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African Crop Science Journal, Vol. 9, No. 1, March 2001, pp. 59-66
Y. Cadersa, K. Wong Yen Cheong1 and N. Govinden1
Code Number: CS01035 ABSTRACT About 80% of the potato produced in Mauritius comes from an intercrop with sugarcane. Recently, several new tropical clones with higher yields have been proposed to growers. Since their higher yields are associated with larger canopies, five trials were conducted in 1997 and 1998 to investigate whether the new clones affect the yield and yield components of intercropped cane. Percent and duration of canopy cover of tropical clones were significantly higher than those of traditional temperate clones. Canopy cover was also higher in intercropping than in pure stands.The potato had no significant effect on tiller density, number of millable cane, sucrose content, cane and sugar yield of intercropped cane. There were no relationships between either potato canopy cover duration or potato yield and relative cane yield. This confirms that even potato with larger canopies and higher yields do not affect intercropped cane growth and yield. Key Words: Mauritius, potato/sugarcane intercrop, sucrose content, tiller density RÉSUMÉ Environ 80% de la production de pomme de terre au Maurice provient de la culture intercalaire de la canne à sucre. Récemment, plusieurs clones tropicaux ayant de meilleurs rendements ont été proposés aux planteurs. Comme ces rendements elevés étaient associés à un plus grand développement foliaire, cinq essais ont été plantés en 1997 et 1998 pour déterminer si ces nouveaux clones tropicaux avaient des effets sur la canne à sucre en culture intercalaire. La couverture du sol des clones tropicaux et leur cycle étaient supérieurs à ceux des clones traditionnels des climats temperés. La couverture du sol était aussi supérieure en intercalaire qu-'en culture pure. La pomme de terre n'a pas eu d'effets sur le tallage, le nombre de tiges usinables, la teneur en sucre et le rendement en canne et en sucre. Il n'y a pas eu de relation entre le rendement relatif de la canne, d'une part, et la durée de la couverture foliaire du sol et le rendement de la pomme de terre, d'autre part. Ces résultats démontrent qu'en dépit d'un plus grand développement foliaire et de meilleurs rendements, les nouvelles variétés de pomme de terre d'origine tropicale n'affectent pas la canne à sucre en intercalaire. Mots Clés: Ile Maurice, canne à sucre, culture intercalaire, pommes de terre, tallage, teneur en sucre INTRODUCTION In Mauritius where sugar cane occupies 92% of the cultivated land, the production of other crops is seriously limited by the scarcity of arable land. Intercropping of the sugar cane is a method used extensively to intensify crop production in line with the national policy of promoting agricultural diversification in addition to, rather than at the expense of sugar production (Antoine, 1973). Potato, tomato and other food crops are grown in interrows of sugarcane. At present, 77% of the total production of potato comes from intercropping with sugar cane (Govinden, 1990). Numerous studies with commercial temperate cultivars have shown that potato does not affect the yield and yield components of sugar cane. With newly developed tropical clones, field trials have shown that they give better yields per plant when intercropped with sugar cane than when grown in pure stands. This has tentatively been attributed to their larger canopies and longer canopy durations (Wong Yen Cheong and Govinden, 1997). Since these new clones have large canopies, some growers have been reluctant to grow them as intercrops for fear of reducing cane yield. The objective of this study was to verify that the new clones have larger canopies and that they do not affect cane when grown as intercrops under a range of conditions. MATERIALS AND METHODS Five trials were planted in Mauritius (20° S latitude, 57° E longitude), four in 1997 and one in 1998. In 1997, the trials in the first season (May-June plantation) were located at Réduit and Union Park Experimental Stations and those in the second season (August-September plantation) were at Highlands and Union Sugar Estates. The 1998 trial was at Union Park Experimental Station and will be referred to as Union Park 2. In 1997, there were two sets of two trials, one in each season. In the first set, two temperate clones were compared to three tropical ones, whereas in the second set, one temperate clone was compared to two tropical ones. The potato was grown in pure stands (sole crops) and in interrows of plant sugar cane (intercrops). In 1998, at Union Park 2, physiological age was varied by removing the potato seeds from the cool store at 150 and 240 days. As in 1997, two temperate clones were compared to two tropical clones. Land was prepared with a disc plough followed by a rotovator. Cane furrows 30-40 cm deep were made with a