Digestibility and growth in rabbits fed a basal diet of sweet potato vines replaced

with cassava foliage meal

Nguyen Kien Cuong, Duong Nguyen Khang and T R Preston*

Nong Lam University
Thu Duc, Ho Chi Minh City, Vietnam
kiencuongvl@yahoo.fr
 duongnguyenkhang@gmail.com
* TOSOLY, AA48 Socorro, Colombia

Abstract

There were two experiments. In the digestibility experiment, four local and four New Zealand White female rabbits were allocated to 4 treatments according to a duplicated 4*4 Latin square arrangement. The treatments were: replacement of sweet potato vines by cassava foliage meal at levels of 0 (CFM0),  20 (CFM20), 40 (CFM40) and 60% in dry matter (CFM60).  The growth experiment (for 56 days) was with the same treatments using 24 crossbred female rabbits, weighing 1,292 – 1,317 g at the start.

In experiment 1, there were: a positive curvilinear relationship between the proportion of CFM in the diet and the total DM intake  R² = 0.95); and negative linear relationships for apparent digestibility coefficients of DM, organic matter (OM), crude protein (CP), NDF and ADF (R² = 0.97-0.99). In Experiment 2, with increasing proportions of CFM in the diet there was a decline in growth rate  (y = 21.3 – 0.070x; R² = 0.99)  and a poorer feed conversion rate  (y= 3.98 +0.023x; R² = 0.99).

It is concluded that cassava foliage meal is not a suitable source of foliage for growing rabbits.

Key words: Feed intake, feed conversion, soybean milk residue

Introduction

The basal diet of most rabbits in Vietnam is based on roadside grass and agricultural by-products that are not suitable for feeding to people. In general, these feeds are nutritionally poor and imbalanced for rabbits. To overcome this problem feed supplements are normally provided. Cereal-based feeds are generally too expensive for use as supplements because of competing needs for the expanding human population and the diminishing food producing capacity of the earth's surface (Brown and Kane 1994). It is argued that a major priority is to develop livestock feeding systems which do not depend on cereal grains (Preston 1995). Considerable progress has been made in the use of non-cereal energy-rich feeds such as cassava (Machin and Nyvold Solveig 1991), molasses (Preston et al 1968), sugar cane juice (Sarria et al 1990), palm oil and palm fruit (Ocampo 1994a) and sugar palm juice (Khieu Borin et al 1995) for monogastric animals; water spinach for rabbits (Nguyen Thi Kim Dong et al 2006a 2006b; Samkol 2004; Samkol et al 2004a, 2004b, 2006). Progress has also been made in the use of tree legumes such as Leucaena and Gliricidia (Leng 1997) and Sesbania (Nguyen Thi Hong Nhan 1998) for ruminants.

Cassava is an important crop in the tropical countries. Cassava production in Vietnam has steadily increased during recent years, mainly because of increases in both area planted and yield per hectare. According to FAOSTAT data (2006), total yield in 2000 was nearly 2 million tonnes grown on 237,600 ha. The area cultivated in 2006 was increased to nearly 400,000 ha, with a total tuber yield of 6 million tonnes. Beside the root, each hectare of cassava can produce a large amount of leaf. The potential yields of cassava leaves as by-products at root harvesting may amount to as much as 4.64 tonnes dry matter per hectare (Ravindran and Ravindran 1987). If cassava is grown as foliage in the dry season, it can give 41,000 kg/ha of fresh leaf, equivalent to about 12,000 kg/ha of cassava foliage meal. The protein content in cassava leaf is about 23 % (Man and Wiktorsson 2001, 2002; Khang and Wiktorsson 2000, 2004a, 2004b, 2005, 2006). Therefore from one hectare of cassava, it is possible to obtain almost 2,500 kg of protein (Wanapat et al 1999). The high protein content of cassava leaves is well documented. However, the current practice, in most instances, is to return this valuable resource directly to the soil as green compost. The reason for it not being used as a feed resource is its potential toxicity, which may affect animal health. Cassava foliage contains the cyanogenic glucosides, linamarin and lotaustralin. After tissue damage, these are hydrolyzed by the endogenous enzyme linamarase to cyanohydrins. Further hydrolysis to hydrogen cyanide is responsible for chronic toxicity. However, sun drying reduces cyanide content in cassava foliage to levels which are safe for animals (Ravindran et al 1987; Man and Wiktorsson 2001, 2002; Khang and Wiktorsson 2000, 2006). Thus, feeding sun-dried cassava foliage as a supplement to rabbits may be an attractive alternative.

The hypothesis behind this study is that sun-dried cassava foliage, including tender stems, could be utilized for rabbit feeding. To test this, studies were initiated with the aims to: 

•  Examine the effect of cassava foliage meal supplement on digestibility in rabbits fed sweet potato vines as the basal diet.

• Determine the level to which cassava foliage meal could replace sweet potato vines in the diet of growing rabbits.

Materials and methods

Experiment 1. Digestibility indices in growing rabbits fed a basal diet of sweet potato vines replaced with cassava foliage meal

Animals and experimental design

The experiment was conducted at the Experimental Farm of Nong Lam University, Vietnam. Four local and four New Zealand White female rabbits, with average live weight of 1.8 kg were allocated to 4 experimental treatments according to a duplicated 4*4 Latin square arrangement.

The treatments (ratios in DM basis) were:

·          CFM0: 20% Soybean milk residue (SMR) + 80% sweet potato vines (SPV)

·          CFM20: 20% SMR + 60% of SPV + 20% of sun-dried cassava foliage ( CFM)

·          CFM40: 20% SMR + 40% of SPV + 40% of  CFM 

·          CFM60: 20% SMR + 20% of SPV + 60%  CFM

Mineral premix and salt (0.2%) were supplied in all the diets. 

