Abstract: The efficacy of wetting sun-dried cassava tuber meal as a method of reducing its hydrocyanide (HCN) content and improving its nutritive value for laying hens was investigated. Cassava tubers were peeled, chopped into pieces, sun-dried and then milled. Part of the Sun-dried Cassava Tuber Meal (SCTM) was soaked in water at the rate of 5 parts of water to 4 parts of the meal, thinly spread on the floor for 5 h and then taken out and sun-dried again. The Raw Cassava Tuber Meal (RCTM), Sun-dried Cassava Tuber Meal (SCTM) and Wetted Sun-dried Cassava Tuber Meal (WSCTM) were analyzed for HCN content. Five diets were made such that diet 1 (control) contained no cassava tuber meal; in diets 2 and 3, 50% of the maize in diet 1 was replaced with SCTM and WSCTM, respectively, while in diets 4 and 5, 100% of the maize was replaced with SCTM and WSCTM, respectively. Each diet was fed to a group of 24 laying hens for 12 weeks. At the end of the feeding trial, 4 birds were randomly selected from each group and used for determination of internal organ weights and haematological indices. Raw cassava tuber meal contained 800 ppm HCN, SCTM contained 50 ppm HCN while WSCTM contained 10 ppm HCN. The group on 100% WSCTM diet consumed significantly (p<0.05) less feed, gained least body weight and recorded least hen-day egg production, possibly due to very powdery nature of the diet. Egg weight and feed conversion ratio were not affected by the treatments (p>0.05). Egg quality indices were also not affected by the treatments (p>0.05). Internal organ weights were not affected by the treatments (p>0.05) but the birds on cassava diets recorded significantly (p<0.05) more abdominal fat. The birds on cassava diets also recorded significantly (p<0.05) less WBC and PCV values relative to the control group.
INTRODUCTION
Nigeria is the world’s largest producer of cassava with an estimated production of about 38.2 million metric tons in 2004 (FAO, 2005). With the current Federal Government emphasis on cassava production, it is expected that its production in Nigeria could double within the next few years. Most of the cassava produced is locally consumed as it is the staple food of the people. Attempts to use it as a source of energy in poultry diets as an alternative to maize have however not yielded encouraging results because of its content of cyanogenic glucosides (Odukwe, 1994; Udedibie et al., 2004). Maize on the other hand has been playing a key role as a source of energy in poultry diets in the country. However, because maize is a major human food and also used for various industrial raw materials in the country, its demand outstrips its supply, leading to over 2000% increase in price within the last 20 years. This has invariably contributed to the high cost of poultry feeds with concomitant increase in the prices of poultry products.
One method of processing cassava tubers for human consumption is sun-drying. Unfortunately, the traditional sun-drying method leaves a large amount of the cyanogenic glucoside, linamarin, remaining in the cassava flour, amounting to 25-33% retention of the cyanide (Bradbury, 2004). Recent studies in Australia (Bradbury, 2004) have, however, shown that wetting sun-dried cassava flour for 5 h at the ratio of 5 parts of water to 4 parts of the flour reduces its cyanide content to about 1/3 of its previous level and therefore makes it safe for human consumption. Follow-up studies in Nigeria (Udedibie et al., 2007; Udedibie and Asoluka, 2008) have also demonstrated that wetting sun-dried cassava tuber meal for 5 h reduced its HCN content to 1/5 of its previous level and improved its nutritive value as energy feed for broilers.
The study herein reported was designed to determine the effects of 5 h wetting of sun-dried cassava tuber meal on its value as source of energy in the diets of laying hens.
MATERIALS AND METHODS
The trial was carried out at the Poultry Unit of the Teaching and Research Farm of University of Uyo, Uyo-Akwa Ibom State, Nigeria.
Source and Processing of Cassava Tubers
The fresh cassava tubers (bitter variety) used for the study were bought
from a local market in Uyo, Akwa Ibom State of Nigeria. They were peeled, chopped
into small pieces and sun-dried until they became crispy. The sun-dried chips
were then milled and sieved to produce Sun-dried Cassava Tuber Meal (SCTM).
Half of the sun-dried cassava tuber meal was soaked in water at the ratio of
4 parts of the meal to 5 parts of water and then spread thinly on the floor
and allowed to stand for 5 h at room temperature. Thereafter, the mash was re-dried
in the sun and milled to produce wetted Sun-dried Cassava Tuber Meal (WSCTM).
