The supplementation of dried cassava chips in growing rabbit diets to improve growth performance and economic returns

Nguyen Thi Kim Dong and Nguyen Van Thu

Cantho University, Cantho City, Vietnam
ntkdong@ctu.edu.vn

Abstract

In a feeding trial, 60 crossbred rabbits with average initial  live weight of 735±4.64g and 8 weeks of age were fed 5 levels of dried cassava chips (0, 10, 20, 30 and 40 g/rabbit/day) as a supplement to Para grass fed ad libitum. In experiment 2, 30 crossbred rabbits at 12 weeks of age with average live weight of 1213±147g were used to measure digestibility and N balance.

Increasing the offer level of dried cassava chips in a basal diet of Para grass fed to growing rabbits led to linear increases in total DM intake, live weight gain, coefficients of apparent digestibility and N retention. There were positive linear relationships between coefficients of apparent DM digestibility and live weight gain and N retention. It was proposed that the determinant of rabbit growth rate in forage-based diets is the overall apparent digestibility of the diet rather than the composition of the diet in terms of the relative proportions of soluble and structural carbohydrates.

Key words: carbohydrates, digestibility. feed conversion, N balance, Para grass

Introduction

Rabbits are raised in almost all developing countries, including Vietnam, and contribute to improving the nutrition and economy of famer smallholders, both as a source of animal protein as well as a source of extra income. Rabbit meat has been also considered as good meat for it's high content of protein , low fat  and low cholesterol levels (Owen 1992).

In the Mekong delta of Vietnam feeding of rabbits is mainly based on natural grasses and agro-industrial by products (Nguyen Thi Kim Dong and Nguyen Van Thu 2009) so the cost for rabbit production is lower than for the other animal species ones. This study aimed to evaluate the potential benefits from supplementing Para grass with dried cassava chips in diets of growing rabbits. 

Materials and methods

Animals and experimental design

Two experiments were carried out at the Experimental farm in Cantho City. For the feeding trial, sixty crossbred rabbits (local x improved breeds) at 8 weeks of age with average initial weight around 735g, were arranged in a completely randomized design with 5 treatments and 3 replications. Four rabbits balanced for sex were the  experimental unit. The treatments were dried cassava chips (DC) supplemented to the Para grass (PG) basal diet at levels of 0 (DC0), 10 (DC10), 20 (DC20), 30 (DC30) and 40g (DC40) / day/ animal. Soybean waste  was offered at 150g/day/animal and extracted soybean meal was fed at 7-11g/day/animal for adjustment of the same protein level of 11.5g/day/rabbit. The experimental period lasted 9 weeks.

In the digestibility trial, the experimental design was similar to that of the feeding trial; 30 rabbits at 12 weeks-old  with live weight around 1200g were used. The animals had two week for adaptation and another week for feces and urine collection. Feeds offered and refusals were  measured daily. Urine was collected for nitrogen analysis to calculate the nitrogen retention.  DM, CP, EE, and NDF apparent digestibility coefficients were calculated according to McDonald et al (2002).   

Feeds, feeding and management

Para grass was collected daily in the areas surrounding the farm. Soybean waste was bought daily from the soybean milk factory in the city;  cassava chips and extracted soybean meal were bought on one occasion from an animal feed shop  for using throughout the trials. The animals were fed three times a day at 7:30h, 14:00h and 18:00h. Fresh water was always  available. The animals were vaccinated against rabbit hemorrhagic and parasite diseases.

Measurements

The measurements taken were  intakes of dry matter (DM), organic matter (OM), ash, neutral detergent fiber (NDF), daily weight gain, feed conversion ratio, economic returns, nutrient digestibility and nitrogen retention. Chemical analyses of DM, OM, CP, EE, CF, NDF and Ash followed the procedure of AOAC (1990) and Van Soest et al (1991).

Statistical analysis

The data were analyzed using the General Linear Model (GLM) option in the ANOVA program of the Minitab software (Minitab 2000). The comparison of differences between two treatments was done by the Tukey option of Minitab  (2000).

Results and discussion

Experiment 1. Feeding trial

The values for ingredient composition (Table 1) are in agreement with those compiled by Gőhl (1971). 

