Effect of processing Taro leaves on oxalate concentrations and using the ensiled leaves as a protein source in pig diets in central Vietnam

Du Thanh Hang and T R Preston*

Faculty of Animal Husbandry and Veterinary Medicine, Hue University of Agriculture and Forestry, Hue City, Vietnam
hangduthanh@yahoo.com.vn
* TOSOLY, AA48 Socorro, Colombia

Abstract

Oxalate levels in 3 species of taro were higher in petioles (range of 1326 to 3567 mg/100 g DM) than in leaves (770 to 2531 mg/100g DM), and within each plant part, values were highest for Alocacia odera and lowest for Xanthosoma nigra, with intermediate values for Colocacia esculenta. Sun-drying, soaking, cooking and ensiling all reduced the concentration of oxalate but the effects were most pronounced (50% reduction) for cooking and ensiling. 

DM and crude protein intakes of pigs fed a basal diet of maize, rice bran, cassava root meal and fish meal did not differ when ensiled taro leaves (ETL) replaced fish meal at up to 30% silage in the diet DM;  however, intake of fiber increased linearly with replacement rate of ETL. Growth rate was reduced only slightly up to the 30% inclusion rate of ETL, at which point the growth rate was markedly reduced. The trends for feed conversion were similar to those for weight gain.

The results suggest that ETL can replace up to 30% of the dietary fish meal (20% ETL in the diet DM) in diets for growing pigs without loss of performance.

Keywords: Alocacia, boiling, Colocacia, cooking, conversion, growth rate, itching, soaking, Xanthosoma

Introduction

Taro (Colocasia esculenta) is a tropical crop widely cultivated in Central Viet Nam usually in small plots near the house. When provided with adequate water, biomass  yields of over 250 tonnes/ha/year have been recorded (Ngo Huu Toan and Preston  2008; Hoang Nghia Duyet 2010, unpublished observations). This ability of taro to grow in waterlogged conditions allows for the utilization of soils with poor drainage capacity, which are unsuitable for other crops. Vietnamese and Cambodian farmers traditionally feed the leaves and petioles to pigs but usually after cooking, a practice which has been observed to overcome the “itching” produced on skin surfaces that come in contact with the fresh foliage, especially the petioles (Hoang Nghia Duyet  2010, unpublished observations; Pheng Buntha et al 2008).

The “itching” characteristic of Taro plants is caused by the presence of crystals of calcium oxalate (Jiang Gaosong 1996), the concentrations of which in Colocacia esculenta have been reported to range from very high values in young leaves (12,576 ± 108mg/100g DM)  (Radek and Savage 2008) to levels of 236 mg/100 g fresh leaves (about 1560 mg/100 g DM) (Savage and Dubois 2006). According to Oscarsson and Savage (2006), there are two forms of  oxalate in taro, one soluble and the other insoluble. He found that the oxalate content was higher in young leaves (589±36 mg/100 g fresh basis) than in older leaves (433±15 mg/100 fresh basis) and that soluble oxalate was 74% of the total oxalate in the leaves. In the study of Mårtensson  and Savage (2008), taro leaves were reported to contain 524±21.3 mg/100 g fresh weight of total oxalates (presumably as calcium salts) and 241±21 mg/100 g fresh weight in the soluble form.

The aim of the present study was: (i) to estimate the oxalate content of the different varieties/species of taro commonly grown by farmers in Central Vietnam; (ii) to evaluate different methods of processing the leaves on the content of oxalate; and (iii) to determine the effects on growth performance of pigs of partial replacement of dietary fish meal by ensiled leaves of taro.

Material and methods

Experiment 1. Composition of taro varieties commonly grown in Central Vietnam

Leaf and petiole samples of the six taro varieties widely cultivated by farmers in Thua Thien Hue province were analysed for oxalate and crude protein. They were harvested on the same day from the garden of a farmer in Thuy An village in Hue city. After drying at 65 °C for 48 h they were analysed for crude protein and oxalate by High-pressure Liquid Chromatography using the method of Martz et al (1990).

