THE EFFICIENCY OF TRANSPLANTING SUGAR CANE SEEDLINGS DIRECTLY IN THE FIELD AND ITS IMPACT ON THE SELECTION CYCLE AND RESOURCES ABSTRACT H. Mungur, K. Ramdoyal and D. Santchurn Mauritius Sugar Industry Research Institute A trial was carried out to study the feasibility and efficiency of transplanting sugar cane seedlings directly in the field without potting and to assess its impact on the selection cycle and on resources. Seedlings of five families were transplanted on one-metre wide raised beds at three spacing: close, intermediate and wide, at population densities of seven, five and three seedlings/m 2 respectively. Selection was practised when the crop was aged ten months and all genotypes that produced enough planting material, including those that would have been rejected on visual assessment, were evaluated in two-metre row plots at the 1 st clonal stage. Mortality rate of seedlings was highest in the closest spacing. Significant differences between spacing were found for stalk number and stalk height but not for stalk diameter. Families differed significantly for all the characters measured and family x spacing interaction was detected for stalk diameter and stalk height. Phenotypic correlation coefficients between the seedling stage and the 1 st clonal stage were very low for stalk number, stalk diameter and stalk height. Broad-sense heritability for stalk number at the seedling stage was very low, while stalk diameter and stalk height were more heritable. Among genotypes that would have been rejected at the seedling stage based on visual appreciation, only a few were re-selected at the 1 st clonal stage. Stalk diameter and stalk height were the most reliable criteria for selecting seedlings. The intermediate spacing was most appropriate for transplanting seedlings directly in the field. At this planting density, 8.5 hectares of land, 890 man-days and other resources such as transport and potting medium can be saved each year. In addition, this new technique of evaluating seedlings shortens the selection cycle by one year and impacts favourably on the next stage of selection with respect to planting and selection periods. Keywords: Potting, spacing, visual selection, genotypes, heritability. INTRODUCTION Sugar cane breeding programmes typically commence by evaluating large numbers of seedlings derived from true seeds. The starting population used for selection varies on breeding stations and ranges from 10 000 to 1.2 million seedlings (Mamet and Travailleur, 1998). These are screened through a series of selection stages and multiplied clonally, their numbers being reduced at each stage, with the best genotypes being tested in larger plots. The time taken to release a sugar cane variety ranges from eight to twenty years (Skinner et al., 1987). The variety selection programme at the Mauritius Sugar Industry Research Institute (MSIRI) extends over eleven to fifteen years, with an initial population of 100 000 seedlings produced every year. Seed is sown in September and seedlings are transferred in polythene bags at the age of six to eight weeks before they can be transplanted to the field in March of the following year at the age of three to four months. Most breeding stations first transfer seedlings to polythene bags, flats, beds or peat pots for better survival and growth before transplanting them in the field. However, this involves transplanting the seedlings twice, which increases the time lapse from sowing to evaluation in the field. Seedlings are also transferred directly from sowing trays to airbricks, as in South Africa (Thomas, 1989) or transplanted in raised beds, as practised in India and Reunion Island. 135
Seedlings at MSIRI are visually screened in April/May, 13 to 14 months after transplanting in the field. Selection criteria are number and size of stalks, growth habit and absence of defects. Individual seedling selection is of low efficiency given the low broad-sense heritability for the majority of traits (Skinner, 1971, Skinner et al., 1987). Selection should therefore be lenient. Genotypes which are selected at the seedling stage are planted in two-metre row plots in the 1 st clonal stage. Selection at this stage is done after 14 to 15 months, in July of the following year. The time at which selection is practised at both the seedling and 1 st clonal stages affects the quality of planting material for establishing the 1 st and 2 nd clonal stages respectively. A lot of resources are devoted to evaluate large populations of genotypes in small plots. At MSIRI, eleven hectares of land are required every year for evaluating the seedling population. Since most activities are carried out