The rabbits were housed in metabolism cages (50 x 50 cm) built to allow the quantitative collection of hard faeces and urine. Each experimental period of 2 weeks consisted of nine preliminary days when the rabbits were adapted to the diets followed by another five days for collection of faeces and feed refusals The rabbits were weighed at the beginning of the trial and at the end of each period.

Feeds, feeding and management

Sweet potato vines were harvested daily at 35 days of growth, in the morning (7:30 h) in the field of the Experimental Farm of Nong Lam University.  Cassava foliage (leaves, petioles and tender green stems) was bought from farmers at the time of harvesting the roots. It was chopped into pieces of 4 to 6 cm length and sun-dried for 2 days by spreading on a plastic sheet placed on the ground. The dried cassava foliage was ground to form cassava foliage meal.  

The rabbits were fed three times daily at 8.00, 15.00 and 19.00h. The amounts of sweet potato vines, cassava foliage meal and SMR were determined on the basis of a DM intake of 8% of live weight and the planned ratios. The quantities were revised weekly according to the live weights of the rabbits. Fresh water was available at all times. The refusals and spillage were collected for the forages separately and weighed daily in the morning to calculate the feed intake. The animals were vaccinated against rabbit hemorrhagic diarrhea and treated with Ivermectin to control parasites.

Data collection

During the 5-day collection period the faeces were collected every day and kept frozen in plastic bags until analysis. At the end of each period, feed refusals and faeces were mixed thoroughly by hand and a representative sample homogenized in a coffee grinder, for analysis of pH, N, DM, ash, ether extract (EE),  NDF and ADF. Organic matter(OM) was calculated as 100 minus % ash in DM basis.

Chemical analyses

Chemical analyses of diets and faeces were undertaken following the methods of AOAC (1990) for DM, OM, CP, EE and ash. NDF and ADF were determined following the procedures of Van Soest et al (1991).

Statistical analysis

The data were analyzed by adjusting linear or polynomial regressions to the data, using the “regression” option in the Minitab software, version 13.20 (Minitab 2000). The independent variable (X) was the recorded level of CFM in the diet; the dependent  variables (Y) were intakes and coefficients of apparent digestibility).

Results

The crude protein contents of all the ingredients were similar (Table 1). ADF was highest in the CFM.

The recorded proportions of CFM in the diet were close to the planned levels except at the highest CFM ratio (CFM60) when observed intake was about 10% less than the planned level (Table 2). There was a positive curvilinear relationship between the proportion of CFM in the diet and the total DM intake

(y = -0.007x² + 0.895x + 98.50;  R² = 0.95).

There were negative linear relationships between the proportion of CFM in the diet and all coefficients of apparent digestibility the decreasing trend being most marked for the cell wall fractions of ADF and NDF (Tables 3 and 4 and Figure 1). 

There were no differences between breeds in the coefficients of apparent digestibility (Table 5)

Experiment 2. Effect of different levels of cassava foliage meal supplementation on the growth rate and economic benefits of growing crossbred rabbits
 

Animals and experimental design

The experiment was conducted at the Experimental Farm of Nong Lam University, Vietnam. Twenty-four crossbred female rabbits at two months of age were arranged in a complete randomized design with 4 treatments and 3 replications. Two rabbits were housed in a wire mesh and wood cage, as the experimental unit. The dietary treatments were levels of cassava foliage meal in the diets of 0 ( CFM0), 25 (CFM25), 50 (CFM50) and 75% ( CFM75) (on DM basis), as replacement fo sweet potato vines.. The amounts of each ingredient were calculated on the basis of a DM intake of 8% of live weight and were adjusted every week according to changes in live weight. The experimental lasted 8 weeks.

Feeds, feeding and management

Collection and processing of feeds and feeding schedule were the same as in Experiment 1. The animals were vaccinated against rabbit hemorrhagic diarrhea and treated with Ivermectin to control parasite diseases.

Measurements

Chemical analysis of feeds offered and refused was the same as in Experiment 1. The rabbits were weighed at the beginning of the experiment and thereafter weekly.

Statistical analysis

Linear or polynomial regressions were adjusted to the data using the “regression” option in the Minitab software, version 13.20 (Minitab 2000). The independent variable (X) was the recorded level of CFM in the diet; the dependent  variables (Y) were feed intakes, live weight gain and feed conversion.

Results

The composition of the dietary ingredients and the relative proportions consumed were similar to what was observed in Experiment 1 (Table 5 and 6). Feed intake increased linearly (y = 0.113x + 85.81; R² = 0.99) with increasing proportion of CFM in the diet.

Growth rates declined and feed conversion deteriorated with a linear trend according to the level of CFM in the diet (Table 7; Figures 2 and 3). 

Discussion

The problem with the cassava foliage meal seems to be in the low digestibility of the OM and the crude protein, and especially the cell wall fraction. Thus although the voluntary feed intake increased with increasing cassava foliage in the diet,  the decline in digestibility more than offset the  increase in intake with the result that intake of apparently digestible DM became the constraint to performance (Figure 4).

In pigs the findings are similar, in that DM and crude protein digestibility also decline with increasing levels of cassava leaves in the diet (Bui Huy Nhu Phuc et al 1996; Du Thanh Hang 1998; Chhay Ty and Preston 2005).   Rabbits have a greater capacity to utilize fiber than pigs due to their relatively larger caecum and the recycling opportunity represented by the process of caeotrophy (Leng 2008), thus it is surprising that the digestibility of the cell wall fractions should decline so markedly with increasing proportion of cassava foliage meal in the diet.

Conclusions

The data on apparent digestibility, growth rate and feed conversion all indicate that  cassava foliage meal is very inferior in nutritive value to sweet potato vines and therefore is not a suitable forage source for growing rabbits.

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