Effect of processing cassava leaves and supplementing them with DL-methionine, on intake, growth and feed conversion in crossbred growing pigs

Chhay Ty, Khieu Borin and T R Preston*

Center for Livestock and Agriculture Development (CelAgrid),
Pras Teat village, Rolous commune, Kandal Stung district, Kandal province. PO Box 2423 Phnom Penh 3, Cambodia
chhayty@celagrid.org
* Finca Ecológica, TOSOLY, AA #48, Socorro, Santander, Colombia

Abstract

Eighteen crossbred (local x landrace) castrated male pigs with initial mean  body weights at 24.5±0.84 kg (range of 19 to 30 kg) were allocated to individual pens in 3 blocks according to body weight, and within blocks to a 2*3 factorial arrangement of 6 treatments. The first factor was with or without supplementary DL-methionine; the second factor was processing cassava leaves (dried, ensiled or fresh). The other dietary ingredients were rice bran, cassava root silage and dried fish. The feeding trial lasted for 90 days from 03^rd November 2008 to February 2009.

There was no health problem in any of the pigs during the whole of experiment. Total DM intake was increased by supplementation with DL-methionine and was highest when cassava leaves were ensiled. Cassava leaves provided close to 50% of the diet DM  and over 60% of the dietary crude protein.  HCN levels were highest on the treatment with fresh cassava leaves and lowest when the leaves were sun-dried. Growth rates were increased by methionine suppplementation and tended  to be lower when the cassava leaves were processed by sun-drying. There were no interactions between methionine supplementation and cassava leaf processing.  Feed conversion ratio was improved when the cassava leaves were fed fresh compared with sun-drying, with intermediate values when the leaves were ensiled. There was no relationship between intake of cyanogenic glucosides (measured in terms of potential release of HCN) and performance traits.

Key words: Drying, ensiling, fresh leaves, HCN, toxicity

Introduction

Early maturing cassava varieties (sweet) are cultivated by most rural families in Cambodia for root production. The leaves are seldom used for other purposes especially for animal feed. Cassava leaves are reported to be rich in lysine but limiting in DL-methionine (Eggum 1970). The cyanogenic glycosides in cassava leaves have been considered to be a limitation to their use for mono-gastric animals (Tewe 1992). Levels of hydrocyanic acid (HCN) produced by the action of hydrolytic enzymes on the cyanogenic glycocides that are present in the plant are influenced by the nutritional status and age of the plant (Ravidran and Ravindran 1988). However, several studies (Chhay Ty and Preston 2005a; Du Thanh Hang and Preston 2005; Du Thanh Hang et al 2009a,b) have shown that cassava leaves can be fed in the fresh state with no apparent toxicity. It has been suggested that the acid condition in the pig stomach may de-activate the enzymes that cause the release of the HCN (R A Leng, unpublished observations). Elemental suphur and the sulphur amino acids, cysteine and methionine, may also play a role in facilitating the detoxification of HCN into the non-toxic thiocyanate (Oke 1978).

The hypothesis that was tested in the following experiment was that in the case of feeding pigs with the fresh cassava leaves, there would be improvements in performance by also providing supplementary DL-methionine. 

The objective of the study was to determine the effect of different ways of processing cassava leaves and supplementing them with DL-methionine, on intake, growth and feed conversion in crossbred growing pigs in Cambodia

Materials and methods 

Location 

The experiment was carried out from 03^rd November 2008 and finished in 01^st February 2009 at the Center for Livestock and Agriculture Development (CelAgrid), located in Phras Teat village, Rolous Commune, Kandal stung district, Kandal province about 25km from Phnom Penh City, Cambodia. 

Experimental animals, treatments and design 

Eighteen crossbred (local x landrace) castrated male pigs with mean body weight of 24.5±0.84 kg (range from 19 to 30 kg) were allocated to individual pens in 3 blocks according to body weight, and within blocks to a 2*3 factorial arrangement of 6 treatments. The first factor was supplementary DL-methionine (with or without) and the second factor was processing cassava leaves (dried, ensiled or fresh).