manually, the selection process at this stage is laborious and expensive. In the context of the new strategic plan for the sugar sector (MAFTNR, 2001), the sugar industry is called upon to operate in an environment of reduced labour. Alternative methods of evaluating seedlings under resource constraints need to be considered without jeopardising the efficiency of selection. This paper investigates the feasibility of transplanting sugar cane seedlings directly in the field without potting and its impact on the selection cycle and on resources. MATERIALS AND METHODS The seedling stage Seeds of five bi-parental families were sown thinly in July to produce vigorous seedlings. These were transplanted without potting, in November of the same year, on one-metre wide raised beds at Réduit Experimental Station at three different spacing: Close - three rows of seedlings at 0.40 m between rows and 0.30 m within rows Intermediate - three rows of seedlings at 0.40 m between rows and 0.40 m within rows Wide - two rows of seedlings at 0.40 m between rows and 0.40 m within rows at population densities of seven, five and three seedlings/m 2 respectively. The statistical design was a split plot with two replicates, with spacing as main plot and families as sub-plots. Each family was represented by 368 seedlings. An assessment of mortality of seedlings was done at regular intervals for the first three months after transplanting and at the time of selection, ten months after the establishment of the crop. At selection time, the following characteristics were measured on all genotypes that produced enough planting material, ten three-eyed cuttings, to establish the 1 st clonal stage: number of millable stalks per stool, stalk diameter (mm), stalk height (cm) and visual grade (select or reject). A total of 717 genotypes, 260 from close spacing, 245 from intermediate spacing and 212 from wide spacing were selected for further evaluation at the 1 st clonal stage. The genotypes selected also included those that would have been rejected on visual assessment but which produced enough planting material for planting in the 1 st clonal stage. 1 st clonal stage At the 1 st clonal stage genotypes were planted in two-metre row plots, using a row and column design with the control variety, R 570, planted every five to six rows of test genotypes. Same criteria were measured as for the seedling stage, ten months after establishment. In addition, Brix (total dissolved solids correlated with sucrose content) was measured in the field, using a hand refractometer, on all genotypes, including the control. Selection was based on a threshold level for Brix and on visual assessment. A total of 173 genotypes were promoted for further evaluation in the 2 nd clonal stage. 136
Statistical analyses With unequal number of progenies assessed for the five families, analysis of the unbalanced data was done by Residual Maximum Likelihood using the software ASREML (Gilmour et al., 2001) and GenStat for windows, Release 4.2. Estimates of phenotypic correlation coefficient for stalk number, stalk diameter and stalk height between the seedling stage and the 1 st clonal stage were obtained through covariance analysis according to the formula: Cov( seedling, clonal) r 12 = Var( seedling) Var( clonal) st where r 12 is the estimate of correlation coefficient for the trait measured at the seedling and 1 clonal stages, C ov( seedling, clonal) is the covariance between the mean of the trait measured at the seedling stage and the mean of the trait measured at the 1st clonal stage. Var(seedling) is the phenotypic variance of the trait at seedling stage and Var(clonal) is the phenotypic variance of the trait st in the 1 clonal stage (Steel and Torrie, 1980). Broad-sense heritability ( 2 h b ) values for stalk number, stalk diameter and stalk height at the seedling stage was estimated using the variance components generated by REML analysis. The formula used was: 2 Vg h b = Vp where Vg is the genetic variance and Vp is the phenotypic variance (Falconer and Mackay, 1996). Estimates of land and labour resources were based on a population of 100 000 seedlings and an average selection rate of 20%. RESULTS AND DISCUSSION Mortality in seedlings Mortality rate was highest (27%) in the closely transplanted seedlings, compared to that observed in intermediate (16%) and widely (8%) spaced ones (Figure 1). This suggests that high competition is detrimental in the high density treatment. It has been observed that vigorous seedlings in sowing trays tend to establish better in the field, indicating that they are more likely to have a significant competitive advantage in small plots. To ensure survival in the field, seed sowing should be done as lightly as possible to produce robust seedlings for direct transplanting without potting. Visual selection Selection rate varied with families and spacing treatments (Figure 2). Highest selection rate was obtained in the wide spacing treatment (43%) followed by intermediate spacing (28%) and close spacing (23%). Out of 717 genotypes selected at the seedling stage, 535 were visually suitable on a select/reject basis (Table 1), whereas the rest, although visually unsuitable, produced enough cuttings for planting at the next stage of selection. 137