pneumatic tractor at 160 cm intervals. Three- bud cuttings of cane were placed in double rows and spaced at 0.2 m within the row. After cane plantation, one potato furrow was dug in every cane interrow. Pre- germinated seed tubers were planted at 30 cm within the row at Réduit, Highlands and Union Park 2, whereas at Union and Union Park 1, density was varied by planting at intra row spacings of 24, 30 and 40 cm. At Réduit and Highlands, the temperate clones were Spunta and Stirling originating from the Netherlands and Scotland, respectively, and the tropical clones were 57-2, 52-2 and 26-2 originating from the Mauritius Sugar Industry Research Institute's (MSIRI's) breeding programme. At Union Park 2, the same clones were used except 52-2. At Union and Union Park 1, Spunta was the only temperate clone used, and the tropical clones were 84-2 and 26-2. Fertilisers were applied at recommended rates (MSIRI, 1995) based on soil analysis. Weeds, diseases and insects were controlled by spraying with chemicals at the rates recommended by Govinden et al. (1986). In potato, the proportion of ground covered with green leaves was measured weekly using a grid as suggested by Burstall and Harris (1983). It consists of a wooden frame divided into 100 equal sections of dimensions 8 cm x 6 cm- a multiple of the planting pattern for potato. The grid was placed on top of the potato at one metre from the ground, and those sections more than half filled with green leaves were counted by observing vertically above to avoid parallax error. In sole potato, the grid was placed half way on each side of the potato row to sample two plants (Fig. 1). In intercropped potato, two grid positions were used, one on either side of the potato row. Canopy cover was calculated as the ratio of the area of grids counted to the area of ground allocated to potato and is expressed as a percentage. In pure stand, canopy cover by this definition cannot exceed 100%. In intercropping, potato canopy sometimes extend beyond the 80 cm allocated to it on either side of the row. Thus, canopy cover can exceed 100% (Fig. 1). Two measurements were made per plot. Canopy cover duration (CCD) was calculated by summing the ground cover at successive weeks from emergence to senescence. The cane plots consisted of 5 rows 7.2 m long. The central 3m portions of the third and fourth rows constituted the experimental area of 9.6 m2. Tiller density was determined by counting all live tillers in the experimental area at monthly intervals. At harvest, number of millable canes was counted, and plot weights were recorded with a field balance. Data on tiller density and millable canes were not collected at Union Park 2. After analysis of variance, treatments were compared with the Least Significance Differences (LSD). Whenever differences between the treatments were not significant, clones were merged, and means of temperate and tropical groups are presented. The relationships between (a) potato canopy cover duration or (b) potato yield and cane yield were explored through regressing the former on relative cane yield calculated as percent of sole cane. This latter transformation was necessary before sites could be merged. RESULTS AND DISCUSSION Potato canopy cover development. The general trend in canopy cover (Figs. 2 & 3) is an increase as from three weeks after planting to reach maximum values between 7 to 11 weeks depending on the clones. Canopy cover declines and finally drops to zero as the crop senesces. This trend is to be expected since canopy cover is a function of leaf growth. Clone effects. Canopy cover was significantly higher in tropical potato clones than in temperate ones due in part to differences in maturity. Tropical clones had longer crop cycles (14 -15 weeks) than temperate clones which senesced by 12 -13 weeks (Figs. 2 & 3). This is in conformity with the findings of Demagante et al. (1996) who found that in the Philippines, tropical clones tuberise later and thus have more developed canopies of longer durations than temperate clones which develop smaller canopies of shorter durations. Cropping systems effects. In general, canopy cover was slightly higher in intercropping than in pure stand (Figs. 2 & 3). At Union for instance, canopy cover in pure stand peaked at 100% while in intercropping, according to the definition, it exceeded 100%. In pure stand,on one hand, potato plants compete with one another for growth resources because of limited space. At canopy closure, the 80 cm interrow space is completely covered by overlapping leaves from adjacent rows. Thus, space for further haulm growth is restricted, especially in clones that achieve complete ground cover in less than nine weeks and maintain it for a sufficiently long period. In intercropping, on the other hand, potato rows are further apart (1.6 m) and the young cane does not make much demand on growth resources available in the interrows (Govinden, 1990). Therefore, by benefiting from growth resources and more space available in the interrows, some clones are able to display a larger canopy, which we have chosen to emphasise by allowing the canopy to exceed 100%. Plant density effects. Intra row spacings of 24, 30 and 40 cm did not significantly affect canopy cover in either temperate or tropical clones at Union and Union Park 1. Data are presented for Union only (Fig. 3). Seed age effects. At Union Park 2, canopy cover of crops grown from both young and old seeds did not reach 100% even in intercropping and attained maximum values of 70%, implying that this may be a site effect. Potato grown from old seeds senesced earlier by one week than those from young seeds (data not presented). This result supports the findings of O'Brien and Allen (1981) that crops grown from old seeds senesce earlier. Tiller density and millable canes. In plant cane, germination usually starts at about 2 weeks, and tillers emerge at first slowly for up to about 30 days, then rapidly as from 60 days to reach peak values at about 150-180 days, depending on various factors such as variety and temperature and soil moisture. Subsequently, tillers die until their number stabilises at 8 months or so. At none of the sites was there a significant effect of treatments on tiller density. At Union Park 1, cane intercropped with temperate clones gave the highest peak tiller number at about 180 days, but at harvest, number of millable canes was not significantly different between intercrop and sole crop treatments (Fig. 4). Sucrose content, cane stalk yield and sugar yield. In the 1997 plantations, no significant differences were observed between intercrop and sole cane treatments in sucrose content, cane stalk yield and sugar yield. In some instances, both cane and sugar yields of intercropped cane were higher than those of sole cane. This could either be a statistical artefact or a real effect such as a beneficial residual effect of fertiliser applied to the potato (Imam et al., 1989). Across the five sites, there was no evidence for an effect of potato on sugar yield and its components and no evidence for a difference between tropical and temperate clones in their effects on cane (Table 1). A non-significant correlation coefficient was obtained for the relationship between potato yield and relative cane yield (Fig. 5a). This indicates that even when potato yields are high, yields of intercropped cane are not reduced. This finding is in agreement with the results of Wong Yen Cheong and Govinden (1997) whereby the effect of potato on intercropped cane was not dependent on potato yields, but it is in contrast to the situation in maize where high maize yields reduced cane yields (Govinden and Ramasamy, 1995). Hence, within the range tested (up to 35 t ha-1), excellent potato yields can be obtained without any decrease in cane yields. Moreover, the absence of a relationship between mean canopy cover duration (CCD) and relative cane yield (Fig. 5b) indicates that even at the high CCD shown by new tropical clones, cane yields were not reduced. The hypothesis that larger canopies of tropical clones could lead to yield reductions in intercropped cane is therefore rejected. CONCLUSION The results with different clones at five sites show conclusively that sugar cane yield and its components are not affected by intercropped potato even when the clones, and more particularly, the new tropical ones, have more developed canopies. Consequently, intercropping of these clones can be recommended without fear of a possible adverse effect on sugar cane. ACKNOWLEDGEMENT The work reported in this paper was supported by the Tertiary Education Commission and the Mauritius Sugar Industry Research Institute through a research fellowship to the first author. They are gratefully acknowledged. Thanks are expressed to Union and Highlands estate agronomists and to field officers of Réduit and Union Park Experiment Stations for help in the management of field trials and to the Director and Deputy Director (Biology) of Mauritius Sugar Industry Research Institute for their comments on the manuscript. REFERENCES
Figure 2. Canopy cover of sole potato (left) and intercropped potato (right) in temperate and tropical clones at Highlands. Figure 3. Canopy cover of tropical and temperate clones (mean of 2 clones) at three intra row spacings in sole potato (left) and intercropped potato (right) at Union. Figure 4. Cane tiller density in sole and intercropped potato at Réduit, Highlands, Union and Union Park 1 in 1997. Figure 5. The relationship between (a) potato yield and (b) potato canopy cover duration and relative cane yield in five trials in 1997/98. TABLE 1. Effect of two types of potao clones - temperate and tropical on the yield and yield components of intercropped sugar cane at five sites in 1997/98.
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