The effects of simple processing methods of cassava leaves on HCN content, intake and performance by growing pigs

Du Thanh Hang,  N Q Linh, T R Preston^*, H Everts^** and A C Beynen^**

Faculty of Animal Husbandry and Veterinary Medicine,
Hue University of Agriculture and Forestry, Hue City, Vietnam
hangtuat@pmail.vnn.vn
^*UTA-Colombia, TOSOLY, AA # 48, Socoro, Santander, Colombia
**Department of Nutrition, Faculty of Veterinary Medicine, Utrecht University, Utrecht, the Netherlands

Abstract

Cassava leaves collected from a high-yielding variety after 60 days of growth and at root harvest (180 days) were wilted for 24 hours for determination of changes in the DM and HCN content. The HCN level was reduced by 58% after 24 hours with no difference between the two sources of leaves. In a comparison of three processing methods (washing, wilting and ensiling) the HCN level in cassava leaves from a high-yielding variety was reduced by 19% by washing, 74% by wilting for 24 hours and 76% by ensiling for 21 days. Three simple methods to reduce the HCN content of cassava leaves before feeding (washing, chopping and washing; and chopping, washing and wilting) were compared in a feeding trial with 6 pigs of 25 kg mean live weight (3 Mong Cai and 3 F1 [Large White x Mong Cai]) in a double Latin square arrangement 2[3*3) with 21-day periods. The leaves were offered ad libitum in one feed trough with a mixture of ensiled cassava roots and rice bran (2:1 fresh basis) at 80% of ad libitum level in a separate trough.

The fresh cassava leaves were readily consumed, providing 38% of the dietary DM and over 70% of the dietary protein with no effect of processing method on total DM intake, which ranged from 27 to 31 g/kg live weight. Levels of HCN were reduced slightly (16%) by washing and substantially (82%) by wilting, resulting in intakes of HCN of between 6.0 and 15 mg/kg live weight, levels considerably higher than previously reported as safe to avoid toxicity (1.4 to 4.4 mg/kg live weight). There were no apparent symptoms of HCN toxicity. Twelve crossbred (Large White x Mong Cai) pigs with an average initial weight 27.2 kg ± 2.71 kg were allocated to 4 treatments in a random design with one pig per pen. Three test diets (with 0, 0.1 and 0.2% DL-methionine) contained fresh cassava leaves (20% in DM) replacing fishmeal on equal protein basis. The control diet contained 15% fish meal. After 80 days the live weight gain on the treatment with cassava leaves and 0.2% DL methionine was higher and feed conversion better, than for other levels of DL-methionine, and similar to the control.

It is concluded that fresh cassava leaves, chopped and washed before feeding, can be included in ensiled cassava root / rice bran diets for growing pigs at levels of up to 40% of the diet DM, and as the sole source of supplementary protein. Adding 0.2% methionine to a diet with 50% cassava leaves improved the live weight gain and feed conversion ratio.

Key words: Ensiling,  feed intake, fresh cassava leaves, HCN, pigs, methionine, washing, wilting

Introduction

In Vietnam, cassava has been changing its role from being solely a "food crop" to use for starch processing and as animal feed. The total annual cassava starch production in Vietnam is estimated at about 500,000 tonnes, 70% of which is for export and 30% used domestically (Hoang Kim et al 2000).

Cassava is traditionally grown for the production of roots. However, the leaves have become increasingly important as a source of protein for monogastric and ruminant animals (Preston 2001; Wanapat 2001). According to Bui Huy Dap (1987), the average yield of cassava leaves in Vietnam is of the order of 7 to 12 tonnes/ha/year, containing from 500 to 1400 kg of crude protein. Cassava leaves are rich in protein but they are low in sulphur amino acids (Gomez et al 1985). The leaf protein is reported to be limiting in methionine and tryptophan but rich in lysine, with an overall biological value of 49 to 57% (Yves Frochlich et al 2001). By the addition of methionine, the biological value of the protein could be increased to 80% according to Roger and Milner (1963) and Eggum (1970). The advantages of supplementing cassava root diets with methionine and cystine have been demonstrated for monogastric species (Maner and Gomez 1973).

Materials and methods

Effect of chopping and wilting on HCN content

Leaves from a high-yielding cassava variety (Cao san) were collected at 09.00h. The petioles were removed, and the leaves chopped into small pieces (2-3 cm) and then spread out on a plastic sheet under a roof and allowed to wilt until 09.00h the next morning. Samples were taken for analysis immediately after collecting the leaves (09.00h) and again at 12.00h, 15.00h, 18.00h and at 09.00h the following morning.