Figure 1 Survival (%) of sugar cane seedlings transplanted at three spacing 100 Wide spacing Percent survival 90 80 Intermediate spacing Close spacing 70 60 Transplanting 1 month 2 months 3 months 10 months (selection) Crop stage Figure 2 Percentage of seedlings selected visually in five sugar cane families transplanted at three spacing 60 50 Selection rate (%) 40 30 20 10 0 F1 F2 F3 F4 F5 Mean Close spacing Intermediate spacing Wide spacing When all the 717 selected genotypes were evaluated on larger plots at the 1 st clonal stage, 174 (33%) of the 535 genotypes (select grade) were again selected and provided most of the genotypes promoted to the 2 nd clonal stage (Table 1). Among those that were visually unsuitable (reject grade) at the seedling stage, only 19 (10%) were promoted for further evaluation at the 2 nd clonal stage (Table 1). This shows that if only the attractive genotypes are selected, there is a risk of losing some good ones. However, due to high environmental influence, selection should be lenient as the consequences of eliminating good genotypes are more serious than continuing to test the inferior ones. 138
Table 1 Number of genotypes categorised on select/reject basis at the seedling and 1st clonal stages and number of genotypes selected for the 2nd clonal stage Genotypes selected 717 Visual grade at the seedling stage Select (S) 535 Reject* (R) 182 S R S R Visual grade at the 1 st clonal stage 174 361 21 161 Genotypes selected for 2 nd clonal stage 154 (29%) 19 (10%) * Genotype that is not visually attractive Cane yield components Stalk number in the seedling population varied from two to eleven, substantiating the large coefficient of variation of 38-40% observed in the three spacing. Lower coefficient of variation of the order of 14-15 and 17-18 were obtained for stalk diameter and stalk height respectively, showing less variability for these two characters as compared to stalk number. Similar coefficients have been reported by Nagarajan (1997) and Nair (1989) in sugar cane seedling populations. Families differed significantly for all the characters measured (Table 2) showing the superiority or inferiority of some families over the others for any of the three characters. Mean values for the cane yield components are given in Table 3. With regards to spacing, significant differences were observed for stalk number and stalk height but not for stalk diameter, implying that planting density did not have an impact on stalk thickness. The majority of genotypes in the close spacing treatment produced two to six millable stalks, as compared to three to seven in the low planting density treatments showing that larger spacing tends to favour the production of stalk. However, mean stalk number observed at intermediate and wide spacing were not significantly different as revealed by an LSD test. Irrespective of the family, stalk height was significantly taller at intermediate spacing. The absence of family x spacing interaction for stalk number shows that the relative performance of sugar cane families for that character would not change with respect to spacing (Table 2). Table 2 Mean square values for stalk number, stalk diameter and stalk height for five families transplanted at three spacing Stalk number Stalk diameter Stalk height Source of variation d.f m.s d.f m.s d.f m.s Family (F) 4 16.84** 4 547.67** 4 40541.5** Spacing (S) 2 25.85** 2 14.63 ns 2 43251.6** F x S interaction 8 2.84 ns 8 41.5** 8 2450.3** Residual 701 2.35 2033 13.87 2031 890.9 *: Significant at P= 0.05 **: Significant at P= 0.01 ns: Not significant 139
