| Livestock Research for Rural Development 13 (2) 2001 | Citation of this paper |
An experiment was designed at laboratory scale to study the
effect of time (0, 14, 28, 42 and 56 days), origin of the cultivar (Kampong Cham
province and Chamcar Daung, Phnom Penh) and levels of added sugar palm syrup
(2.5, 5 and 7.5% in fresh basis) for ensiling of cassava foliage. Average air
temperature was 27 °C, and crude protein content of both sources of cassava
foliages was 18.1 and 22.2% in dry basis, respectively.
There was no significant interactions between factors studied.
The overall pH profile of the ensiled material showed a sharp drop at 14
days after ensiling ( from 6.10 to 3.73), with slight fluctuations thereafter. A
significant effect (P<0.001 ) of sugar palm syrup concentration
on
pH values was found in favour of 5% addition of the source of
carbohydrate.
There was a significant correlation (R² = 0.31; P<0.001) between pH values
and neutralizing capacity of the preserved material.
The cyanide concentration of the silages exhibited a significant negative (P<0.001) linear dependence on time. In spite of the fact that the Kampong Cham variety had a higher initial HCN content (86 mg/kg fresh foliage) as compared to that from Chamcar Daung (73 mg/kg fresh leaves), there were no differences in HCN content after 56 days of ensiling (overall value, 20 mg/kg fresh foliage).
It is suggested the use of sugar palm syrup at the level of 5%
in the preparation of cassava foliage silages in Cambodia. According to results
from the present experiment, the HCN content is in the range of safe utilization for
monogastric animals such as pigs, after 56 days of ensiling the material.
Managing cassava as a perennial forage has been claimed to be
a very important component of integrated farming systems in tropical countries
(Preston et al 1999). 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 succesfully after ensiling (Ravindran 1992). On the other hand,
cassava leaves silage has been used in metabolism studies (Du
Thanh Hang et al 1997;
Nguyen and Rodríguez 1998) and growth trials (Du
Thanh Hang 1998) indicating a promising
use in tropical pig production.
The aim of this study was to
evaluate different ensiling
conditions for preservation of cassava foliage in Cambodian farms.
60 laboratory silos were made by adding 2.5, 5 and 7.5% in fresh basis of palm (Borassus flabellifer L.) syrup to freshly harvested cassava foliage. The cassava foliage was from periodical cuts of plantations at the University. About one metre of the top of the plants was cut and there was no separation of leaves, stems and petioles. Two cultivars of unknown origin were utilized, from Kampong Cham (KCh) and Chamcar Daung (ChD) respectively, from which the only apparent difference was the color of leaf petioles: red in KCh and white in ChD. Some characteristics of the harvested cassava foliage are set out in Table 1. The ChD cultivar contained higher DM content than that from KCh. On the other hand, the ChD cultivar was better than the KCh in crude protein (Nx6.25) and HCN concentration.
|
Table 1:
Proximal analysis of the cassava foliage |
||
|
|
Kampong
Cham |
Chamcar
Daung |
|
Dry matter, % |
21.3 |
23.8 |
|
Water
soluble DM, % |
- |
51.4 |
|
Nx6.25, % in DM |
18.1 |
22.2 |
|
Organic matter, %
in DM |
93.8 |
92.7 |
|
NDF, % |
23.7 |
26.7 |
|
HCN. mg/kg fresh basis |
86.6 |
73.0 |
The laboratory silos consisted of cylindrical plastic cans of
350 cm3 capacity where the leaves with petioles and
green stems were
pressed after chopping (particle size less than 1
cm) and mixed with palm syrup,
then covered with a sheet of polyethylene before sealing with the cap.
The contents of each silo weighed 200 g approximately. The palm syrup was obtained in the local
market, and was mixed with water (2:1 by weight) to achieve a brix
(soluble sugars) of near 40°.
.The silos were opened at random after 0, 14, 28, 42 and 56 days of
storage at an average air temperature of 27°C
.
Samples of fresh materials or silages were analysed for DM by
drying to constant weight by microwave radiation (Undersander
et al
1993). The N and ash content in fresh leaves were determined according to AOAC
(1990) recommendations, whereas the NDF fraction was estimated following Van
Soest et al (1991). In the fresh silages the pH was recorded by a glass
electrode and the neutralizing capacity was determined in aqueous extracts (1:4
by weight) by titrating with 0.1 N KOH until pH = 6.5.
The cyanide content was determined in another aliquote of the silage by
steam distillation and thereafter titrating with AgN03 after boiling
the sample in chloroform and reaction with KOH.
The data were subjected to analysis of variance (Steel and
Torrie 1980) following a general linear model procedure of the Minitab
statistical package (version 10.2). In addition, an analysis of regression was
conducted in the required cases.
