African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 9, Num. 1, 2001, pp. 83-96

African Crop Science Journal, Vol. 9, No. 1, March 2001, pp. 83-96

Code Number: CS01038

ABSTRACT

Three potato (Solunum tuberosum) varieties and one maize variety were intercropped in six spatial arrangements, viz., sole crops, 2:1, 2:2, 1:1, 1:2 potato: maize row arrangements and one additive mixture. Intercropping influenced some growth parameters of potato but not of maize. For instance, the rate of potato stem extension and leaf formation rates were hastened by intercropping. Branching in potato influenced leaf area development, especially during the second season when the additive mixture supported the least leaf area. Potato yield differed significantly among the spatial arrangements with the highest yield in the sole crop followed closely by the 2:1 and 2:2 potato: maize mixtures. However, these yield differences depended on potato variety, with Kisoro being the most responsive to changes in spatial arrangement. Assessment of biological efficiency of intercropping using the Land Equivalent Ratio (LER) method, revealed that yield advantages increased slightly with increase in the proportion of potato in the mixtures. However, it was only the additive mixture with a significant overall LER of 1.58, indicating a 58.2% yield advantage for intercropping. The contribution of maize to total LER was greatest in the 1:2 and additive mixtures.

Key Words: Biological efficiency, inter-cropping, land equivalent ratio, Solanum tuberosum, spatial arrangement, yield advantage

RÉSUMÉ

Trois variétés de pomme de terre (Solanum tuberosum) et une variété de maïs ont été associées dans six arrangements spatiaux à savoir: la monoculture, 2:1, 2:2, 1:1, 1:2 arrangement en lignes de pomme de terre et de maïs et un mélange additionel. L'association des cultures a influencé certains paramètres de croissance de la pomme de terre et ne pas ceux du maïs. Ainsi le taux d' élongation de la tige de la pomme de terre et le taux de formation des feuilles ont été favorisés par l'association. Le branchement de la pomme de terre a favorisé le développement de la surface foliaire particulièrement durant la deuxième période de pluies et le mélange additionel a supporté la surface foliaire. Les rendements de la pomme de terre ont differé significativement entre les différents arrangements spatiaux et le rendement le plus elevé a été obtenu dans la monoculture étroitement suivi par l'arrangement 2:1 et 2:2 de maïs et de pomme de terre. Cependant ces différences dépendaient de la variété et Kisoro fut la variété qui a répondu le plus aux changements d' arrangement spatial. L' évaluation de l' éfficacite de l' association par la méthode de Land Equivalent Ratio (LER), a montré que les avantages de rendements ont augmenté partiellement avec l'augmentation de la proportion de pomme de terre dans les associations. Mais, c' était seulement le mélange additionel qui a eu un important LER de 1.58 montrant un avantage de rendement de 58.2% pour l'association culturale. La contribution du maïs sur le LER total était la plus grande dans l'arrangement 1:2 et le mélange additionel.

Mots Clés: Efficacite biologique, association culturale, land equivalent ratio, Solanum tuberosum, arrangement spatial, avantage de rendement

INTRODUCTION

Potato (Solanum tuberosum L.) originated in the high altitude areas of the Andes and so reasonable potato yields are usually obtained in the hilly areas of the tropics (Ahmad, 1977; Horton, 1987). However, there is also considerable potential for potato production at mid-altitude elevations. For example, experiments conducted at Mityana in Uganda (1300 m above sea level) have shown that yields of up to 21.8 t ha^-1 are possible. Therefore, attempts to popularise potato production in the mid and lowland tropics, at elevations of 1000-1500 m above sea level (Bhagsari et al., 1994) are not ill-founded. However, high temperatures (>25 °C) encountered in the lowland humid tropics place a major constraint on potato yields (Tizio, 1978; Menzel, 1983; Manrique, 1993). Thus, while a 21.8 t ha^-1 tuber yield of potato is possible at Mityana, which has a relatively cooler weather, only 8.9 t ha^-1 is realised at Kabanyolo where the weather is relatively hotter.

Intercropping has been used in some countries to modify unsuitable climatic conditions including high temperatures and low soil moisture (Harverkort, 1989). This is achieved when farmers sandwich potato plants between slower growing cereals like maize.

In mixed cropping, the major factors that are important in plant interaction include growth habits (particularly stature of intercrops, rooting systems and maturity periods), growth rates and competitive ability of species (Willey, 1979a; Beets, 1982). Thus, a more vigorous and longer season maize variety reduced potato yields compared to the less vigorous and short season maize variety (CIP, 1985). Similarly, maize yields were significantly reduced by a delay in potato maturity associated with greater potato plant height and increased competition with maize for light (CIP, 1990). High yield plasticities and shade tolerance are also important characteristics for short statured plants in crop mixtures (Beets, 1982).

De Wit (1960) stated that an advantage of mixing two species is likely to occur when the individual species use slightly different environments. For instance, yield advantage in bean+maize mixture reported by Osiru and Willey (1974) was attributed to the crop species having different growth cycles so peak demand for environmental resources occur at quite different times. Also, yield advantage reported from sorghum+pigeon pea mixture was attributed to differential nutrient demand periods (Rao and Willey, 1979). In addition, Willey (1979a) linked improved water use in intecrops with better rooting patterns. Nevertheless, Osiru and Willey (1974) reported that two species should sufficiently compete to make them fully use their respective parts of the environment and to maximise the advantage of intercropping.