Table 3 Mean and S.E for stalk number, stalk diameter (mm) and stalk height (cm) for five families transplanted at three spacing Family Stalk number Spacing Close Intermediate Wide Mean F1 3.46 ± 0.18 3.89 ± 0.22 4.34 ± 0.34 3.90 ± 0.14 F2 3.86 ± 0.18 4.61 ± 0.23 4.31 ± 0.21 4.26 ± 0.12 F3 3.17 ± 0.15 3.66 ± 0.19 3.63 ± 0.19 3.49 ± 0.10 F4 3.56 ± 0.21 3.91 ± 0.21 4.38 ± 0.31 3.95 ± 0.14 F5 4.12 ± 0.22 4.06 ± 0.21 4.92 ± 0.28 4.37 ± 0.14 Mean 3.63 ± 0.09 4.03 ± 0.10 4.32 ± 0.12 3.99 ± 0.06 Stalk diameter F1 23.4 ± 0.55 25.4 ± 0.56 24.5 ± 0.64 24.4 ± 0.34 F2 25.0 ± 0.46 25.5 ± 0.46 25.6 ± 0.46 25.4 ± 0.27 F3 25.1 ± 0.43 24.5 ± 0.52 24.5 ± 0.48 24.7 ± 0.27 F4 23.2 ± 0.39 23.7 ± 0.43 23.7 ± 0.46 23.5 ± 0.24 F5 22.8 ± 0.36 22.1 ± 0.38 22.4 ± 0.41 22.4 ± 0.22 Mean 23.9 ± 0.21 24.2 ± 0.22 24.1 ± 0.23 24.1 ± 0.13 Stalk height F1 153.4 ± 3.3 170.4 ± 4.3 151.2 ± 4.3 158.3 ± 2.4 F2 135.1 ± 2.4 154.8 ± 3.6 139.3 ± 3.9 143.1 ± 2.0 F3 166.8 ± 4.1 175.8 ± 5.6 167.3 ± 4.4 170.0 ± 2.7 F4 160.3 ± 4.2 167.7 ± 3.8 149.3 ± 3.8 159.1 ± 2.4 F5 159.2 ± 3.7 166.1 ± 2.8 149.2 ± 3.1 158.2 ± 1.9 Mean 155.0 ± 1.7 167.0 ± 1.8 151.3 ± 1.8 157.7 ± 1.1 In light of these results and taking into consideration land resources, the intermediate spacing appears to be the optimum planting density for transplanting seedlings directly in the field. Heritability The broad-sense heritability for stalk number was very low (0.04) at the seedling stage, indicating a very high component of environmental variation. Julien (1988) reported that stalk number is generally the character which is mostly affected by environment in a seedling population. Low to moderately low narrow-sense heritability values of 0.24 and 0.48 have been reported by (Singh and Singh, 1994) and (Singh et al., 1995) in seedlings evaluated in rows 0.60 m apart with 0.40 m within row. Stalk number is not a reliable selection criterion in seedlings in the range of spacing investigated in this study. Stalk diameter and stalk height were more heritable with heritability values of 0.54 and 0.58 for the two traits respectively. A narrow-sense heritability value of 0.59 has been observed by (Singh et al., 1995) for stalk diameter and stalk height while Singh and Singh (1994) reported values of 0.71 and 0.91 for the two characters. Phenotypic expression of these two traits can constitute reliable selection criteria for selection purposes. 140
Correlation between seedling and 1 st clonal stages Correlation coefficients between the seedling and the 1 st clonal stages were generally significant for all the traits, though they were very low (Table 4). Consistent correlation coefficients of 0.24, 0.24 and 0.23 for stalk number between the two selection stages were obtained in the close, intermediate and wide spacing treatments respectively, showing no advantage to any of the spacing. Higher correlation coefficients for stalk number, ranging from 0.32 to 0.61, between seedlings transplanted directly in the field and the 1 st clonal stage have been reported by Tripathi et al. (1977), Sundaresan et al. (1979), Nagarajan et al. (1983) and Nair (1989). However, these studies were carried out at larger spacing at the seedling stage. Low correlation coefficients for stalk diameter were obtained between the seedling stage and the 1 st clonal stage. The highest repeatability for this trait (0.32) was observed in the intermediate spacing treatment. However, the repeatability values were lower than those (0.39 to 0.63) reported by Tripathi et al. (1977), Sundaresan et al. (1979), Nagarajan et al. (1983) and Nair (1989) between the first two selection stages. A better expression of stalk diameter can therefore be expected when genotypes are widely spaced in the field. Table 4 Phenotypic correlation coefficients and S.E for yield components between the seedling and the 1 st clonal stages for genotypes planted at three spacing at the seedling stage Spacing Stalk number Stalk diameter Stalk height Close 0.24** ± 0.06 0.15* ± 0.06 0.25** ± 0.06 Intermediate 0.24** ± 0.06 0.32** ± 0.06 0.25** ± 0.06 Wide 0.23** ± 0.07 0.09 ns ± 0.07 0.38** ± 0.06 Pooled population 0.21** ± 0.04 0.24** ± 0.04 0.26** ± 0.04 *: Significant at P= 0.05 **: Significant at P= 0.01 ns: Not significant Correlation coefficients of 0.25 to 0.38 were observed for stalk height between the two selection stages in the three spacing treatments. Tripathi et al. (1977) reported coefficients ranging from 0.13 to 0.65 in progenies evaluated at a spacing of 0.6 m in rows 0.9 m apart in the first two stages of the selection programme. The correlation coefficients observed for the three characters in the intermediate spacing treatment were consistent and indicate that selection for the three yield components can be favourably considered at that spacing. Impact on resources A seedling which is directly transplanted in the field at the intermediate spacing will occupy 0.2 m 2 compared to 0.9 m 2 with the conventional method of transplanting potted seedlings. Only 2.5 hectares of land area will be required every year, instead of eleven hectares to accommodate a typical seedling population of 100 000 in the MSIRI selection