There was no significant interaction (P>0.05) concerning
any of the factors studied.
The profile of DM content in the
silages is shown in Figure 1.
Figure
1:
Dry
matter concentration changes after ensiling
Both types of silages showed a slight decrease in DM concentration from 0 to 14 days, thereafter being practically invariable until 56 days of ensiling, when a certain decrease in DM content appeared to be evident. On the other hand, silages made with the KCh cultivar had always significantly higher DM concentration (P<0.001) than the ChD cultivar. The DM concentration in the ensiled cassava foliage was lower than that reported by Du Thanh Hang (1998) which was as high as 35%. Cassava leaves were wilted during one day prior to ensiling with 5% sugar cane molasses in the experiment of Du Thanh Hang 1998).
Figure 2 shows the rapid decrease in the overall pH value of
cassava foliage between 0 and 14 days of ensiling (from 6.10 to 3.73). A similar
sharp decrease in pH of the experimental silos was observed by Du
Thanh Hang (1998) when
sugar cane molasses or rice bran were used for ensiling cassava leaves.
Between 42 and 56 days of ensiling a slight, not significant increase in
pH values of the media was encountered. However, a significant effect
(P<0.001) of sugar palm syrup concentration in silos on pH values was found
in favour of 5% addition of the source of
carbohydrate.
|
Figure
2:
pH value of cassava foliage after ensiling |
The neutralizing capacity of the
cassava foliage was only measured after 28 days of ensiling (Figure 3). No
effect of the level of sugar palm syrup was observed in any case.
However, a
significant effect (P<0.001) of cultivar source was found in the samples
examined. In this opportunity, the ChD cultivar showed a higher neutralizing
capacity value.
There was a significant correlation (R2 = 0.31; P<0.001) between pH values and the neutralizing capacity of the preserved material, the equation being Y = 4.821 – 0.079 X, where Y was pH and X the neutralizing capacity (meq NaOH/100 g material). In this case, the value of Syx was ± 0.272.
Figure 3: Neutralizing capacity of ensiled cassava foliage
Figures 4 and 5 describes the change in the HCN content in
the ensiled foliage. The cyanide concentration of silos exhibited a significant
(P<0.001) linear dependence on time. In spite of the fact that the KCh
foliage was higher in initial HCN content (86 mg/kg fresh material) as compared
to that from ChD (73 mg/kg fresh material), there were no differences in HCN
content after 56 days of ensiling (overall value, 20 mg/kg fresh foliage).
Wilting of cassava leaves during one day was shown by Du
Thanh Hang (1998) to reduce
considerably the HCN content, as compared to the data presented in this paper.
It is known that the destruction of cell wall structures by different methods
favours the intracellular reaction of linamarase with the cyanoglucosides
present in cassava, thus contributing to a rapid HCN elimination from the
material (Ravindran 1992). Even so, the ensiling process permited the steady
elimination of these compounds to low levels of this antinutritional factor.
Figure 4: Changes in HCN content during the ensiling period (fresh matter
basis) |
|
Gómez (1991) has suggested that 100 mg HCN/kg feed
[on
dry matter basis] as indicated by the Council of the European
Community, could be the permissible maximum level.
However, there is no clear evidence concerning either
toxic levels of HCN for pigs or the influence of this acid on production traits
of economic importance in pigs fed cassava foliage. This could be so, perhaps
because of the well known amino acid imbalance in cassava leaves (Eggum
1970) and the considerable proportion of cell wall (González
et al 1999) present in the aerial part of cassava could be major constraints to
the utilization of considerable amounts of cassava foliage in the feed for pigs.
In fact, neither in growing-fattening pigs (Mahendranathan 1971; Hutagalung
1972; Sarwat et al 1988) nor in breeding sows (Tewe and Maner
1981) are there reports of death of animals
caused by HCN intoxication. This has been recognized in several reviews
concerning this subject (Oke 1978; Gómez 1991; Tewe 1992; Ravindran 1993; Ly
1999). On the other hand, the use of ensiled cassava leaves with a relatively
high level of HCN (147 mg/kg DM) resulted in better digestive indices than
sun-dried cassava leaves (22.5 mg/kg DM) in growing pigs, as
was encountered
by Bui Huy Nhu Phuc et al (1996).
It seems that more research concerning the interaction
between HCN and monogastric animals performance in the context of well balanced
diets is necessary. This appears to be evident from the point of view of the
methionine content of the diet, taking into account the participation of this
amino acid in the detoxification process of cyanide.
It is suggested to use sugar palm syrup at the level of
5% in the preparation of cassava foliage silages in Cambodia. According to
results from the present experiment, HCN content is probably in the range of
safe utilization for monogastric animals such as pigs, after 56 days of ensiling
the material.
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Received 7 Februay 2001
