Ileal and total tract digestibility in growing pigs fed ensiled taro leaves as partial replacement of fish meal, maize and rice bran

D T Hang, V V Hai, P V Hai, T T Tra, N D Qui¹ and L D Ngoan

Faculty of Animal Husbandry and Veterinary Medicine, Hue University of Agriculture and Forestry, Vietnam
hangduthanh@gmail.com
¹ Institute of Biotechnology, Hue University, Vietnam

Abstract

Taro leaves (Colcacia esculenta) were ensiled alone or with equal parts of taro petioles or 5% molasses. After 21 days, the lactic acid concentrations (% in DM) were 1.27, 1.45 and 1.49, and pH was 4.5, 4.0 and 3.9, for leaves alone, leaves plus petioles and leaves plus molasses, respectively. Ensiling the taro leaves reduced oxalate concentrations by 30% on leaves alone and by 50% when petioles or molasses were added.

When growing pigs equipped with a T-cannula in the distal ileum were fed a control diet (8% fish meal) or the control diet with 50% of the DM replaced by ensiled taro leaves (2% fish meal) , N retention decreased from 19 to 17 g/day but the biological value of the protein was the same on both diets (86 and 87%, respectively). It is concluded that ensiled taro leaves are a protein source of high biological value for growing pigs.

Key words: biological value, oxalate, toxicity

Introduction

Taro (Colocasia esculenta) is well-adapted to the Vietnamese climate and can grow almost everywhere with minimal maintenance, under either dryland or wetland conditions. Taro is one of the few crops that can be grown under flooded conditions. Yields of 370 tonnes/ha/year of foliage (leaves and petioles) were reported by Toan and Preston (2010) with 50% of the dry matter (DM) as leaf. Taro leaves are rich in protein (21-26% of DM) while the petioles are rich in soluble carbohydrates (Hang and Kien 2012; Hang and Preston 2009; Rodríguez and Preston 2009).

Taro contains high levels of oxalates which are important anti-nutritional compounds (Oscarsson and Savage 2007) which cause irritation and burning sensation in the throat and mouth on ingestion (Akpan and Umoh 2004). According to Hang et al (2011), the total oxalate ranged from 2400 to 4420 mg/100g of the DM in petioles and 2021 to 6342 in leaves. Total oxalate can be reduced by simple processing methods such as ensiling, boiling, soaking, wilting and washing (Hang et al 2011; Hang et al 2013). Oxalates can form non-absorbable insoluble salts with Ca²⁺, Fe²⁺, and Mg²⁺, rendering these minerals unavailable (Savage et al 2009).

Ensiling of the leaves with molasses (Tiep et al 2006) or by combining them with the petioles (Hang and Preston 2010) resulted in a considerable reduction in total oxalates. The benefit of ensiling petioles with the leaves appears to be due to the high content of soluble sugars in the petiole (Rodríguez and Preston 2009).

The objectives of this study were to evaluate the changes in chemical composition of taro leaves ensiled alone, with the petioles, or with molasses, and to determine the nutritive value of diets in which ensiled taro leaves provided 50% of the diet for growing pigs replacing a combination of fish meal, rice bran and maize.

Materials and methods

Experiment 1:

Taro leaves were ensiled alone (TL), with the petioles (TLP) or with molasses (TLM). Taro leaves and petioles were harvested from farmers’ gardens and ensiled in plastic bags with capacity of 1.0 kg. Each treatment was replicated three times. Samples were taken at 0, 7, 14, 21 and 56 days for analysis of DM, crude protein (CP), lactic acid, pH and oxalic acid.

Experiment 2:

Ten castrated Large White male pigs (45±2.55 kg) were fed a rice bran basal diet or the basal diet replaced 50% by taro leaves ensiled with molasses (Table 1). They were fitted with simple T-cannulas (Stein et al 1998) and housed individually in metabolism cages. Chromium oxide was used as a digesta flow marker.