Light intensity influences plant growth and development, often affecting biomass accumulation and distribution. Reduced light intensity in potato results in enhanced stem elongation and biomass partitioning in the vines; it also results in reduced total biomass accumulation in the tubers and therefore low yields compared with potato grown at high irradiance (Gawronska and Dwelle, 1989). Shaded plants produce small and irregular tubers (Sale, 1976; Menzel, 1985).

However, shading by companion or relay crops and mulching lowers soil temperature and conserves water (CIP, 1983). Midmore et al. (1983) found a linear reduction of soil temperatures (at 7cm soil depth) with an increase in shade and this significantly hastened potato emergence in most cases. Likewise, Burton (1989) observed that heat stress in potato in the tropics can be reduced by growing potato in the shade of other crops. This reduces the light available for photosynthesis, but if properly managed, heat reduction offsets the effect of low light intensity (Midmore et al., 1983). This may result in improved productivity in the potato crop.

A study was therefore initiated to address some of the above problems in potato production in mid-altitude areas of central Uganda. The specific objectives of the study were to determine the effect of intercroping potato with maize on the growth and yield of these crops, and to select potato agronomic attributes that are suitable for intercropping with maize.

MATERIALS AND METHODS

The experiment was conducted during the long and short rains of 1995 at Namulonge Agricultural and Animal Research Institute (NAARI), Uganda (O° 32' N, 32° 35'E; 1150 metres above sea level). Three potato genotypes of contrasting agronomic attributes and one maize variety (Longe 1) were used in the study. Longe 1 is a relatively short <1.5 m open pollinated maize variety with 120 days maturity period. The potato variety, Sangema (CIP code # 374080.5) is early maturing (75 days) and erect, Victoria (CIP code # 381381.20) is also early maturity (75 days) and erect, while Kisoro (CIP code # 381379.9 is medium-to-late maturity (80-90 days) and semi-erect.

The three potato varieties were interplanted with maize in different proportions to form seven spatial arrangements including two pure stands of the component crops (Table 1 ). Four mixtures: 2:1, 2:2, 1:1 and 1:2 were formed using the De Wits "Replacement Series" technique as described by Wiley and Osiru (1972). In this method, one maize plant is equivalent to one potato plant. There was also one additive mixture, which was formed by adding the optimum sole crop population of one species onto that of the other. In this case it means adding a row of maize between potato rows sown at its optimum sole crop population, which amounts to doubling the potato:maize rows total plant population.

The treatments were arranged in a split plot design with varieties randomised in the main plots and spatial arrangements in the subplots. Each main plot measured 26 m long and 4.9 m wide and subplot 4.5 m x 4.9 m.

The component crops were planted simultaneously, potato tubers being planted on ridges and maize on flats except in the additive mixture where maize was planted in the furrows between potato rows. Spacing was 70 cm between rows and 30 cm within rows for both species giving the same plant population for both intercrop and control pure stands, except in the additive mixture.

Freshly sprouted tubers for planting were obtained from Kalengyere Highland Crops Research Centre. Maize was thinned two weeks after emergence to one plant per hill. Weeding and earthing-up were carried out whenever necessary. Apart from one spray of fungicide (Dithane M45) which was used to control late blight (Phytophthora infestans), no other chemicals were applied.

Data collection and analyses. The leaf area of potato was measured at 60 days after planting (DAP) on four plants randomly selected from the inner rows of each plot. The leaf area was determined following the dry weight-area ratio method (Edje and Osiru, 1987). In the case of maize, three leaves comprising of ear leaf, the leaf above and one below, per plant were used in the estimation of leaf area. Five plants per plot were used. The leaf area was determined following the procedures of Edje (1980).

Plant height for both component crops was measured at 30 and 60 (at anthesis) DAP. Height was taken from ground level to the base of the fully opened maize leaf and from ridge top to the highest point in the potato canopy.

To compare growth in the different potato varieties, the rate of canopy development was measured by determining rate of leaf formation and stem extension. These parameters were obtained by tagging the top most fully expanded leaf, two weeks after potato emergence. Two weeks later, the distance between the tagged leaf and the topmost fully expanded leaf was measured and the leaves formed above the tagged one were counted as described by Yao et al. (1988).

Tuber initiation time was determined to identify which variety initiated tubers earlier. Ten plants were uprooted for this exercise at two weeks intervals, to determine the number of tubers formed. Potato dry matter production was also assessed 60 DAP, when foliage production was at its peak. Four plants were sampled from each plot. Each plant had its tubers, leaves and stems separated and oven-dried at 65°C for four days, and dry weights taken on a sensitive digital weighing scale. Based on the above measurements, harvest index, a measure of tuber storage efficiency, was calculated as the ratio of the tuber dry matter to total biological yield. Harvest index could not be determined later because of rapid leaf senescence.

