The transference of nitrogen from pasture legumes to an associated grass under several systems of management in pot culture. Aust. J. Agric. Res., 16: 915-926. Sinthuprama, S., 1979. Cassava and cassava-based intercrop system in Thailand. In: E. Weber, B. Nestel and M. Campbell (Editors), Intercropping with Cassava. Proc. Int. Workshop, Trivandrum, India, 27 November-1 December 1978. IDRC, Ottawa, pp. 57-65. Tan, S.L. and Cock, J.H., 1979. Branching habit as a yield determinant in cassava. Field Crops Res., 2: 281-289. Thung, M. and Cock, J.H., 1979. Multiple cropping cassava and field beans: status of present work at the International Centre of Tropical Agriculture (CIAT). In: E. Weber, B. Nestel and M. Campbell (Editors), Intercropping with Cassava. Proc. Int. Workshop, Trivandrum, India, 27 November-1 December 1978. IDRC, Ottawa, pp. 103-113. Tsay, J.S. Fukai, S. and Wilson, G.L., 1985. Soybean response to intercropping with cassava. In:

239 S. Shanmugasundaram and E.W. Sulzberger (Editors), Soybean in Tropical and Subtropical Cropping Systems. AVRDC, Tainan, Taiwan, pp. 13-24. Tsay, J.S., Fukai, S. and Wilson, G.L., 1987. The response of cassava (Manihot esculenta) to spatial arrangements and to soybean intercrop. Field Crops Res., 16: 19-31. Weber, E., Nestel, B. and Campbell, M. (Editors), 1979. Intercropping with Cassava. Proc. Int. Workshop, Trivandrum, India, 27 November-1 December 1978. IDRC, Ottawa, 142 pp. Willey, R.W., 1979a. Intercropping - its importance and research needs. Part 1. Competition and yield advantages. Field Crop Abstr., 32: 1-10. Willey, R.W., 1979b. Intercropping - its importance and research needs. Part 2. Agronomy and research approaches. Field Crop Abstr., 32: 73-85. Zamora, O.B., Wilson, G.L. and Fukai, S., 1984. Profiles of photosynthesis, solar irradiance and leaf area index at three stages of growth of cassava (Manihot esculenta Crantz) in the field. Philipp. J. Crop Sci., 9: 53-59.