Samples of the Raw/fresh Cassava Tuber Meal (RCTM), Sun-dried Cassava Tuber
Meal (SCTM) and Wetted Sun-dried Cassava Tuber Meal (WSCTM) were analyzed for
HCN content, using the Picrate Paper kits method of Bradbury
et al. (1999).
Experimental Diets
Five experimental laying diets were made such that diet 1 (control) contained
no cassava tuber meal. In diets 2 and 3, 50% of the maize in the control diet
was replaced with SCTM and WSCTM, respectively, while in diets 4 and 5, 100%
of the maize was replaced with SCTM and WSCTM, respectively. Other ingredients
were adjusted in such a way as to make the diets fairly isonitrogenous and iso-caloric.
Ingredient composition and calculated chemical composition of the diets are
shown in Table 1.
Experimental Birds and Design
One hundred and twenty laying hens at 2 months of laying life were divided
into 5 groups of 24 birds each and each group randomly assigned to one of the
5 experimental diets in completely randomized design. Each group was further
sub-divided into 4 replicates of 6 birds each and each replicate housed in a
pen measuring 1½x1½ m with wood shavings as litter material. Feed
and water were provided ad libitum. The trial lasted 12 weeks.
Data Collection
Data were collected on initial and final body weights, feed intake, feed
conversion ratio, egg production, egg weights, egg quality indices, internal
organ weights and haematological indices of the birds.
The birds were weighed at the beginning and end of the trial to determine their body weight changes. Feed intake was determined by subtracting the weight of left-over feed from the weight of the feed fed the previous day.
| Table 1: | Ingredient composition of the experimental diets |
| *To provide the following per kg of feed; Vitamin A: 10,000 iu; Vitamin D3: 1500 iu; Vitamin E: 3 iu; Vitamin K: 2 mg; Riboflavin: 3 mg; Pathothanic acid: 6 mg; Niacin: 15 mg; Vitamin B12: 8 mg; Choline: 350 mg; Folic acid: 4 mg; Mg: 56 mg; Iodine: 1.0 mg; Iron: 20 mg; Cu: 10 mg; Zn: 0.5 mg | |
Eggs were collected twice daily. At the end of each week, the eggs from each pen were weighed to determine average egg weights. Feed conversion ratio was determined as kg feed intake per kg eggs produced. Hen-day egg production was determined by dividing total egg production by the total number of layers. Egg quality indices were determined accordingly: egg yolk index according to Funk (1948); albumen index according to Heiman and Carver (1936); Haugh unit according to Haugh (1937) as modified by Brant et al. (1951).
At the end of the feeding trial, 4 birds were randomly selected from each treatment (one per replicate) and used for determination of the haematological indices and internal organ weights. They were weighed before they were killed by severing their necks from which about 2 mL of blood were carefully collected into labeled Bijon bottles containing EDTA as the anticoagulant. The blood samples were analyzed for Haemoglobin (Hb) level, Red Blood Cells (RBC), White Blood Cells (WBC), Packed Cell Volume (PCV), Mean Cell Haemoglobin Concentration (MCHC) and Mean Cell Volume (MCV) using the methods described by Monica (1984).
After the blood collection, the birds were de-feathered and eviscerated and their internal organs (liver, heart, gizzard and kidneys) as well as their abdominal fat weighed and expressed as percent of dressed weight.
Statistical Analysis
Data generated were subjected to analysis of variance (ANOVA) using SPSS
(2004). Where ANOVA detected significant treatment effects, means were compared
using New Duncan Multiple Range Test (NDMRT) as outlined by Obi
(1990).
RESULTS AND DISCUSSION
Hydrogen Cyanide Content
The fresh raw cassava tuber contained 800 ppm HCN; the sun-dried cassava
tuber meal contained 50 ppm HCN while the wetted sun-dried cassava tuber meal
contained 10 ppm HCN. This meant about 94% reduction in HCN by sun-drying and
about 99% reduction when wetting process was applied.
| Table 2: | Performance of the experimental birds |
| Means within a row with different superscripts are significantly different (p<0.05) | |
This result tended to confirm the earlier observation of Bradbury (2004) that wetting sun-dried cassava tuber meal as described above is a simple method that reduces HCN content of the meal to about 1/3 of the previous level. The reduction observed in this study was about 1/5 of the previous level which corresponds with the value recently reported by Udedibie and Asoluka (2008). The WHO safe standard for cyanide is 10 ppm (FAO/WHO, 1991). Wetting method therefore appears to be very effective as a method of processing cassava tubers for human consumption as well as a source of energy in poultry diets.