At the first harvest, four rows per plot (8.4 m²) were harvested, for the sole crops after the haulm had dried. The number of rows harvested for the mixture treatments varied with mixture but the total area harvested remained constant. Plants in the harvest area were counted and recorded before harvest. The tubers were sorted into three categories; small (< 50 gm), medium (50-100 gm) and large (>100 gm). Tuber number and fresh weight were determined and recorded accordingly.

Maize cobs were harvested immediately after the stalks dried and weighed. Subsequently, 200 gm samples were oven-dried at 65° C for four days and used to determine grain moisture content and hence the dry grain weight. From 200 gm dried samples, 100-grain weight was determined.

Environmental factors monitored were soil temperatures (at 5 and 10-cm depths), relative humidity (%) and percentage shading. For the measurement of soil temperature, ordinary thermometers were inserted into the centre of potato ridge and left for 10^-15 minutes before temperature was recorded. Dry and wet bulb thermometers were hanged from a support to allow the thermometers come into contact with potato foliage. Respective dry and wet bulb temperatures were recorded. Relative humidity was obtained from the differences between the temperatures using a slide rule designed specifically for the purpose.

Shading of potato by maize plants was determined using the grid method described by Calavan and Weil (1988), but dimensions of cardboard changed to 30 cm x 30 cm to cover the canopy of potato plants. All the data obtained were subjected to Analysis of Variance to separate significant treatments effects. Multiple regression was used to determine the relationship between various microclimatic variables and yield. The Land Equivalent Ratio (LER) index was used to determine the biological efficiency of intercropping (Willey, 1979b).

RESULTS

Effect of variety and spatial arrangement on potato and maize growth. Both potato leaf formation and stem extension rates were significantly different among the varieties (Table 2 ). Victoria was more vigorous in growth than Sangema and Kisoro and Kisoro had the slowest stem extension rate. These parameters were significantly influenced by spatial arrangement. For example, potato stem extension rate was greatest in the additive mixture followed by the 1:1, 2:2, 2:1, 1:2 mixtures and least in the sole crop. The number of main stems, however, did not differ significantly between the spatial arrangements. Branching was lowest in the additive mixture. The sole crop produced most branches but these were not significantly different from those in the 2:1 and 2:2 mixture.

Higher leaf area index (LAI) (Table 3 ) was achieved in the first than in the second season with Victoria having the highest LAI of 2.75 and 2.46, for the first and second seasons, respectively. Kisoro had the least LAI. Maize LAI was significantly higher in mixtures with Kisoro than when combined with Sangema or Victoria in first season, but the differences were not significant (P>0.05) in the second season.

The effect of spatial arrangement on Potato LAI was not consistent especially during the first season (Table 3 ). Overall, there was a tendency for potato LAI to decrease as population of maize increased. Spatial arrangement did not have significant effect on maize LAI in both seasons although maize in the 2:2, 1:2 and 1:1 mixtures tended to have higher LAI.

Plant heights were not significantly different among the varieties (Table 3 ). Similarly, spatial arrangement did not have significant influence on plant height during both seasons.

Time to tuber initiation varied significantly (P=0.05) among the varieties. By 40 DAP 5.4, 4.6 and 1.6 tubers per plant had been formed by Sangema, Kisoro and Victoria, respectively. However, at 60 DAP Kisoro had the highest number of tubers per plant (11.8) followed by Sangema (6.0), and Victoria had the lowest number (4.4). At harvest, average number of tubers obtained per plant were 6.6 (Kisoro), 3.8 (Sangema) and 3.6 (Victoria) (Table 4 ).

Dry matter production and partitioning in potato. The trend in total biomass accumulation during both seasons is shown in Table 4 . Victoria accumulated the highest amount of total dry matter in both seasons, and Kisoro produced the least. However, varietal differences were not significant in the second season.

In the first season, there were no significant differences in dry matter (DM) production patterns between spatial arrangements, but significant differences were observed in the second season (Table 5 ). In general, less potato DM was accumulated as population of maize increased in the mixtures. Potato plants in the 2:1 mixtures gave the highest DM production, amounting to 95.6 gm and 3.4 gm plant^-1 in the first and second season, respectively.

Biomass partitioning in potato showed seasonal variation and closely followed the trend in the total biomass production pattern. Overall, there was more dry matter in the first compared to the second season. Except for the case with leaf dry weight where the sole crop had the highest value, the 2:1 potato: maize mixture was superior in dry matter partitioning to potato stems and tubers (Table 4 ).

The proportion of total dry matter partitioned to the tubers (Harvest index) was significantly different among the varieties in both seasons (Table 4 ). Sangema had the highest harvest index of 0.56 and 0.51 at 60 DAP for the first and second seasons, respectively.

Potato and maize yield and yield components. Total tuber yields (Table 5 ) were not significantly different in either season. The first season average tuber yield was 4935, 4790 and 4414 kg ha^-1 for Victoria, Kisoro and Sangema, respectively, and 3726.8, 3399.5 and 3376.3 kg ha^-1 for the second season. In pure stands, Kisoro yielded highest (8854.2 and 7896.8 kg ha^-1) and Sangema least (7366.1 and 7103.2 kg ha^-1).