For the determination of ileal digestibility, a total of 12 digesta samples from each pig were taken during 2 days of collection. On each collection day, samples were taken every 2 hours between the morning and afternoon feeding, kept on ice and then frozen at -20⁰C. Feces were collected four times per day and stored at -20⁰C. Digestibility of a dietary nutrient/component at each sampling site was calculated using chromium as marker (Sauer et al 2000). Urine was collected in a bucket via a funnel below the cage. To prevent nitrogen losses the pH was kept below 4 by adding 50ml sulphuric acid 25% in the collection bucket.

The chemical composition of diets and ileal and fecal samples was determined by the standard methods (AOAC 1990). NDF and ADF were determined according to Van Soest et al (1991). Cr ₂O₃ in feed, feces and ileal digesta was determined by atomic absorption spectrometry after ashing (Fenton and Fenton 1979). Lactic acid was determined by the method of Ranfft (1973) and oxalate according to Savage et al 2000).

Statistical analysis

The data were analyzed by ANOVA using the General Linear Model (GLM) of Minitab Statistical Software, version 15.1 (Minitab 2000). Tukey pair-wise comparisons were used to determine differences between treatment means at P<0.05.  

Results and discussion

Effect on silage quality

The pH of the silage fell, and of lactic acid increased, more rapidly when the leaves were supplemented with petioles or molasses, reflecting the effect of addition of soluble sugars from molasses and the petioles (Figures 1 and 2). The increase in pH and decrease in lactic acid in the 56-day samples indicate that taro leaf silages should be used not later than 21 days after ensiling.

Figure 1. Changes in pH when ensiling taro leaves alone, with the petiole or with molasses	Figure 2. Changes in lactic acid when ensiling taro leaves alone, with the petiole or with molasses

After ensiling for 21 days, the concentration of oxalic acid was reduced by half (Figure 3), the effect being more pronounced when the petioles or molasses were included along with the leaves.

Figure 3. Changes in oxalic acid content when ensiling taro leaves alone, with the petiole or with molasses

At the optimum stage for ensiling (21 days), there were no differences in pH, nor in concentrations of lactic and oxalic acids between leaves ensiled with petioles and those ensiled with molasses, both being superior to ensiling of the leaves alone (Table 2).

Nutritive value of ensiled taro leaves for pigs

The values for apparent NDF digestibility at the ileum and rectum were lower, when ensiled taro leaves replaced 50% of the control diet (Table 3). There were similar, slight,ly negative, effects for apparent digestibility of crude protein for the ETL diet compared with the control.

Figure 4. N retention for the control and ensiled taro leaf (ETL) diets	Figure 5. N retention as % of N digested for the control and ensiled taro leaf (ETL) diets

The similar values for N retained as percent of N digested in pigs fed the ETL and control diets indicates that the protein in the diet with 50% ensiled taro leaves has a high biological value (Table 4; Figure 5). There were marked decreases in oxalate concentration from ileal to rectal locations (Table 5). This can be explained by the soluble oxalates combining with Na⁺, K⁺ or NH₄⁺ (Noonan and Savage 1999) and the absorption of the soluble oxalate salts in the small intestine via active transport and passive absorption (Hatch and Freel 2005). Oxalate solubility within the small intestine appears to be a critical factor, as evidenced by the propensity of concomitant calcium ingestion to reduce oxalate absorption in the small intestine (Liebman and Chai 1997; Liebman and Costa 2000), presumably by chelating with oxalic acid. A 30% reduction of insoluble oxalate in the rectum may be related to the action of microorganisms in the large intestine.

Conclusions

• Taro leaves can be successfully ensiled by themselves or in combination with petioles or supplementation with 5% molasses.
  
  
• The ensiling process improves the product markedly by reducing oxalate concentration by up to 50% of the original values.

Acknowledgments

This research was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) (Grant number 106-NN.05-2013.31).

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Received 14 April 2016; Accepted 29 May 2016; Published 2 June 2016

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