Performance of the Experimental Birds
Data on the performance of the laying hens are presented in Table
2.
Feed Intake and Body Weight Changes
The group on 100% WSCTM diet gained significantly (p<0.05) less body
weight and consumed significantly (p<0.05) less feed. The poor body weight
gain was believed to be due to the reduced feed intake by the group. The low
feed intake was also believed to be due to the very powdery nature of the feed.
Poor feed intake of birds on cassava flour diet had earlier been observed by
various researchers (Tewe and Bokanga, 2001; Udedibie
et al., 2004). Recent studies at our station (Udedibie
et al., 2008) had shown no differences in feed intake of layers when
cassava tuber meal was processed into pellets, confirming the observation that
the powdery nature of the diet was responsible for the reduced feed intake.
Egg Production and Egg Weight
There were significant (p<0.05) differences in egg production among the
groups. The group on 50% SCTM diet had the highest hen-day egg production, followed
by the group on 50% WSCTM diet. Egg production of the groups on 100% SCTM and
WSCTM diets were significantly (p<0.05) lower than that of the control group.
Therefore, it appears that 100% replacement of dietary maize with sun-dried
or wetted sun-dried cassava flour is not ideal for egg production.
There were no significant differences in egg weights among the treatment groups (p>0.05). There were also no significant differences in feed conversion ratio among the groups (p>0.05).
Egg Quality Indices
There were no significant differences among the groups in yolk and albumen
indices (p>0.05). The control group, however, recorded significantly (p<0.05)
lower Haugh unit. The reason for this was not clear. Egg shell thickness was
also not effected by the treatments (p>0.05).
Organ Weights and Haematological Indices
The weights of the internal organs expressed as percent of dressed weight
were not affected by the treatments (p>0.05). This tended to suggest that
the cassava tuber meals were not toxic to the laying hens. Atuahene
et al. (1986) and Bamgbose and Niba (1995)
have associated significantly heavier liver weight of birds fed diets containing
high levels of raw cottonseed meal with presence of toxic factor(s) in the diets.
The birds on cassava-based diets however developed significantly (p<0.05)
more abdominal fat than the control.
The treatments had no effect on haemoglobin concentration (p>0.05). The values were within the range of 7-18 g dL-1 recommended by Mitruka and Rawnslay (1977) as the normal values for chickens. The groups on cassava tuber diets had significantly (p<0.05) reduced WBC and PCV values relative to the control group. This seems to be in agreement with the earlier observation by Udedibie and Asoluka (2008) on broiler chickens. There were no observable trends in the values of MCHC and MCV. Haematological constituents usually reflect the physiological responsiveness of the animal to its external or internal environments and thus serve as a veritable tool for monitoring animal health. The haematological indices observed in this study are therefore indications that total replacement of dietary maize with sun-dried or wetted sun-dried cassava tuber meal had no serious deleterious effects on the internal physiology of the birds.
CONCLUSION
The study has revealed that sun-drying of cassava tubers and 5 h wetting of sun-dried cassava tuber meal can reduce the HCN content of the fresh cassava tuber meal by 94 and 99%, respectively. It would therefore follow that poor performance of animals fed diets based on such products would not be due to HCN content. The study has also shown that cassava tuber meal can replace 50% of the maize in the diets of laying hens if sun-dried. Even though wetting sun-dried cassava tuber meal eliminated almost all the HCN in the sun-dried cassava tuber meal, it did not attract any advantage over the sun-dried cassava tuber meal as source of energy in the diets of the laying hens. The relatively poor performance of the hens on 100% WSCTM diet was believed to be due to the dusty nature of the diet and not HCN, since the group on 50% SCTM diet relatively consumed more HCN.
Therefore, it is recommended that 50% of dietary maize for laying hens can be effectively replaced with sun-dried cassava tuber meal if the diet is balanced for protein in view of the disparity in crude protein content between maize and cassava. Higher replacements may need processing the cassava into pellets in view of the powdery nature of the diet at higher dietary cassava tuber meal.