Total yields per hectare varied according to the proportion of the different species in the mixture (Table 5 ). In the first season, sole cropped potatoes had the highest yield. Among the mixtures the 2:1 potato:maize had the highest yield and the 1:2 potato:maize mixture had the lowest potato yield. The same trend was observed in the second season. It should be noted that although the potato sole crop and the additive mixture have the same potato population density, the tuber yields were significantly higher in the former than the latter case. Tuber yield reductions were greater as the proportion of maize in mixtures increased and this was more pronounced in the second than in the first season. However, sole maize total grain yield was not significantly better than maize yield in the additive mixture, in both seasons. On the other hand, maize yield in the 2:2 and 1:1 potato:maize row arrangements (both containing 50% maize) were not significantly different from each other.

Weight per tuber varied among the varieties with Kisoro producing the lightest tubers compared to Sangema and Victoria (Table 6 ). Victoria produced the heaviest tubers. Generally, weight per tuber decreased as the proportion of maize in the mixtures increased. The 2:1 potato:maize mixture produced heavier and larger tubers while the additive mixture produced the smallest tubers compared to the other mixtures. In the case of maize the 100-grain weight was significantly different in the various spatial arrangments (P<0.01) but only in the first season. The 2:2 and 2:1 potato:maize mixture had higher weights per grain than the other mixtures and sole maize (Table 6 ).

Biological efficiency of intercropping. The biological efficiencies of the different spatial arrangements were compared using the Land Equivalent Ratios (LER). Total LER did not show any specific trends among the mixtures. It was, however, high at high population of the component crops, i.e., in the additive mixture. In all the other spatial arrangements the average intercropping advantage was less than 10%. On average, partial LERs obtained were higher for maize than potato and LER tended to increase with increase in maize population (Table 7 ).

Potato micro-environment and its relation with potato yields. Spatial arrangement significantly (P<0.001) influenced light transmission to potato, with shading being highest in the additive mixture (Table 8 ). Overall, there was an increase in shading on potato as the population of maize increased, and there was a strong negative correlation (r= -0.88, P=0.001) between level of shading and potato yield (Table 9 ).

Spatial arrangement also signigicantly influenced RH and soil temperature (Table 8 ). The RH was highest in the additive mixture, followed by the 1:1 mixture, sole crop and then the 2:1 mixture.

Temperatures at 5 and 10 cm soil depths were significantly influenced by spatial arrangements. The highest and lowest temperatures were recorded in sole crop and additive mixture, respectively, and these differences were significant (Table 8 ). Temperature was more uniform at 10 cm depths (range = 1.9 °C) than at the 5 cm depth (range = 2.6 °C). In all spatial arrangements soil temperature was higher at the 5 cm than the 10 cm depths.

DISCUSSION

Intercropping potato with maize in different spatial arrangements influenced some aspects of growth and yield of potato. Certain growth parameters such as number of stems and tubers varied only across varieties but uniformly under different spatial arrangements.

According to Gawronski et al. (1989), competition for light in potatoes results in tall plants. In the present study, this was apparent at 60 DAP, during the first season, when potato plants in the additive mixture were taller than in other spatial arrangements including the sole crop. However, in the second season the etiolated stems produced in the 1:2 and the additive mixtures lodged resulting in low plant heights.

Maize plant growth was not affected by its association with potato. The taller maize plants in the sole maize, 2:2 and 1:2 potato: maize mixtures was probably due to intra-specific competition that is sometimes more intense than inter-specific competition (Beets, 1982).

Shading is known to depress branching (Acock and Acock, 1987). Furthermore, the intensity of branching in varieties was inversely related to the number of main stems produced. Therefore, higher branching in Kisoro and Victoria was probably a compensatory growth response to the few main stems they produced. Reduced branching observed in the additive mixture was associated with low potato LAI and few leaves.

The number of main stems produced is dependent on the tuber size or variety (Allen and Wurr, 1992). Therefore, the differences in tuber numbers observed among varieties were probably due to the varietal differences. Furthermore, sprouts that developed into stems were formed well before planting and emerged in the same environment before it was modified by intercropping treatments. So simultaneous planting of potato and maize in a mixture did not have a significant effect on stem production. This also explains the uniformity in the number of tubers among the different spatial arrangements. Similar results were reported by Kuruppuarachchi (1990).

Dry matter production patterns were closely related to light transmission. This relationship was closer in the second than in the first season probably due to the north-south row orientation. With this orientation there is more shading in mixtures especially at low sun angles (Pendeleton et al., 1972). Seasonal differences in dry matter accumulation and partitioning could also be a result of differences in magnitude of stress factors that were apparently more intense in the second season. The high dry matter partitioning to stems in second season was probably a reaction to high level of shading. Potato plants, in an attempt to place their leaves in light position partitioned more assimilates for stem growth at the expense of tuber bulking.

The performance of different varieties in the intercrops depended partly on their rates of growth and development. Probably, the more vigorous Victoria exploited environmental growth resources effectively well before shading by maize plants intensified. Hence, it accumulated more biomass than Kisoro and Sangema. Coupled with high LAI and therefore light interception, Victoria produced larger and more numerous tubers in comparison with both Sangema and Kisoro. Interestingly, Sangema, though not very vigorous in growth, matched Victoria in potato yield. This is probably due to its early tuber initiation which allowed early tuber bulking before shading became intense enabling it to store assimilates produced in the tubers (Milthorpe and Moorby, 1975). Thus, the 60 DAP harvest index (HI) for Sangema was significantly higher than for both Victoria and Sangema. Secondly, Sangema being non- spreading did not lodge and therefore intercepted more solar radiation at around 60 DAP, which is the critical time for tuber bulking. The above findings are in consonance with the observation by Liu and Midmore (1990) that potato varieties with wide-spreading canopies are least suited for intercropping with maize.

