Effect of variety and wilting on HCN content of cassava leaves and on intake, digestibility and N retention by growing pigs

Chhay Ty, T R Preston^* and Khieu Borin

Centre for Livestock and Agriculture Development (CelAgrid), PO box 2423, Phnom Penh, Cambodia
chhayty@celagrid.org
^*UTA (Colombia), AA #48, Socorro, Santander, Colombia

Abstract

Two experiments were carried out to study the effect of duration of wilting (0, 24 or 48 h) on HCN content of leaves from "sweet" and "bitter" varieties of cassava and on their nutritive value for pigs as measured by digestibility and N balance. In the first experiment, the duration of wilting was 0, 24 or 48 h. In the second experiment, the leaves from the bitter and sweet varieties were fed to pigs, either fresh or after 24 h wilting, as the only supplementary source of protein in diets based in broken rice and sugar palm juice.

The bitter variety was higher in DM, N and HCN than the sweet variety. However, there were significant interactions between variety and time of wilting. DM increased and HCN decreased at a faster rate in the bitter than in the sweet variety. Wilting cassava leaves for up to 48 h reduced HCN content to minimal values, the rate of decrease being more pronounced in the bitter than in the sweet variety, with no differences between varieties after 48 h. There were no differences in the nutritive value for pigs, between bitter and sweet varieties, nor were there benefits from wilting for 24 hours, as measured by apparent digestibilities of DM, crude protein and crude fibre, and by N retention. The fresh cassava leaves provided about 20% of the diet DM, which resulted in HCN intakes of from
3.08 to 3.19 mg/kg LW for the diets with fresh cassava leaves, and from 1.85 to 2.16 when the leaves were wilted 24 h.

It is concluded that amino acid imbalance, levels of cell wall constituents and condensed tannins are more important than HCN levels in limiting the usefulness of cassava leaves for pigs.

Key words: Cassava leaves, HCN, varieties, wilting

Introduction

Varieties of cassava are traditionally designated as "bitter" or "sweet" considered to reflect different levels of cyanogenic glucosides, the precursors of the highly toxic cyanic acid (HCN). Normally, the roots of the sweet variety are used for human consumption and are boiled before being eaten. The bitter variety is considered to require some additional form of processing, such as sun-drying, before it is used as animal feed. Cassava leaves have been claimed to be a very important component of integrated farming systems in tropical countries (Preston 2001). In this connection, some constraints to cassava utilization in pig feeding, such as the cyanogenic glucosides present in the plant (Gómez and Valdivieso 1985; Ravindran and Ravindran 1988) can be neutralized successfully after ensiling (Ravindran 1992, Chhay Ty et al 2001) and sun drying (Bui Huy Nhu Phuc et al 1996). However, these methods can be time-consuming and in the case of sun-drying are difficult in the rainy season. Wilting under a roof promises to be a simpler and more reliable procedure.

The aim of this study was to study the effects of wilting under a roof on the content of HCN in two local cassava varieties, characterized by being either "sweet" or "bitter", and to evaluate possible effects on digestibility and N retention in growing pigs.
 

Materials and methods

Location and climate

The experiments were conducted in CelAgrid (Centre for Livestock and Agriculture Development), which is located in Kandal village, Rolous commune, Kandal Steung districk, Kandal province in Cambodia in June 2006. During the study, the mean temperatures in the shade were 29 ⁰C (8.00am), 34 ⁰C (12.00am) and 30 ⁰C (6.00pm)

Experiment 1

Two local varieties of cassava, one considered to be "bitter" (supposedly high in HCN precursors) and the other "sweet" (low in HCN precursors) were planted in the experimental cropping area of CelAgrid. Fertilization was with effluent from a biodigester loaded with pig manure. Both varieties were harvested after 2-2.5 month's growth when they had reached about 1.2m in height. Following removal of the petioles and stems, a representative sample of the leaves from each variety was analyzed immediately for content of HCN, DM, N and water-extractable DM and N. About 5 kg of fresh leaves of each variety were then chopped finely and a sample taken and analyzed immediately for HCN and DM. The remaining chopped leaves were spread out in a layer of 5 cm in the open air and left to wilt for 48 hours. At 4 hour intervals during the daytime the leaves were turned to facilitate exposure to sunlight. Samples were taken after 24 and 48 hours to determine HCN, DM and water extractable DM and N.

