Effect of variety, soil type and harvest interval on biomass yield and soluble and insoluble oxalates in taro (Colocasia esculenta L.) foliage

Du Thanh Hang, Phan Vu Hai and Geoffrey Savage¹

Faculty of Animal Husbandry and Veterinary Medicine, Hue College of Agriculture and Forestry, Hue University, Vietnam
duthanhhang@huaf.edu.vn
¹ Food Group, Department of Wine, Food and Molecular Biosciences, Faculty of Agriculture and Life Sciences, Lincoln University, Canterbury, New Zealand.

Abstract

The biomass yield and total and soluble oxalate contents of leaves and petioles of two taro cultivars (Ao Trang and Mon Ngot), grown on clay or sandy soils, were determined when the plants were subjected to repeated harvests at 14-day intervals.

The biomass yield of the petioles and the leaves of the two taro cultivars increased linearly over three harvests at 14-day intervals following the initial harvest 30 days after planting. A significant drop in biomass yield was observed for the harvest taken after 86 days of growth. For both cultivars the dry matter yield of the leaves was larger than the dry matter yield of petioles.

Levels of insoluble oxalates were almost twice as high as those for soluble oxalates independent of variety, soil type and age at harvest. The overall levels and degree of difference (insoluble/soluble) were much greater for petioles than for leaves. Oxalate levels were consistently higher in the Ao Trang than in the Mon Ngot variety but did not differ between plants grown in sandy or clay soils. Soluble and insoluble oxalate levels in petioles and soluble oxalate in petioles were highest in the first harvest (30 days after planting) and then decreased linearly over the five succeeding harvests at 14-day intervals. This trend was less uniform for insoluble oxalates in leaves.

Key words: clay soil, leaves, petioles, repeated harvest, sandy soils

Introduction

In Central Vietnam, there are seven main cultivars of taro ( Colocasia esculenta L) grown extensively as tubers for human consumption. Two cultivars, Ao Trang and Mon Ngot, are more commonly fed to pigs (Hang et al 2017). Taro leaves and stems have high oxalate content as non-absorbable salts with Ca⁺⁺, Fe⁺⁺ and Mg ⁺⁺, rendering these minerals unavailable (Oscarsson and Savage 2007; Savage et al 2000).

The oxalate contents of the regrowth of maturing petioles and leaves of taro have not been investigated, and it is possible that the levels may change as the leaves mature. Studies carried out earlier (Oscarsson and Savage 2007; Savage et al 2000) showed that the total oxalate content of older taro leaves was higher compared to freshly-harvested young leaves (589 mg compared to 443 mg/100 g fresh weight). Related observations were made by Watanabe et al (1994) who showed that the oxalate concentration in spinach leaves harvested in winter (growing more slowly and more mature) were higher than in rapidly growing leaves harvested in autumn (740 mg/100 g versus 530 mg/100 g FW).

Taro plantations in Central Vietnam are frequently harvested by cutting out the main plant and allowing the daughter plants to regrow for the next harvest. As taro plants grow rapidly in this region this harvesting can occur every 14 days. The biomass yield and soluble and insoluble oxalate content of the repeated regrowth of the stems and petioles of taro has not previously been measured in controlled experiments. Therefore, the objective of this study was to measure the biomass yield and the oxalate content of the petioles and leaves of two commonly grown taro cultivars when these were harvested repeatedly at 14-day intervals.

The objective of this study was to measure the oxalate content of the petioles and leaves of two commonly used taro cultivars when these were harvested repeatedly at 14-day intervals.

Materials and methods

Two taro (Colocasia esculenta L.) cultivars (Ao Trang and Mon Ngot) were grown at two locations having either sandy or clay soils. Initial planting was with the head, and a longitudinal section (20-23cm) of the corm about 5 cm thick, taken from “good” plants in a nearby field. Each cultivar was randomly assigned to four 5 m x 4 m plots in each soil type. There were thus 8 plots with a total area of 200 m² at each location. Each plot was fertilized with 200 kg pig manure, 10 kg NPK (10:12:15) at the beginning and with 200 liters of wash water from the pig pens after each harvest. During the experiment the soil was always kept moist. The first harvest (Photo1) was carried out 30 days after planting and then every 14 days after that. The plants were harvested by cutting the mature petioles and leaves at 20 to 30 mm above ground level (Photo 2) and leaving only one or two daughter shoots to regrow (Photo 3).