Experiment 2. Effect of processing method on chemical composition of the leaves of taro (Ao trang) (Colocasia esculenta)

The leaves of Ao trang taro variety (Colocasia esculenta) were harvested from a pond in Thuy An village at 7.00am and chopped into 1-2 cm pieces prior to applying the following treatments:

Drying: Chopped leaves were dried for 48h at in an oven at 65⁰C (control)

Cooking:  Chopped leaves were put in boiling water with ratio: 3 part leaves: 2 parts water (fresh weight) for 4-5 minutes and then placed in a basket for wilting for 15 minutes.

Sunlight: Chopped leaves were dried under sunlight for 2 days with temperature around 34⁰C.

Soaking: Chopped leaves were soaked in water with ratio: 3 parts water: 1 part leaves (fresh weight) during 3 hours and then wilted in a basket.

Ensiling: Chopped leaves were wilted overnight under a roof of house, then mixed with 3% molasses (fresh weight) and packed in air-tight plastic bags for 21 days.

There were three replicates of each treatment.

After processing all samples were dried in an oven at 65⁰C for analysis of crude protein, crude fiber, lipids and ash following AOAC (1990) methods. Oxalate was determined by the method of Martz et al (1990).

Experiment 3. Replacing fish meal with different levels of ensiled taro leaves in diets of growing pigs

Location

The experiment was carried out in the experimental farm of the Hue University of Agriculture.

Experimental design, diets and feeding

The control diet consisted of rice bran, maize, cassava root meal and fish meal (FM). The three test diets were formulated by replacing part of the fish meal of the control diet with 0, 10, 20 and 30% of ensiled taro leaves (DM basis) (Table 1).  

Animals and management

Sixteen crossbred pigs (Mong Cai female x Large White male) aged about 65 days and with mean body weight of 18 ± 1.7 kg (mean ± SD) were housed in individual cages. They were vaccinated against hog cholera and pasteurellosis, and de-wormed 2 weeks before being randomly allocated to the four dietary treatments.

The taro leaves were from the Ao Trang variety that is most common in central Vietnam. They were chopped and wilted under a roof for 12h and ensiled with 3% molasses then stored 21 days before feeding.

The meal and silage components were offered separately in amounts that led to minimum refusals of the ensiled taro leaves.. Feeding times were 6:00, 10.30 and 17.30 for the ensiled taro leaves and 8:00, 13:00 and 19:00h for the other components of the diets. The refusals were collected before each new meal was offered. Drinking water was freely available.

Measurements

Records were kept of amounts of feeds offered and refused. Live weights were taken every 20 days and growth rates calculated from the slope of the linear regression of weight against days on experiment.

Chemical analyses

Samples of feed and refusals were dried at 60⁰C for 24h and ground through a 1 mm sieve prior to chemical analysis according to the standard methods of AOAC (1990). Dry matter (DM) was measured by drying fresh samples at 105⁰C for 24h. Crude protein was determined on fresh samples by the Kieldahl method (N* 6.25). Ether extract (EE) was determined by Soxhlet extraction. Ash was the residue after ashing the samples at 600⁰C. Calcium and phosphoruus were determined according to AOAC (1990) using the dry method and alkalimetric ammonium molybdo-phosphate method, respectively.

Statistical analysis

Analysis of variance was performed using the general linear model (GLM) procedure of Minitab Version 14 (Minitab 2000). Sources of variation were: treatments and error. Regression analysis was applied using the regression function in the Minitab software.

Results  and discussion

Experiment 1. Composition of taro varieties commonly grown in Central Vietnam

Oxalate levels were higher in petioles than in leaves, and within each plant part, values were highest for Alocacia odera and lowest for Xanthosoma nigra, with intermediate values for Colocacia esculenta (Table 2). The order of the different varieties for oxalate corresponded to farmer observations as to which varieties caused the most and the least itching (Hoang Nghia Duyet and Du Thang Hang 2010, unpublished observations).