Individual treatments were

• DCL: Dried cassava leaves without supplementary DL-methionine
• ECL: Ensiled cassava leaves without supplementary DL-methionine
• FCL: Fresh cassava leaves without supplementary DL-methionine
• DCL-M: Dried cassava leaves plus 0.3% DL-methionine
• ECL-M: Ensiled cassava leaves plus 0.3% DL-methionine
• FCL-M: Fresh cassava leaves plus 0.3% DL-methionine

The other dietary ingredients were rice bran, cassava root silage and dried fish (Tables 1 and 2).

The pigs were housed in individual pens (1.5m and 2m) with concrete floor and provided with feeders and drinking nipples. They were vaccinated against salmonella and swine fever and were de-wormed with ivomectin prior to being adapted to the feeds and the housing for 10 days before starting the experiment.  

Feeds, feeding and management 

Rice bran was purchased from the rice mill near CelAgrid center and cassava leaves and roots from farmers who plant cassava along the banks of the Mekong River. Cassava leaves were harvested when the plant was 3-4 months of age; roots were harvested after 6 months. The cassava variety, known locally as "Damlong Kor" was a "sweet" variety. Dried fish and DL-methionine were purchased from animal feed shops in Phnom Penh city. 

Stems and petioles were removed from the cassava foliage prior to processing by sun-drying (3-4 days exposure to sunlight) or ensiling. The sun-dried leaves were ground in a hammer mill. Ensiling of the leaves was done with 10 kg of rice bran plus 0.5g of salt for 100kg of fresh leaves and stored for one month before feeding. Cassava roots were cleaned with water and chopped and ensiled with 0.5 kg of salt for 100 kg of fresh roots. On the FCL treatments, the fresh cassava leaves, after removal of the stems and petioles, were chopped into small pieces and offered immediately to the pigs. The daily feed allowance was given in three meals at 7:30, 12.00 and 16.00h. The rice bran, cassava root silage, dried fish and DL-methionine were mixed together and given first followed by the cassava leaves which were placed in separate troughs. The total quantities offered were based on an expected total daily intake of 40 g DM per 1 kg LW.  The basal  diet (excluding the cassava leaves) was offered at approximately 28 g/kg LW and was fed first. When the pigs had eaten all the basal diet the cassava leaf component was fed at about 12 g DM/kg live weight. These offered levels were varied according to recorded intakes with the objective to minimize refusals and maintain the balance between basal diet and the cassava leaf fraction.
 

 Data collection and analyses 

The pigs were weighed every 10 days during the 90 days of the experiment. Feeds offered and residues were recorded daily. Representative samples of feeds offered and residues were taken one time per 10 days to estimate content of DM, N and HCN.  DM was determined using the micro-wave procedure of Undersander et al (1993). N and HCN were analyzed following procedures of AOAC (1990).  

Statistical analysis 

Data of DM feed intake, weight gain and feed conversion rate were analyzed using the general linear model (GLM) option of the ANOVA software of Minitab 2000 version 13.31 (Minitab 2000). The sources of variation were blocks, processing,  methionine, interaction processing * methionine and error. When the "F" test was significant (P<0.05) the means were separated by the Tukey test in the Minitab software. The model used was:

Y_ij= μ+ρ_j + α_i+e_ij

Where Y_ij: dependent variable,

μ: overall mean,

ρ_j: effect of block, j: 1-3,

α_i: effect of treatment,  i: 1-6 

e_ij: experiment error

Results and discussion 

Feed intake 

Total DM intake was increased by supplementation with DL-methionine and was highest when cassava leaves were ensiled. Cassava leaves provided close to 50% of the diet DM (Figure 1)  and over 60% of the dietary crude protein (Figure 2). Earlier experiments with cassava leaves in pig diets were  focused on reducing the risk of toxicity from hydrocyanic acid, arising from the cyanogenic glucosides present in high concentrations in the fresh leaves of most cassava varieties (Bui Huy Nhu Phuc et al 2001).  In this experiment, HCN levels were highest on the treatment with fresh cassava leaves and lowest when the leaves were sun-dried. Expressed as daily intakes per kg LW, the HCN levels on the treatment with fresh cassava leaves were higher than the levels  of 1.4 (Getter and Baine 1938) and 2.1 -  2.3  (Johnson and Ramond 1965) but slightly lower than the level recommended by Butler (1973) (4.4)  although similar to that recommended by Tewe (1992) (3.5 mg/kg LW).  However, in contrast to the reports by these authors, in this experiment fresh cassava leaves were fed with no signs of toxicity (Table 3) despite the high HCN levels.   