programme. In addition, direct transplanting does not require the preparation of the soil medium (soil, farmyard manure, filter mud and factory ash) for the potting of seedlings. An economy of 50 tonnes of soil can therefore be made each year. Similarly, the labour that is used for potting and that required for the transport of potted seedlings from the nursery to the field is no longer needed. Estimates of labour requirements at the seedling stage under the conventional and direct transplanting methods show that a substantial saving of 890 mandays can be achieved with the direct transplanting method (Table 5). In addition, seedling trays occupy less space and are more conveniently transported to the fields. Therefore, about 70 lorry trips are no longer required for the transport of potted seedlings to the field. 141
Table 5 Estimated labour requirements (mandays) for the main activities at the seedling stage for the conventional (potting) and direct transplanting methods of transplanting seedlings Activities Potting Direct transplanting Potting 520 - Management practices 250 230 Field preparation 110 250 Transplanting in field 570 180 Selection of seedlings 550 450 Total 2000 1110 Impact on selection cycle The calendar of activities for the first two selection stages of the MSIRI selection programme, for both the conventional and direct transplanting methods of transplanting seedlings, shows a gain of one year in the selection cycle (Figure 3). For the direct transplanting method, the best time for sowing seed is July for transplanting seedlings in the field in November of the same year, compared to March of the following year with potted seedlings. Selection in seedlings planted without potting can be done in September when the crop is aged 10 months instead of 13 to 14 months, in April/May of the following year, for seedlings transplanted by the conventional method. At this crop age, planting material of better quality is obtained for establishing the 1 st clonal stage compared to that obtained when selection is practised on seedlings transplanted from pots. Concurrently, the change in the planting calendar for the 1 st clonal stage enables selection at this stage to be done on a 10 months crop compared to 14 to 15 months for the conventional method. Planting material of good quality is thus obtained for establishing the 2 nd clonal stage. It has been observed that with the conventional method, many attractive genotypes are lost every year on account of poor quality of canes (bulging buds, sprouting) by the time selection is carried out. 142
Figure 3 Calendar of activities for the conventional and direct transplanting methods of transplanting seedlings for the first two stages of selection Conventional method Direct transplanting method Year Month Month Year 1 May - July CROSSING May - July 1 1 September SOWING SOWING July 1 1 November - December POTTING Transplanting seedlings directly in field November 1 2 March Field transplanting Selection of seedlings & planting 1 st clonal stage September 2 3 April - May Selection of seedlings & planting 1 st clonal stage Selection of 1 st clonal stage July 3 4 July Selection of 1 st clonal stage CONCLUSION This study has shown that direct transplanting of sugar cane seedlings without potting in the initial stage of the selection programme at MSIRI is viable. Sowing of seed should be done as thinly as possible to produce robust seedlings to ensure a good establishment in the field. Intermediate spacing at a population density of five seedlings/m 2 appears to be the optimum spacing for evaluating seedlings with this method. Visual selection integrating stalk number, stalk diameter and stalk height as selection criteria can be practiced but should be lenient at this stage of selection. However, less emphasis should be placed on stalk diameter and only clones with thin stalks should be rejected at this stage. Selection efficiency is improved as selection at both the seedling and the 1 st clonal stages can be done on ten months old crops, ensuring a better quality of planting material for establishing the next stages of selection. Concurrently, substantial saving on land, labour, transport and other resources can be realised yearly with this transplanting technique. This also impacts on the calendar of activities for the first two stages of selection and results in the shortening of the selection cycle by one year. ACKNOWLEDGEMENTS The authors are grateful to the personnel of the Plant Breeding department for their contribution and thank the Director of the MSIRI for his constant support and for reviewing this paper. 143
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