Increasing the proportion of maize in the mixtures, and reduced spacing in the additive mixture inevitably increased shading on potato. The weight per tuber differences among spatial arrangements are attributed to the differences in light interception during later stages of growth. Improved performance (yield per plant) in some spatial arrangements especially those with low maize populations are attributed to species interface effects. Interface or edge row effects have been shown to confer yield superiority in narrow strip cropping systems (Lai and Wen, 1990). This effect enables intercropped species to exploit a wider growth environment with minimum intraspecific competition. Hence, per plant yields in sole crops were lower than in 2:2 and 2:1 mixtures in the first season. Similarly, in maize the consistent low 100-grain weight in sole maize and additive mixture could have been due to mutual shading during the grain filling period. However, the uniformity in the maize yield component across seasons suggests that the variation observed in yield per plant resulted from differences in grain numbers. Grain number has been reported to be more sensitive to shading than weight per grain (Reed et al., 1988). Thus, this could explain the low grain yield per plant in sole maize and additive mixture during the second season.

There was no significant differences in tuber number per plant among the spatial arrangements. This is because tuber initiation is influenced by temperature and the amount of assimilates available. Since shading which affects temperature and photo assimilation production became pronounced after the initiation of the tubers, the numbers of tubers in the different spatial arrangements patterns were not affected. A similar observation was reported by Vander Zaag and Demagante (1990). Tuber number is also determined by the number of stems produced which in turn depend on the tuber size and variety. However, the reduction in the number of tubers formed in the additive mixture during the second season could have been due to failure of the tubers initiated to develop or having been re-absorbed (Burton, 1989).

Earlier work (Gawronska et al., 1989) suggested that the rates of photosynthesis under a range of irradiance levels varied with varieties. Also, yields of vigorous and early maturing varieties are less affected by intercropping (Midmore, 1990). Therefore, Victoria which had a faster canopy development (i.e., leaf formation and stem extension) was least affected by intercropping in both seasons. Total tuber yields were related to the amount of dry matter produced. This in turn was closely influenced by the light intercepted by potato canopy. As a result, there was a notable decrease in total tuber yields in mixtures compared with sole crop. This was attributed to shading of potato by maize. Tuber yields among varieties were not significantly different, in spite of significant differences in the yield components. Kisoro, which produced mostly small tubers compensated by producing large number of tubers. On the other hand Victoria with few tubers had bigger tubers. Though the interaction between variety and cropping system was not significant, Kisoro was most affected by shade in the mixtures with more maize because of its spreading canopy. This further reduced its weight per tuber.

Yield advantages were consistently high in the additive mixture in both seasons. This is attributed to the high population density of both crops in this mixture (Infenkwe et al., 1989). Seasonal variation in intercrop benefit was perhaps due to climatic and edaphic factors which were not constant. Similar reports of yield advantage varying seasonally in spatial arrangements have been reported (Lai and Wen, 1990).

Significant differences were observed among varieties in growth rates and dry matter accumulation. Growth rate variation among potato varieties is important in selecting for intercropping with taller crop species like maize. Faster growth is an indication of the ability of the low-storey species to compete for positional and unidirectional growth resources especially light. This attribute is pronounced in variety Victoria. In addition, maintaining upright posture by potato plants helps to reduce species height difference and therefore enhances competitiveness in potato. This was characteristic of Sangema variety used in this study.

The yields per plant of potato in the 2:1 and 2:2 potato: maize mixtures were higher than for the sole crop. These mixtures seem to be promising spatial arrangements and need further evaluation. However, higher efficiencies in the 2:2 mixture, compared with 1:1 mixture of similar species population, show that wider spacing of the shade crop is needed to sustain productivity of the intercrops. Thus, strip cropping may provide a more practical approach to multiple cropping. The use of alternate rows or very narrow strips may require some other slow growing crop to replace maize.

ACKNOWLEDGEMENT

The study was financed by USAID through a collaborative research grant to Fort Valley State College, Georgia, USA and Makerere University, Kampala, Uganda.

REFERENCES

Acock, B. and Acock, M.C. 1987. Periodic shading and the location and timing of branches in soybean. Agronomy Journal 79:949-952

Ahmad, K.U. 1977. Potatoes for the tropics. Mumtaj Kamal Publisher, Bangladesh

Allen, E.J. and Wurr, D.C.E. 1992. Plant density. In: The Potato Crop. Harris, P. (Ed.), pp. 292-330. 2nd ed. Chapman and Hall, London.

Beets, W.C. 1982. Multiple Cropping and Tropical Farming System. Westiview Press, Manila.

Burton, W.G. 1989. Challenging heat stress in potato production. In: The Potato. Burton, W.G. (Ed.). Longman, Scientific, and Technical, Longman House, London.