Other samples of cassava leaves were taken after 60 days of growth and at the time of harvesting the roots after 180 days. For the 60-day growth (CL60), the leaves were collected at a point on the stem about 2/3 of the height of the plant. The leaves taken at the harvesting of the root (CL180) were from the top of the cassava plant.

Washing, wilting and ensiling to reduce HCN content

Leaves from the high-yielding cassava variety (Cao San) were collected, chopped into 2-3 cm lengths and then washed two times in a plastic basin (each time for about 5 minutes with 10 litres water per 2 kg of leaves) and analysed immediately. Leaves from the same variety were chopped and wilted overnight under a roof and then analysed immediately. Other leaves were chopped and ensiled with 5% molasses in air-tight plastic bags (10 kg capacity) for 21 days.

Feed intake of cassava leaves by growing pigs

Treatments and design

Three treatments were compared in a double 3*3 Latin square arrangement using 3 pigs of each of two breeds (Mong Cai and F1 Large White x Mong Cai):

• W: Fresh cassava leaves, washed and fed immediately

• CW: Fresh cassava leaves, chopped into small pieces (2-3 cm) then washed prior to feeding

• CWW: Fresh leaves, chopped, washed and wilted for 24 hours

The basal diet was a mixture of ensiled cassava roots and rice bran (2:1 ratio on fresh basis) with an estimated crude protein content of 6.2% in DM.

Feeds and feeding system

The cassava leaves without petioles were collected at the time of root harvesting (180 days). For the "W" treatment, they were washed two times in a plastic basin (each time for about 5 minutes with 10 litres water per 2 kg of leaves). For "CW" the leaves were chopped into small pieces (2-3 cm) and then washed as in "W". For "CWW" processing was as for "CW" followed by wilting over-night under a roof.

Whole cassava roots were purchased from a local farmer. They were washed, ground and mixed with 0.5% NaCl prior to ensiling in plastic bags under anaerobic conditions for 21 days. Rice bran was purchased from the market. Prior to feeding, the ensiled cassava root and the rice bran were mixed in a ratio of 2:1 (fresh basis).

During a 7-day adaptation period, the cassava leaves and the mixture of ensiled cassava root and rice bran were fed ad libitum in separate feed troughs to establish the expected level of intake of the ensiled cassava root and rice bran mixture.

In the subsequent experimental period of 21 days the offer level of the mixture of ensiled cassava root / rice bran was set at 80% of the intake observed during the period of adaptation, while the cassava leaves continued to be fed ad libitum, both feeds in separate feed troughs. The mixture of ensiled cassava root / rice bran was offered three times daily at 06.00, 12.00 and 18.00 h. The cassava leaves were given 4 times at 08.00, 10.00, 14.00 and 16.00 h.

Animals

The Mong Cai and F1 (Large White x Mong Cai) pigs had an average initial weight of about 23 kg. They were allocated to the three treatments in an arrangement of two Latin squares, with periods of 14 days (Table 1).

Table 1.  Allocation of diets and animals
Periods/animals	MC1	MC2	MC3	LM1	LM2	LM3
1	CW	W	CWW	CW	W	CWW
2	W	CWW	CW	W	CWW	CW
3	CWW	CW	W	CWW	CW	W

Effect of different levels of DL-methionine on the performance of growing pigs fed cassava leaves as partial replacement for fish meal

Animals and management

Twelve growing pigs housed in individual pens were allocated to four treatments. The pigs (Mong Cai X Large White) were about 80 days old with mean body weight of 27.2 ± 1.7 kg (mean ± SD). They were vaccinated against hog cholera and Pasteurellosis, and de-wormed 2 weeks before starting the experiment.

Experimental design, diets and feeding

The experiment was a completely randomized design with four treatments and three replicates. The control diet consisted of rice bran, maize, cassava root meal and ensiled cassava root supplemented with fish meal (FM). The three test diets were based on the control diet with replacement of FM by fresh cassava leaves (Table 2), and with 0, 0.1 or 0.2% DL-methionine. The basal diets were fed according to a restricted feed allowance (NRC 1988) and distributed equally into 3 meals per day at 6:00, 12:00 and 17:00 h. The cassava leaves were offered at 8:00, 10:00 and 15:00 h. The refusals were collected before offering the next meal. Drinking water was available ad libitum.

The cassava roots were ensiled with 0.5% common salt (Nguyen Thi Loc et al 1996) and stored 21 days before feeding. The leaves were from a high-yielding cassava variety that is commonly planted in central Viet Nam. The fresh leaves were chopped and wilted under a roof before feeding. The composition and analysis of the diets are in Tables 2 and 3, respectively.