Treatments

There were six treatments with two varieties and three methods of processing

Varieties

S: Sweet

B: Bitter

Wilted

W0: not wilted

W24: wilted 24 hours

W48: wilted 48 hours

Chemical analyses

DM was determined by microwave radiation (Undersander et al 1993). N was determined according to AOAC (1990) procedures and water extractable DM and N following the method of Ly and Preston (1997). The HCN was determined by titrating with AgN0₃ after boiling the sample in chloroform and reaction with 0.1N KOH.

Statistical analysis

The data were subjected to analysis of variance according to the general linear model of the Minitab software (Minitab 2000). Sources of variation were: varieties, wilting time, interaction varieties*wilting time and error.

Experiment 2

Treatments and design

The experiment was done with four castrated male pigs (Local*Large White) weighing on average 20 kg. The four treatments in a 2*2 factorial within a 4*4 Latin square arrangement (Table 1) were:

Varieties

• Bitter (BC)
• Sweet (SC)

Wilting

• 0h (no wilting)
• 24h (wilted for 24 h under shade)

The pigs were housed in metabolism cages, installed in an open stable and constructed so as to facilitate the quantitative collection of faeces and urine. The characteristics of the cages have been described elsewhere (Chhay Ty et al 2003). Each experimental period consisted of 12 days, of which the first seven days were for adaptation to the diets followed by another five days for collection of faeces, urine and feed refusals.

Experimental feeds

The daily dry matter (DM) allowance was calculated at 4-5% of the body weight according to observation of the intake (Table 2) and was divided between the cassava leaves (fresh or wilted 24 hours), broken rice (offered at 2% of live weight) and sugar palm juice (offered at 1% of live weight (all on DM basis).

Leaves from both cassava varieties were harvested every day beginning 3 months after planting. Stems and petioles were removed from the leaves, which were then chopped into small pieces and offered immediately (0h) to the pigs or were wilted in the shade for 24 h, after chopping. Broken rice was purchased from the local rice mill and sugar palm juice was purchased from the market.

The pigs were fed thrice daily with approximately equal rations at 8:00 am, 12:00 am and 3:00 pm. Water was permanently supplied through drinking nipples.

Data collection

The animals were weighed at the beginning of the trial and every 12 days. Feed refusals and faeces were collected every day and were kept frozen in plastic bags until analysis. A representative sample (10% of amount voided) was obtained from every animal. At the end of each period, samples of feed refusals and faeces were mixed thoroughly by hand and then homogenized in a coffee grinder, prior to taking representative samples that were analyzed for DM, N, crude fibre, HCN and ash.. Urine was collected in a plastic bucket to which sulphuric acid was added to maintain the pH below 4.0 (10% of concentrated H₂SO₄). The volume of urine was measured every day and 10% preserved until the end of each period when the samples were mixed together and analyzed for N.

Chemical analyses

Chemical analyses of the feed ingredients and faeces were undertaken following the methods of AOAC (1990) for ash, N and crude fibre (Table 3).

The DM content was determined using the microwave method of Undersander et al (1993). The N content of urine was determined by the AOAC (1990) procedure. Fresh and wilted cassava leaves were analyzed for HCN by the procedure of AOAC (1990).

Statistical analyses

The data were subjected to analysis of variance according to the general linear model of the Minitab software (Minitab 2000). Sources of variation were: pigs, period, variety, wilting time, interaction between varieties*wilting time and error.

Results

Experiment 1

The bitter variety was higher in DM, N and HCN than the sweet variety (Table 4), on the basis of the average of all wilting times,.

However, there were significant interactions between variety and time of wilting (Table 5).

DM increased (Figure 1) and HCN decreased (Figure 2) at a faster rate in the bitter than in the sweet variety.

The proportions of the DM and the N extracted by washing (Table 6; Figure 3) did not differ between varieties but decreased markedly with increased time of wilting.

Experiment 2

DM intake was not affected by variety nor by wilting (Table 7).

However, there were significant interactions between variety and wilting, with higher intakes when sweet cassava leaves were fed fresh and lower values when the leaves were wilted for 24 h (8). Overall intakes of the cassava leaves accounted for some 20% of the diet DM (Figure 4), which was only about half of the planned proportions of 39.5% (Table 3). As a result the crude protein content of the diets (9.5 to 10.6% in DM) was lower than what has been indicated as the optimum (12 to 14% in DM) for growing pigs of 20 kg live weight fed diets in which all the supplementary protein is from vegetative sources (Thim Sokha et al 2007).