Calavan, K.M. and Weil, R.R. 1988. Peanut-corn intercrop performance as affected by within row corn spacing at a constant row spacing. Agronomy Journal 80:635-642.

Dewit, C.T. 1960. On competition. Versel. Landbouwk. Onderz. 66, 8.

Edje, O.T. 1980. Comparative development and yield and other agronomic characteristics of maize and groundnuts in monoculture and in association. In: Intercropping, Proceedings of the second symposium on intercropping in semi-arid areas, held at Morogoro, Tanzania 4-7 August 1980, pp. 17-26. FAO.

Edje, O.T. and Osiru, D.S.O. 1987. Methods for determining leaf area in some crop plants. In: First annual meeting of the collaborative group in cassava-based cropping systems research. 16^-19 November 1987. International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria. pp. 237-245.

Gawronska, H. and Dwelle, R.B. 1989. Partitioning of photoassimilates by plants (Solanum tuberosum) as influenced by irradiance. II. Partitioning patterns by four clones grown under high and low irradiance. American Potato Journal 67:163^-176.

Harton, D. 1987. Potatoes: Production, marketing, and programmes for developing countries. Westview Press, Boulder, USA. 243 pp.

Harverkort, A.J. 1989. Potato cropping practices in North and West Africa with special reference to intercropping. In: Proceedings of the First Triennial General Meeting and Conference, June 21-24 1987. Nairobi, Kenya.

Ifenkwe, O.P., Odurekwe, S.O., Okonkwo, J.C. and Nwokocha, H.B. 1989. Effect of maize and potato populations on tuber and grain yields, net income and land equivalent ratios in potato/maize intercropping. Tropical Agriculture 25:73-82.

International Potato Center (CIP), 1983. Annual Report. CIP, Lima, Peru. 148 pp.

International Potato Center (CIP), 1985. Annual Report. CIP, Lima, Peru. 175 pp.

International Potato Center (CIP), 1990. Annual Report. CIP, Lima, Peru. 166 pp.

Kuruppuarachchi, D.S.P. 1990. Intercropped potato (Solanum spp.): Effect of shade on growth and tuber yield in the north west regional belt of Sri Lanka. Field Crops Research 25:61-72.

Lai, Z. and Wen, G. 1990. Row ratios and plant density in potato/maize strip cropping. Field Crops Research 25:51-59.

Liu, J. and Midmore, D.J. 1990. A review of potato intercropping practices in western Hubei, China. Field Crops Research 25:41-50.

Manrique, L.A. 1993. Constrains of potato production in the tropics. Journal of Plant Nutrient 15:2679-2726.

Menzel, C.M. 1983. Tuberisation in potato at high temperature: interactions between shoot and root temperatures. Annals of Botany 52:65-69.

Menzel, C.M. 1985. Tuberisation in potato at high temperature: Interaction between temperature and irradiance. Annals of Botany 55: 35-39.

Midmore, D.J. 1990. Scientific basis and scope for further improvement of intercropping with potato in the tropics. Field Crops Research 25:3-24.

Midmore, D.J., Accatino, P. and Barios, D. 1983. Potato production under shade in hot climates. In: Proceedings Research for Potato in the year 2000. Hooker, W.J. (Ed.), pp. 132. International Potato Center, Lima, Peru.

Midmore, D.J., Berrios, D. and Roca, J. 1988. Potato (Solanum spp.) in the tropics. IV. Intercropping with maize and influence of shade on micro environment and crop growth. Field Crop Research 18:141^-157.

Milthorpe, F.L. and Moorby, J. 1975. An introduction to crop physiology. Cambridge University Press, Cambridge, London. 202 pp.

Osiru, D.S.O. and Willey, R.W. 1974. Studies on mixture of dwarf sorghum and beans with particular reference to plant population. Journal of Agricultural Sciences 79:531-540.

Pendeleton, J.W., Bolen, C.D. and Seif, R.D. 1972. Alternating strips of corn and soyabean vs. solid plantings. Agronomy Journal 55:293-295.

Rao, M.R. and Willey, R.W. 1979. Sorghum in rainfed intercropping systems. Paper presented for the Golden Jubilee Symposium, Sorghum Research Station, Darbhani.

Reed, J.A., Singletary, G.W., Schussler, J.R., Williamson, D.R. and Christy, A.L. 1988. Shading effects on dry matter and nitrogen partitioning, kernel number, and yield of maize. Crop Science 28:819-825.

Sale, P.J.M. 1976. Effect of shade at different times of growth and yield of potato. Australian Journal of Agricultural Research 27:567-574.

Tizio, R.M. 1978. Potential of potato production for food and seed in the lowland tropics. FAO, Rome. 71pp.

Vander Zaag, P. and Demagante, A.L. 1990. Potato (Solanum spp.) in an isohyperthermic environment. V. Intercropping with special reference to tropics and sub-tropics. Potato Research 26:323-362.

Willey, R.W. 1979a. Intercropping: Its importance and research needs. Part 1. Competition and yield advantages. Field Crop Abstract 32:1^-10.

Willey, R.W. 1979b. Intercropping. Its importance and research approaches. Field Crop Abstract 32:73-85.