Experiment 2. Effect of processing method on chemical composition of the leaves of taro (Ao trang) (Colocasia esculenta)

All methods of processing reduced the concentration of oxalate but the effects were most pronounced (50% reduction) for cooking and ensiling (Table 3).

Chai and Liebman (2005) reported that  boiling markedly reduced the soluble oxalate content in taro leaves (by 30-87%)  and was more effective than steaming (5-53% reduction); the losses of insoluble oxalate during cooking varied greatly, ranging from 0 to 74%. Soaking taro for 18 h resulted in a 26% reduction in the soluble oxalate content of the raw leaves (Savage and Dubois 2006). According to these authors, boiling the taro leaves was the most effective way of reducing the soluble oxalate content.

There appear to be few reports on the effect of ensiling the leaves on oxalate content other than observations that the “itching” was no longer a problem when taro leaves were ensiled (Pham Sy Tiep et al 2008; Du Thang Hang and Preston 2009) and that intakes were almost two times higher for ensiled compared with fresh leaves (Du Thang Hang and Preston 2009).. For feeding of live stock, ensiling is obviously more convenient and less energy-intensive than cooking and appears to be equally effective in reducing the oxalate content.

Experiment 3. Replacing fish meal with different levels of ensiled taro leaves in diets of growing pigs

DM and crude protein intakes did not differ among treatments; however, intake of fiber increased linearly with replacement rate of ETL (Table 4) as ETL with 17.1±2.3% fiber replaced fish meal with zero fiber. 

Weight gain decreased in curvilinear fashion as the level of ETL in the diet increased (Figure 1).  It was not much affected by replacement of fish meal by ETL until the 30% inclusion rate of ETL when growth rate was markedly reduced. It would appear that growth rate was constrained by the lower digestibility of the ETL compared with the fish meal, especially of the protein component (see Rodríguez et al 2009). The trends for feed conversion were similar to those for weight gain (Figure 2). 

Figure 1.  Trend in rate of live weight gain according to rate of substitution of fish meal by ETL	Figure 2.  Trend in feed conversion rate according to rate on substitution of fish meal by ETL

Preliminary observations (Du Thanh Hang, unpublished data), in the village where the taro was sourced, indicate that the farmers are enthusiastically applying the ensiling of taro foliage as a simpler and less energy-intensive procedure compared with cooking.  Training courses on the ensiling technology are planned to disseminate the research findings in neighboring villages

Conclusions

• Oxalate levels in 3 species of taro were higher in petioles (range of 1326 to 3567 mg/100 g DM) than in leaves (770 to 2531 mg/100g DM), and within each plant part, values were highest for Alocacia odera and lowest for Xanthosoma nigra, with intermediate values for Colocacia esculenta.

  
  

•  Sun-drying, soaking, cooking and ensiling all reduced the concentration of oxalate but the effects were most pronounced (50% reduction) for cooking and ensiling.

  
  

• DM and crude protein intakes of pigs fed a basal diet of maize, rice bran, cassava root meal and fish meal did not differ when ensiled taro leaves (ETL) replaced fish meal at up to 30% silage in the diet DM;  however, intake of fiber increased linearly with replacement rate of ETL.

  
  

• Growth rate was reduced only slightly by replacement of fish meal by ETL until the 30% inclusion rate, at which point growth rate was markedly reduced. The trends for feed conversion were similar to those for weight gain.

  
  

• The results suggest that ETL can replace up to 30% of the dietary fish meal (20% ETL in diet DM) without loss of performance.

Acknowledgements 

Support for this research from the MEKARM program, financed by SARECSida, is gratefully acknowledged.  We are grateful to the staff of the Industrial Food Institute of Hanoi for the analyses of oxalate.