Table 2. Ingredient composition of the experimental diets (%, DM basis)
Ingredient	Control	FCL*	FCL+0.1	FCL+0.2
Rice bran Maize ECR* CM* Fish meal FCL Premix Methionine Soybean oil	29.0 25.0 20.0 10.0 15.0 0 1.0 0 0	13.4 25.0 20.0 9.7 9.0 20.0 1.0 0 1.9	13.4 23.0 20.0 11.6 9.0 20.0 1.0 0.1 1.9	12.0 23.0 20.0 12.9 9.0 20.0 1.0 0.2 1.9
* FCL: Fresh cassava leaves; ECR: Ensiled cassava root; CM: Cassava root meal

Table 3. Chemical composition of  the experimental diets (% in DM except for ME and HCN)
	Control	FCL	FCL+0.1	FCL+0.2
Crude protein Lipid Crude fiber Ash Ca P ME (MJ/kg DM) Lysine Methionine Cystine HCN (mg/kg DM)	14.2 7.6 3.8 6.9 0.63 0.63 13.8 0.68 0.27 0.21 27.8	14.7 7.6 5.9 6.4 0.68 0.53 14.0 0.74 0.29 0.20 97.2	14.7 7.5 5.9 6.4 0.68 0.52 14.0 0.74 0.39 0.20 98.5	14.6 7.3 5.8 6.3 0.68 0.51 14.0 0.73 0.48 0.19 99.4

Carcass measurements

For the evaluation of carcass traits, the pigs were starved for 24 hours and weighed prior to slaughter. The P2 back fat thickness was measured on the partitioned carcass 10 cm from the midline behind the 10^th rib using a trace paper and a ruler; the loin eye area was measured at the same point. Lean percentage was calculated as the ratio of lean mass to hot carcass weight according to the following equation: Lean mass = 7.23 + (hot carcass weight, Ib x 0.437 + (loin area, In² x 3.877) - (P2 back fat thickness, In x 18.746) (Kaufman and Epley 1996).

Chemical analyses

The feed samples were dried at 60⁰C for 24 h and ground with a 1mm sieve prior to chemical analysis, according to standard methods (AOAC 1984) for DM, N, ether extract and ash. Amino acids were analysed according to Spackman et al (1958) on an ion-exchange column (HPLC).

Statistical analysis

Analysis of variance was performed using the general linear model (GLM) procedure of Minitab Version 14. When the F test was significant (P<0.05), the Tukey test for pair-wise comparisons with confidence levels of 95 was used to determine significant differences between treatment means.

Results and discussion

Effect of chopping and wilting on HCN content

The 58% reduction in the HCN content of the cassava leaves after wilting 24 hours in the shade (Table 4 and Figure 1) is similar to that (60%) reported by Bui Van Chinh and Le Viet Ly (1996, but much less than when wilting is under sunlight, when up to 90% reduction in HCN has been reported (Gomez and Valdivieso 1985; Ravindran et al 1987). The faster rate of drying of the leaves sampled from the 60-day growth compared with the leaves from the mature plant was not reflected in the changes in HCN content, which were similar for both sources of leaves.

Figure 1: Variation in DM and HCN in cassava leaves, from mature and immature plants,
 analysed immediately or after wilting 3, 6, 9 and 24 hours

Effect of washing, wilting and ensiling on HCN

The HCN content was reduced 19% by washing, and by 74-76% by ensiling or wilting (Table 5).

Table 5. Effect of washing, wilting and ensiling on DM and  HCN concentration
	Fresh leaves	Washing	Wilting	Ensiling	SEM	P
DM (%)	24.1	18.8	51.6	32.8	1.58	0.001
CP (% in DM)	27.8	27.7	27.5	25.7	0.56	0.033
HCN (mg/kg DM)	1491	1183	373	356	120	0.001
Reduction in HCN (%)	0	19	74	76

Washing alone or washing after chopping the leaves reduced slightly (by 16 and 21%) the HCN level (Table 6); however, the pink color in the urine observed when fresh cassava leaves were fed was no longer visible when the leaves were washed prior to feeding. Wilting for 24 hours after chopping and washing reduced the HCN content by 82%.