Willey, R.W. and Osiru, D.S.O. 1972. Studies on mixtures of maize and beans (Phaseolus vulgaris) with particular reference to plant population. Journal of Agricultural Science, Cambridge 79:517-529.

Yao, N.R., Goue, B., Zeller, B. and Monteny, B. 1988. Effect of drought on leaf development and dry matter production of cassava (Manihot esculenta Crantz) plant. Tropical Agriculture (Trinidad) 65:84-88.

TABLE 1. Potato: maize spatial arrangements used in the study

Treatments	Description	Proportions potato : maize (%)	Total plant population
T1	Sole potato	100 : 0	47,600
T2	Double potato rows alternating with single maize rows (2:1)	67 : 33	47,600
T3	Double potato rows alternating with double maize rows (2:2)	50 : 50	47,600
T4	Single potato row alternate with single maize row (1:1)	50 : 50	47,600
T5	Single potato row alternating with double maize rows (1:2)	33 : 67	47,600
T6	Additive mixture	50 : 50	95,200
T7	Sole maize	0 : 100	47,600

TABLE 2. Effect of spatial arrangement on leaf formation, stem extension, stem and branch numbers of potato plants in potato + maize intercrop

Treatments	Leaf formation day-1	Stem extension day-1	Number of main stems	Number of branches
Spatial arrangments
Sole crop	0.53	1.78	3.4	5.0
2 : 1 potato : maize mixture	0.61	1.88	4.0	4.5
2 : 2 potato : maize mixture	0.57	1.89	3.1	4.5
1 : 1 potato :maize mixture	0.50	1.97	3.9	3.8
1 : 2 potato : maize mixture	0.58	1.82	4.1	3.4
Additive mixture	0.58	2.00	4.0	2.1
LSD0.05	0.01	0.09	NS	1.8
CV %	2.90	5.02	18.4	46.83
Varieties
Kisoro	0.62	1.72	3.6	7.4
Sangema	0.49	1.87	4.7	2.4
Victoria	0.57	2.08	3.4	3.5
LSD0.05	0.06	0.25	NS	2.2
C.V. %	2.86	5.02	35.7	53.7

TABLE 3. Effect of spatial arrangements on the plant height and leaf area index of potato and maize in potato + maize intercrop grown during the first and second seasons of 1995

Treatments	Potato			Maize
	Plant height 30 DAP (cm)	Plant height 60 DAP (cm)	LAI	Plant height 30 DAP (cm)	Plant height 60 DAP (cm)	LAI
Spatial arrangements
Sole potato	37.6	48.0 (53.5)	2.56 (2.4)	35.7	182.6	2.94
2 : 1 potato : maize mixture	32.2	48.5 (52.9)	2.65 (3.3)	33.0	169.7	2.87
2 : 2 potato : maize mixture	37.8	48.7 (54.4)	2.57 (2.1)	35.0	171.8	2.96
1 : 1 potato : maize mixture	34.8	50.2 (50.3)	2.14 (2.3)	34.0	175.0	3.11
1 : 2 potato : maize mixture	35.8	44.1 (53.4)	2.45 (2.5)	33.5	176.2	2.98
Additive mixture	36.1	45.8 (53.8)	1.61 (2.2)	35.7	167.0	2.78
LSD0.05	NS	NS	0.548	NS	9.22	NS
CV %	12.0	12.6	24.4	9.1	7.5	10.8
Varieties
Kisoro	32.1	48.1	2.17	33.6	173.9
Sangema	36.4	51.5	2.17	34.9	176.1
Victoria	38.7	51.3	2.61	34.9	171.5
LSD0.05	NS	NS	NS	NS	NS
CV %	11.9	9.8	18.4	11.9	7.5

TABLE 4. Effect of intercropping potato + maize on potato dry matter production and partitioning 60 days after planting during the first and second seasons of 1995

Treatment	Leaf dry weight (g plant –1)	Stem dry weight (g plant –1)	Tuber dry weight (g plant –1)	Total dry weight (g plant –1)	Harvest index	Tuber number plant-1
Spatial arrangments
Sole potato	35.5 (15.0)	21.9 (16.8)	31.0 (29.1)	88.4 (60.9)	0.59 (0.46)	5.3
2 : 1 potato:maize mixture	39.7 (14.7)	26.4 (18.7)	29.5 (30.0)	95.6 (63.4)	0.48 (0.47)	5.5
2 : 2 potato:maize mixture	33.4 (14.2)	21.8 (17.2)	28.6 (28.7)	83.8 (60.1)	0.56 (0.46)	5.6
1 : 1 potato:maize mixture	35.7 (11.8)	21.1 (14.4)	27.7 (27.2)	84.5 (53.4)	0.49 (0.49)	4.9
1 : 2 potato:maize mixture	39.5 (13.2)	24.8 (14.7)	24.8 (21.9)	89.1 (49.8)	0.41 (0.43)	4.7
Additive mixture	35.5 (8.4)	21.8 (11.5)	28.9 (17.7)	86.3 (37.6)	0.52 (0.45)	5.1
LSD0.05	NS (2.8)	NS (3.4)	NS (7.9)	NS (12.3)	NS (NS)	NS
Varieties
Kisoro	33.8 (12.4)	17.8 (14.2)	22.0 (19.6)	72.6 (46.2)	0.45 (0.41)	6.2 (6.6)
Sangema	37.2 (13.0)	24.2 (14.8)	31.4 (30.3)	92.9 (58.1)	0.56 (0.51)	4.9 (3.8)
Victoria	39.6 (13.3)	26.9 (17.6)	31.8 (27.4)	98.3 (58.3)	0.52 (0.46)	4.3 (3.6)
LSD0.05	3.8 (NS)	NS (NS)	7.6 (3.65)	16.2 (7.03)	0.07 (0.04)	14.9 (0.63)
CV %	24.0 (22.2)	41.1 (23.0)	28.3 (31.2)	22.1 (22.5)	46.4 (17.2)	12.4 (14.9)