Photo 1. Taro plants ready for harvest	Photo 2. Harvesting the mature plants; leaving the daughter plants	Photo 3. Daughter plants at the start of the next growth cycle

At each harvest, the weights of the petioles and the leaves of the two cultivars were recorded. These samples were mixed together and chopped into 10–20 mm pieces. The chopped pieces were then mixed for an additional 20 min. Three representative subsamples of the fresh samples were dried in an oven at 65°C for 18h. Three hundred g of dried material from each variety were then sealed in plastic bags and stored until analysis commenced. Each sample was ground to a fine powder using a Sunbeam Multi-Grinder (Model No. EMO 400, Sunbeam Corporation Limited, Botany, NSW, Australia) and the residual moisture was determined in triplicate by drying to a constant weight in an oven at 105°C for 24 h.

Sample analysis

The contents of total and soluble oxalate in each finely ground sample (~0.5 g) were determined in duplicate using the method outlined by Savage et al (2000). The insoluble oxalate content was calculated by difference (Holloway et al 1989). The final oxalate values of all the samples were converted to g/100 g DM of the original material.

Crude protein

Total nitrogen was determined using the Dumas method and a factor of 6.25 was used to calculate the crude protein content of the petioles and leaves (AOAC, 2000).

Statistical analysis

Data for biomass yield, soluble and insoluble oxalate were analysed using a general linear model in the ANOVA program in Minitab version 16 (Minitab Ltd., Brandon Court, Progress way, Coventry, UK). Sources of variation were: variety, soil type, date of harvest and error.

Results

Biomass yield

Biomass yield of petioles and leaves increased linearly over three repeated harvests at 14 day intervals, following the initial harvest which was 30 days after planting (Table 2; Figure 1). However, at the 4^th harvest, yield decreased relative to the previous one. There is no obvious explanation for the initial increase in yield and the final decrease (at 86 days) as the soil was always maintained in moist condition by irrigation and fertilization with waste water from the pig pens was at similar rates after each harvest.

Figure 1. DM yield of petioles and leaves of taro subjected to repeated harvests at 14 day intervals (first harvest was 30 days after planting)

The levels of crude protein in leaves declined with a curvilinear trend (R ²=1.00) from 25% in DM at the first (at 30 days) to 18% at the last harvest at 86 days (Figure 2). Crude protein in petioles followed a similar pattern, with much lower levels, declining from 10 to 8% in DM with repeated harvests.

Figure 2. Crude protein content of petioles and leaves of Taro harvested at 14-day intervals (1st harvest was 30 days after planting).

Levels of insoluble oxalates were almost twice as high as those for soluble oxalates independent of variety and soil type (Tables 3 and 4; Figure 3), and age at harvest (Figure 4). The overall levels and degree of difference (insoluble/soluble) were much greater for petioles than for leaves. Oxalate levels were consistently higher in the Ao Trang than in the Mon Ngot variety but did not differ between plants grown in sandy or clay soils (Table 2).

Figure 3. Concentrations of soluble and insoluble oxalate in leaves and petioles of two varieties of Taro (average of 5 successive harvests at 14 day intervals)

There was a major effect of harvest date on soluble and insoluble oxalate levels in petioles and on soluble oxalate in leaves, which were highest in the first harvest (30 days after planting) and then decreased linearly over the five succeeding harvests at 14-day intervals (Figure 4). This trend was less uniform for insoluble oxalate contents in leaves.

Figure 4. Concentrations of soluble and insoluble oxalate in leaves and stems of Taro harvested at 14-day intervals after first harvest 30 days from planting

There was a close negative relationship between biomass yield and levels of soluble oxalate in petioles (Figure 5) and leaves (Figure 6). There was a similar close relationship between biomass yield and insoluble oxalate in petioles (Figure 7) but not in leaves (Figure 8).