Intake of fresh cassava leaves

Although there were major differences in the HCN content of the cassava leaves (Table 6) as between washing with and without chopping and 24 hour wilting, this did not appear to affect the DM intake of the ensiled cassava root / rice bran mixture nor of the cassava leaves (Table 7). The intake of cassava leaves accounted for over 30% of the total DM intake (Figure 2), resulting in intakes of HCN of 373 and 337 mg/day with washed and chopped / washed leaves, compared with 146 mg/day for the wilted leaves (Table 7). Reported toxic levels of HCN (mg/kg live weight) for pigs are 1.4 (Getter and Baine 1938), 2.1 to 2.3 (Johnson and Ramond 1965), 4.4 (Butler 1973) and 3.5 (Tewe 1995). In the present study the HCN intakes were much higher (from 6.0 to 15 mg/kg live weight), yet no signs of toxicity were observed. .

 

Figure 3. Effect of processing method of the leaves on the proportions of cassava leaves and
the ensiled cassava root/ rice bran mixture in the overall DM intake of Mong Cai and F1 pigs.

The cassava leaves contributed 37 to 39% of the diet DM and from 71 to 74% of the total protein, providing an overall level of 15% crude protein in the diet DM (Table 8). These proportions of dietary crude protein provided by fresh cassava leaves are much higher than has previously been reported in the literature (Bui Huy Nhu Phuc 2001; Ly and Rodríguez 2001). However, they are comparable with results of a recent experiment in Cambodia (Chhay Ty and Preston 2005) in which fresh cassava leaves supplied 42% of the diet DM and 70% of the protein in a basal diet of broken rice fed to growing pigs.

Effect of supplementary DL-methionine in cassava based diets on the performance of growing pigs

DM and CP intakes were lower, and growth rate and feed conversion better, on the diet with 0.2% supplementary DL-methionine than on the other diets (Table 9). These results are similar to those reported by Maner and Gomez (1973).

Table 9. Effect of  supplementary DL-methionine on live weight gain (LWG) and feed conversion ratio (FCR)
	Control	FCL	FCL+0.1	FCL+0.2	SEM	P
Dry matter intake (kg/day)	1.62	1.62	1.6	1.49	0.033	0.012
Crude protein intake (g/day)	236	236	232	216	4.812	0.008
Initial weight (kg)	26.9	27.3	28.3	27.3	1.13	0.8
Final weight (kg)	85.1	71.3	77.4	86.7	1.495	0.001
LWG (kg/day)	0.73	0.55	0.61	0.74	0.014	0.001
FCR (kg)	2.81	3.83	3.38	2.66	0.138	0.001

Back fat was reduced and lean percentage was higher on the diets with cassava leaves compared with the control (Table 10). DL-Methionine supplementation of the diets with cassava leaves led to decreases in the weights of liver and thyroid as percentage of live weight at slaughter.

Table 10. Effect of supplementary DL-methionine on carcass traits of fattening pigs fed high levels of ensiled cassava roots and fresh (wilted) cassava leaves
	Control	FCL	FCL+0.1	FCL+0.2	P	SEM
Live weight, kg	80.6 cd	68.1 a	75.3 abc	82.5 d	0.001	1.32
% carcass	81.4	78.7	80.1	81.5	0.314	1.11
% liver in LW	1.82 ab	2.35 d	2.13 c	1.98 bc	0.003	0.066
Loin eye area (cm2)	28.5 b	27.0 ab	28.5 b	28.5 b	0.014	0.288
Back fat (cm)	3.63 b	2.80 a	2.90 a	2.93 a	0.007	0.131
Lean	24.7 c	22.2 ab	25.2 c	28.2 d	0.001	0.924
% Lean	41.8 a	46.4 b	46.3 b	45.7 b	0.012	0.815
Carcass length, m	87.0	89.0	88.0	90.3	0.482	2.053
Thyroid gland (g)	33.9	32.8	34.1	36.9	0.253	0.253
Thyroid gland as % LW	0.042 a	0.048 ab	0.045 ab	0.045 ab	0.05	0.012
a,b,c,d Mean values in the same row without common superscript are different at P<0.05

Conclusions

• Pigs given restricted amounts (80% of observed voluntary intake) of a 2:1 mixture of ensiled cassava roots and rice bran, and free access to fresh cassava leaves, which had been washed, chopped and washed or wilted for 24 hours, consumed similar amounts of total DM (range of 27 to 32 g DM/kg live weight).

• Levels of HCN were reduced slightly (16%) by washing and substantially (82%) by wilting, resulting in intakes of HCN from 6.0 to 15 mg/kg live weight, levels considerably higher than previously reported as safe levels to avoid toxicity (1.4 to 4.4 mg/kg live weight).

• The fresh cassava leaves were readily consumed, providing 38% of the dietary DM and over 70% of the dietary protein.

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