Live weight gain and feed conversion

Growth rates were increased by methionine supplementation and tended (P=0.10) to be lower when the cassava leaves were processed by sun-drying (Table 4). There were no interactions between methionine supplementation and cassava leaf processing. The growth curves (Figures 3 and 4) for the pigs on the different treatments suggest that the beneficial effects of DL-methionine and of fresh compared with dried cassava leaves were enhanced the longer the pigs were fed the diets. 

 Feed conversion ratio was improved when the cassava leaves were fed fresh compared with sun-drying, with intermediate values when the leaves were ensiled. The improved growth rate that resulted from DL-methionine supplementation supports the findings of Du Thanh Hang et al (2009a) and Chhay Ty et al (2009). The lack of interaction between DL-methionine supplementation and processing was in contrast to a previous study, when DL-methionine supplementation was only effective with fresh cassava leaves and not with leaves wilted for 24h (Chhay Ty et al 2009). The lack of relationship between intakes of cyanogenic glucosides and performance traits confirmed several findings that fresh cassava leaves can be fed safely to growing pigs (Chhay Ty and Preston 2005a; Chhay Ty et al 2009; Du Thanh Hang and Preston 2005; Nguyen Thi Hoa Ly 2005; Nguyen Thi Loc and Le Khac Huy 2003). The fact that growth rate and DM conversion rates were best for the treatment with fresh leaves and poorest for sun-dried leaves, is in marked contrast with estimated HCN intakes which were highest on fresh leaves (3.52 mg/kg live weight) and lowest on sun-dried leaves (1.04 mg/kg live weight). These findings indicate that sun-drying or ensiling of cassava leaves may have negative effects on the biological value of the leaves. The fermentation taking place in ensiled forages converts soluble carbohydrates to organic acids and also promotes proteolysis (McDonald 1981). Oshima and McDonald (1978) considered that, after ensiling, 10 to 25% of the total nitrogen in fresh forages would be non-protein nitrogen.  There appear to be no comparable data on the ensiling of cassava leaves but it is probable that some loss in protein quality occurs during the ensiling process.

The detoxification of HCN in the animal body needs a sulphur-donor to facilitate the conversion of  hydrogen cyanide to non-toxic thiocyanate (Oke 1973). DL-methionine would fulfill this purpose and this could be the explanation for the better growth rate when this synthetic amino acid was added to the diets. However, the lack of an interaction between methionine supplementation and cassava processing, does not supportive such an hypothesis, especially when HCN intakes were almost 3 times higher when the cassava leaves were fed fresh compared with the sun-dried form. The improvement in the amino acid balance resulting from methionine supplementation would seem to be a more probable explanation for the beneficial effects of methionine supplementation on growth rates for all the diets (Figure 5).  

Conclusions

• Growth rates were higher for DL-methionine supplementation and were better when the cassava leaves were fed fresh compared with being sun-dried.

• DM feed conversion was not influenced by DL-methionine supplementation but was best when cassava leaves were fed fresh with poorest conversion registered for the treatment with sun-dried leaves.

• There was no relationship between intake of cyanogenic glucosides (measured in terms of potential release of HCN) and performance traits.

Acknowledgments

The Center for Livestock and Agriculture Development (CelAgrid) would like to express gratitude to the CIAT project, which provided the fund for conducting this experiment. We also thank Dr Reinhardt Howler and Dr Tin Mauna Aye for their support. 

References 

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Received 11 August 2010; Accepted 5 March 2011; Published 1 April 2011

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