Figure 5. Relationship between biomass yield and soluble oxalate for taro petioles	Figure 6. Relationship between biomass yield and soluble oxalate for taro leaves
Figure 7. Relationship between biomass yield and insoluble oxalate for taro petioles	Figure 8. No relationship between biomass yield and insoluble oxalate for taro leaves

Discussion

The increase in taro biomass yield with successive harvests is similar, although of a greater order of magnitude, to experiences in the southern region of Lao PDR (Vivasane et al 2012) where taro yielded 574, 887 and 863 kg DM/ha in successive harvests at 84, 112 and 140 days following planting.

The lower concentration of oxalate in the regrowth harvests (from the 30 ^th day through to the 84^th day) compared with the first harvest at 30 days is supported by the observations of Savage et al (2000) and Oscarsson and Savage (2007) that oxalate leaves were lower in young than in older plants. However, the degree and rate of decline in oxalate with repeated harvesting, especially in petioles, have not previously been reported.

We have no explanation for these major changes in the concentration of oxalates due to repeated harvesting of the above ground biomass. This observation merits further research as repeated harvests of the regrowth of the vegetative part of taro plants is a standard method used by farmers in Central Vietnam. Frequent harvesting is done over periods between one and two years. Taro leaves provide a protein-rich fodder which allows farmers to reduce the use of expensive imported soya bean meal which is the most widely used protein source in pig production in tropical countries.

Conclusions

• Biomass yield of petioles and leaves increased linearly over three repeated harvests at 14-day intervals, following the initial harvest which was 30 days after planting. However, at the 4^th harvest, yield decreased relative to the previous one.
  
  
• Levels of insoluble oxalates were almost twice as high as those for soluble oxalates independent of variety, soil type and age of the plant at harvest.
  
  
• The overall levels and degree of difference (insoluble/soluble oxalate) were much greater for petioles than for leaves.
  
  
• Oxalate levels were consistently higher in the Ao Trang than in the Mon Ngot variety but did not differ between plants grown in sandy or clay soils.
  
  
• Soluble and insoluble oxalate levels in petioles and soluble oxalate in leaves were highest in the first harvest (30 days after planting) and then decreased linearly over the five succeeding harvests at 14-day intervals. This trend was less uniform for insoluble oxalates in leaves.
  
  
• There were negative relationships between biomass yield and content of soluble and insoluble oxalate in petioles and between yield and soluble oxalate in leaves. There was no relationship between yield and insoluble oxalate in leaves.

Acknowledgements

The authors would like to thank the farmers Nguyen Thi Xuan and Nguyen Thi Nhung who looked after the taro and the students Nguyen Van Bau, Le Thuong, Nguyen Van Hue and Le Thi Thao Nguyen who harvested the taro forage, prepared the silage and took samples for analysis. This research was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) (Grant number 106-NN.05-2013.31).

References

AOAC 2000 Official Methods of Analysis International, 17th edition. Gaithersburg, MD: Association of Official Agricultural Chemists.

Hang D T, Hai P V, Hai V V, Ngoan L D_, Tuan L M and Savage G P 2017 Oxalate content of taro leaves grown in Central Vietnam. Foods 6 (1):2.

Holloway W D, Argall M E, Jealous W T, Lee J A and Bradbury J H 1989 Organic acids and calcium oxalate in tropical root crops. Journal of Agriculture, Food and Chemistry 37: 337-341.

Oscarsson K V and Savage G P 2007 Composition and availability of soluble and insoluble oxalates in raw and cooked taro (Colocasia esculenta var. Schott) leaves. Food Chem 01(2): 559-562.

Savage G P, Vanhanen L, Mason S M and Ross AB 2000 Effect of cooking on the soluble and insoluble content of some New Zealand foods, J Food Comp Anal 13(3): 201-206.

Vivasane S, Southavong S, Vyraphet P and Preston T R 2012 Effect of biochar and biodigester effluent on growth performance of taro ( Colocasia esculenta). Livestock Research for Rural Development. Volume 24, Article #107. http://www.lrrd.org/lrrd24/6/viva24107.htm

Watanabe Y, Uchiyama F and Yoshida K 1994 Compositional changes in spinach (Spinacia oleracea L.) grown in the summer and the fall, J Jap Soc Hort Sci 2: 889-895.

Received 5 March 2018; Accepted 13 April 2018; Published 1 May 2018

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