TABLE 5. Total yield (kg ha^-1) in different spatial arrangements of the potato + maize intercrops during the first and second seasons of 1995

Spatial arrangements	Potato		Maize
	First season	Second season	First season	Second season
Sole crop	8110	7434	4715	3640
2 : 1 potato : maize mixture	6388 (21.2)	3902 (47.5)	1177	1130
2 : 2 potato : maize mixture	3927 (51.6)	2877 (61.3)	2529	2050
1 : 1 potato : maize mixture	3165 (61.0)	2506 (66.3)	2696	2060
1 : 2 potato : maize mixture	1418 (82.5)	1726 (76.8)	3542	3060
Additive mixture	5273 (35.0)	2560 (65.6)	4587	3350
LSD0.05	1156	802.9	840	212
CV %	25.47	23.8	21.3	27.0
Varieties
Kisoro	4790.0	3399.5
Sangema	4414.1	3376.3
Victoria	4935.3	3726.9
LSD0.05	NS	NS
CV %	25.5	23.8

TABLE 6. Effect of different spatial arrangement on yield components in potato + maize intercrop, first season 1995

Spatial arrangements	Potato		Maize
	Yield Plant-1 (gm)	Weight tuber-1 (gm)	Yield Plant-1 (gm)	100 grains weight (gm)
Sole crop	191.2	38.5	147.4	28.8
2 : 1 potato : maize mixture	201.2	39.8	147.2	30.7
2 : 2 potato maize mixture	195.3	35.5	158.0	31.8
1 : 1 potato : maize mixture	155.7	35.5	168.5	30.2
1 : 2 potato : maize mixture	139.3	33.0	147.6	30.0
Additive mixture	126.4	27.4	143.4	27.9
LSD0.05	34.0	8.0	NS	1.8
CV %	30.0	23.8	18.51	5.0
Varieties
Kisoro	164.6	27.2
Sangema	165.8	34.6
Victoria	174.7	43.0
LSD0.05	34.0	NS
CV %	20.9	23.7

TABLE 7. Biological efficiency of different spatial arrangements in potato + maize intercrop (means of the two seasons)

Spatial arrangements	Yield (ton ha-1)		Expected partial LER		Obtained partial LER		Total LER
	Potato	Maize	Potato	Maize	Potato	Maize
Sole crop	7.77	4.18	-	-	-	-	-
2 : 1 potato : maize	5.15	1.16	0.67	0.33	0.69	0.30	0.99
2 : 2 potato : maize	3.63	2.29	0.50	0.50	0.46	0.58	1.04
1 : 1 potato : maize	2.98	2.38	0.50	0.50	0.39	0.59	0.98
1 : 2 potato : maize	1.65	3.33	0.33	0.67	0.21	0.86	1.07
Additive mixture	3.97	3.97	0.50	0.50	0.60	0.98	1.58
LSD0.05	0.98	0.53	-	-	0.11	0.18	0.29
C.V. %	24.64	24.15	-	-	20.67	25.49	23.0

TABLE 8. Percentage shading, relative humidity and soil temperature (12:00) on potato in various spatial arrangements, second season 1995

Spatial arrangements	Percentage shading		Relative humidity (%)	Soil temperature (0°C)
	30 DAP	60 DAP1		5 cm	10 cm
Sole potato crop	0.00	0.00	71.2	30.1	27.8
2 : 1 potato : maize mixture	0.11	0.44	71.1	29.2	26.8
2 : 2 potato : maize mixture	0.13	1.96	-	-	-
1 : 1 potato : maize mixture	0.00	3.56	78.5	29.6	26.7
1 : 2 potato : maize mixture	1.72	5.73	-	-	-
Additive mixture	20.13	51.50	80.1	27.5	25.9
LSD0.05	1.75	6.03	6.21	1.2	1.2
C.V. %	39.70	49.04	11.06	3.4	3.7

TABLE 9. Regression relationship between light transmission (shading) and crop performance (yield per plant) of three potato cultivars

Variety	Intercept, a	Regression coefficient, b	Correlation coefficient, r	Probability p
Percentage shade
Kisoro	181.9	-2.1	0.72	0.106
Sangema	199.1	-2.3	0.89	0.018
Victoria	195.4	-2.3	0.86	0.026
Soil temperature
Kisoro	-2238	89.53	0.99	0.008
Sangema	-2060	83.38	0.97	0.032
Victoria	-2078	83.91	0.97	0.0288