Feed intake and growth performance

Intake of Para grass declined as the offer level of cassava chips was increased (Table 2). Actual intake of cassava chips was slightly less than the offer level, but was strongly and positively related to total DM intake and growth rate (Table 3; Figures 1 and 2). Strangely, there was no relationship between cassava chip intake and feed conversion.  Under the conditions in Vietnam, economic benefits were increased by almost 70% by supplementing Para grass with cassava chips (Table 3). The results are in marked contrast with those reported by Hongthong Phimmmasan et al (2004),  where there was no improvement in growth rate of rabbits when a water spinach basal diet was supplemented with broken rice, a carbohydrate source of similar composition to cassava chips.  The reason may be that the response to supplementation with a starch-rich carbohydrate is more a function of the effect on the digestibility of the diet, than of the nature of the carbohydrate. In this respect, the digestibility of Para grass is considerably inferior to that of water spinach (Nguyen Thi Kim Dong et al 2006).  Supporting evidence for this hypothesis is presented in the results from Experiment 2.

Figure 1. Relationship between daily intake of cassava chips and total DM intake	Figure 2. Relationship between daily intake of cassava chips and growth rate

Carcass traits of growing rabbits

Cassava chip supplementation had no effect on the proportions of total lean meat and thigh lean meat in the carcass but increased the meat: bone ratio (Table 4).  When  corrected for final live weight, the weight and length of the caecum was decreased by cassava chip supplementation, presumably reflecting the reduction in dietary fiber as cassava chips replaced Para grass.

Experiment 2. Digestibility trial

The ingredients used in this experiment were similar in composition (Table 5) to those used in the feeding trial.

Apparent digestibility and N balance

The trends in intake of Para grass and total DM were similar to those in Experiment 1 (Table 6). Coefficients of apparent digestibility of all dietary components increased linearly as the offer level of cassava chips was increased. N  retention expressed as a function of metabolic live weight showed the same trend.

In the discussion of experiment 1, it was hypothesized that the positive effect of supplementation with starch-rich carbohydrate was probably the result of increasing diet digestibility as Para grass was replaced by cassava root chips. The results from experiment 2 appear to confirm this hypothesis, as observed in the positive linear relationships between percentage apparent DM digestibility (experiment 2) and live weight gain in experiment 1 (Figure 3) and with N retention in experiment 2 (Figure 4). In trials with water spinach replacing Para grass, similar positive relationships  were observed between apparent  DM digestibility and live weight gain (Nguyen Thi Kim Dong et al 2006). Replacing 75% of the Para grass in he diet with water spinach increased apparent DM digestibility from 63 to 73% and growth rate from 13 to 19 g/day.

Figure 3.  Relationship between DM apparent digestibility (from experiment 2)  and live weight gain (from Experiment 1)	Figure 4.  Relationship between DM apparent digestibility ( and N retention (from Experiment 2)

Conclusions

• Increasing the offer level of dried cassava chips in a basal diet of Para grass fed to growing rabbits led to linear increases in total DM intake, live weight gain and coefficients of apparent digestibility.
• There were positive linear relationships between coefficients of apparent DM digestibility and live weight gain and N retention
• It is proposed that the determinant of rabbit growth rate in forage-based diets is the overalll apparent digestibility of the diet rather than the composition of the diet in terms of the relative proportions of soluble and structural carbohydrates.

Acknowledgments

The authors acknowledge the support for this research received from the MEKARN program, financed by Sida.

References

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Nguyen Thi Kim Dong, Nguyen Van Thu, Ogle R B and Preston T R 2006: Effect of supplementation level of water spinach (Ipomoea aquatica) leaves in diets based on para grass (Brachiaria mutica) on intake, nutrient utilization, growth rate and economic returns of crossbred rabbits in the Mekong Delta of Vietnam. Workshop-seminar "Forages for Pigs and Rabbits" MEKARN-CelAgrid, Phnom Penh, Cambodia, 22-24 August,  2006. Article # 20. RetrievedDecember 15, 111, from http://www.mekarn.org/proprf/kimd2.htm

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