References  

AOAC 1990  Official Methods of Analysis. Association of Official Analytical Chemists. 15th edition (K Helrick editor). Arlington pp 1230

Chai W and Liebman M 2005 Effect of different cooking methods on vegetable oxalate content Journal  Agriculture  Food Chemistry 53(8):3027-30

Du Thang Hang and Preston T R 2009 Taro (Colocacia esculenta) leaves as a protein source for growing pigs in Central Viet Nam. Livestock Research for Rural Development. Volume 21, Article #164. http://www.lrrd.org/lrrd21/10/hang21164.htm

Jiang Gaosong, Ramsden  L and Corke H 1996  Multipurpose uses of Taro (Colocasia esculenta). Review. In Proceedings of the International Symposium held in Nanning, Guangxi, China. November 11-15, 1996. pp.262-265.

Mårtensson  L and Savage G P 2008  Composition and bioavailability of oxalates in baked taro (Colocasia esculenta var. Schott) leaves eaten with cows milk and cows milk and coconut milk. International Journal of Food Science and Technology  43, 2213–2218

Martz F A, Weiss M F and Belyea R L 1990  Determination of oxalate in forage by reverse-phase high pressure liquid chromatography. Journal of Dairy Science 73; p474-479 http://jds.fass.org/cgi/reprint/73/2/474.pdf

Minitab 2000 Minitab reference Manual release 13.31. User’s guide to statistics. Minitab Inc., USA

Ngo Huu Toan and Preston T R 2008 Taro as a local feed resource for pigs in small scale household condition. Proceedings MEKARN Regional Conference 2007: Matching Livestock Systems with Available Resources (Editors: Reg Preston and Brian Ogle), Halong Bay, Vietnam, 25-28 November 2007 http://www.mekarn.org/prohan/toan_hue.htm

Oscarsson K V and Savage G P 2006 Composition and availability of soluble and insoluble oxalates in raw and cooked taro (Colocasia esculenta var. Schott) leaves. Food Chemistry. Volume 101, Issue 2, 2007, Pages 559-562

Pham Sy Tiep, Nguyen Van Luc, Trinh Quang Tuyen, Nguyen Manh Hung and Tran Van Tu 2006  Study on the use of Alocasia macrorrhiza(roots and leaves) in diets for crossbred growing pigs under mountainous village conditions in northern Vietnam. Workshop-seminar "Forages for Pigs and Rabbits" MEKARN-CelAgrid, Phnom Penh, Cambodia, 22-24 August,  2006. Article # 11. Retrieved July 30, 107, from http://www.mekarn.org/proprf/tiep.htm

Pheng Buntha, Khieu Borin, T R Preston and B Ogle 2008 Survey of taro varieties and their use in selected areas of Cambodia.  Livestock Research for Rural Development. Volume 20, supplement.  http://www.lrrd.org/lrrd20/supplement/bunt1.htm

Radek M and Savage G P 2008  Oxalates in some Indian green leafy vegetables. Institut National Polytechnique de Lorraine, Ecole Nationale Supérieure d'Agronomie et des Industries Alimentaires de Nancy, Vandoeuvre-les-Nancy, France.

Rodríguez Lylian, Peniche Irina, Preston T R and Peters K 2009  Nutritive value for pigs of New Cocoyam (Xanthosoma sagittifolium); digestibility and nitrogen balance with different proportions of fresh leaves and soybean meal in a basal diet of sugar cane juice.  Livestock Research for Rural Development. Volume 21, Article #16.  http://www.lrrd.org/lrrd21/1/rodr21016.htm

Savage G P and  Dubois M 2006 The effect of soaking and cooking on the oxalate content of taro leaves.  International journal of food sciences and nutrition Volume 57 (issue 5-6)  pp 376-381

Received 23 January 2010; Accepted 16 February 2010; Published 1 April 2010

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