The intakes of HCN, as dietary concentrations and per unit live weight of the pigs (Table 7), were reduced by wilting but did not differ between bitter and sweet varieties. There were no interactions between the effects of wilting and variety for these traits (Table 8).

Coefficients of apparent digestibility were not affected by wilting nor by variety (Table 9).

Intake of N was higher, as was urinary excretion, for 24 h wilting compared with fresh leaves but there were no differences in N retention. Variety had no effect on the N balance components and there were no interactions between wilting and variety (Table 10).

Discussion

According to the study of Du Thanh Hang (1998), it is convenient to wilt cassava leaves one day before ensiling as this helps to reduce the HCN content. It is known that the destruction of cell wall structures by different methods favors the intracellular reaction of linamarase with the cyanogenic glucosides present in cassava, thus contributing to a rapid HCN elimination from the material (Ravindran 1992). It has been suggested that 100 mg HCN/kg feed, as indicated by the Council of the European Community (Gómez 1991), should be the permissible maximum level. However, there is no clear evidence concerning the supposedly toxic levels of HCN for pigs. In fact, neither in growing-fattening pigs (Du Thanh Hang and Preston 2005; Chhay Ty and Preston 2005, 2006; Sarwat et al 1988) nor in breeding sows (Tewe and Maner 1981) are there reports of death of animals caused by ingestion of fresh cassava leaves.

The reported toxic levels of HCN (mg/kg live weight) for pigs are variable among different authors. Proposed toxic levels are 1.4 (Getter and Baine 1938), 2.1 to 2.3 (Johnson and Ramond 1965), 4.4 (Butler 1973) and 3.5 (Tewe1992). In the present study the HCN intakes were from 3.08 to 3.19 mg/kg LW for the diets with fresh cassava leaves, and from 1.85 to 2.16 when the leaves were wilted 24 h. The HCN intakes on the diets with fresh cassava leaves are at the high end of the levels reported to be toxic in pigs. However, there were no symptoms of ill-health and no apparent relationship between intake of HCN and production response as measured by DM intake and N retention. These findings are supported by the results of Du Thanh Hang and Preston (2005) and Chhay Ty and Preston (2005, 2006) that production responses in pigs fed fresh cassava leaves are not related with levels of ingestion of HCN.

It is suggested that other factors limiting usefulness of cassava leaves for pigs, of greater importance than HCN levels, are the well-known amino acid imbalance in cassava leaves (Eggum 1970) and the relatively high proportion of cell wall constituents (González et al 1999) and condensed tannins (Reed et al 2006). This hypothesis is supported by the report of improved growth rate and feed conversion in pigs, when fresh cassava leaves were mixed with equal parts of fresh water spinach (Ipomoea aquatica) foliage (Chhay Ty and Preston 2006), as water spinach protein is relatively richer in methionine-cystine, and there is a lower content of fibre, compared with cassava leaves. Du Thanh Hang et al (2007 Missing) also reported improved production performance of pigs when diets with 20% fresh cassava leaves (DM basis) were supplemented with 0.2% DL-methionine.

The proportion of the DM extracted from dried samples after 90 minutes in a semi-automatic washing machine has been found to be a good predictor of N retention in goats fed a range of tree foliages (Daovy et al 2007). However, such a relation was not apparent in the present study, where the marked decrease in the water extractable DM as wilting time increased (Experiment 1; Figure 3), was not reflected in the performance of the pigs in Experiment 2 when N retention was not affected by wilting time.
 

Conclusions

• The HCN content in fresh cassava leaves was higher in the "bitter" variety compared with the "sweet" variety.
  
  
• Wilting cassava leaves for up to 48 hours reduced HCN content to minimal values the rate of decrease being more pronounced in the bitter than in the sweet variety, with no differences between varieties after 48h
  
  
• There were no differences in the nutritive value for pigs, between bitter and sweet varieties, nor were there benefits from wilting for 24 hours, as measured by feed intake, apparent digestibilities of DM, crude protein and crude fibre, and by N retention.
  

Acknowledgments

The senior author would like to express his gratitude to the International Foundation for Science (IFS), for financing the research as part of the project B 3759-1 and to the Center for Livestock and Agriculture Development (CelAgrid), for providing resources for conducting the experiments.

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Received 22 July 2007; Accepted 17 August 2007; Published 5 September 2007

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