Effects of paddy rice on feed utilization and growth of New Zealand White rabbits fed basal diets of water spinach (Ipomoea aquatica) or sweet potato vines (Ipomoea batatas)

Nguyen Thi Duong Huyen, Nguyen Xuan Trach and T R Preston*

Hanoi University of Agriculture, Hanoi, Vietnam
nghuyen.hua@gmail.com
* Center for Research and Technology Transfer
Nong Lam University, Thu Duc District, Ho Chi Minh City, Vietnam

Abstract

A total of 40 male growing rabbits were equally divided into 2 main groups of 20 each to be fed on either of the two basal diets: water spinach (Ipomoea aquatica) or sweet potato vines (Ipomoea batatas). Each of the two main groups were divided into 5 sub-groups (treatments) of 4 rabbits each to receive one of 5 graded levels of paddy rice, which were 0, 1, 2, 3 and 4% of live weight (LW).

Level of paddy rice had no effect on total DM intake, reduced digestibility but improved feed conversion and  LW gain. Compared to sweet potato vines, water spinach was more digestible and supported better feed conversion than sweet potato vines.  Under most circumstances the improvement in growth performance due to paddy rice supplementation is unlikely to be economical, when high quality forages such as water spinach and sweet potato vines are the basis of the diet.

Key words: carbohydrate, digestibility, energy, protein, supplementation

Introduction

Rabbits can efficiently utilize fibrous feed by courtesy of their feeding and digestive strategies (Leng 2006).  Among other forages, water spinach (Ipomoea aquatica) and sweet potato (Ipomoea batatas) vines are protein-rich vegetables which have been commonly used as a basal diets for rabbits (Nguyen Thi Kim Dong et al 2006a,b; Pok Samkol et al 2006a,b,c; Doan Thi Giang et al 2006). To improve the nutritional balance in diets based on such protein-rich vegetables, different supplements rich in digestible starch or in fiber have been carried out (Hongthong Phimmmasan et al 2004; Nguyen Huu Tam et al 2008).  However, results obtained from those studies have been in marked contrast, with no growth response to broken rice (Hongthong Phimmmasan et al 2004); but with positive effects from paddy rice (Nguyen Huu Tam et al 2008; Chhay Ty et al 2013;  Hang et 2013; Huyen et al 2013; Phuong et al 2013; Tam et al 2013).  The present study was aimed to test  production responses of growing rabbits to graded levels of paddy rice, and to relate these to effects on digestibility in basal diets of water spinach or sweet potato vines.

Materials and methods

The experimental design was a  5 x 2 factorial arrangement with 5  levels of paddy rice (0, 1, 2, 3, and 4% of live weight) supplemented to water spinach (Ipomoea aquatica) or sweet potato (Ipomoea batatas) vines fed as basal diets for growing New Zealand White rabbits.  A total of 40 male growing rabbits at 8 weeks of age were equally divided into 2 main groups of 20 each to be fed on either of the two basal diets. Each of the two main groups was divided into 5 sub-groups (treatments) of 4 rabbits each to receive one of the 5 graded levels of paddy rice.

The experiment lasted for 10 weeks following 7 days of adaptation at the experimental farm of the Faculty of Animal Science and Aquaculture, Hanoi University of Agriculture, Hanoi, Vietnam, during the period from late March to end of May 2010. Before the experiment began the rabbits were vaccinated against hemorrhagic diarrhea and drenched against intestinal parasites. During the experiment, rabbits were housed and fed in individual cages. The animals were fed either water spinach or sweet potato vines ad libitum three times a day at 8:00, 14:00, and 20:00h. Paddy rice was fed once per day at 11:00am. Drinking water was made available at all times.

All animals were individually weighed at the beginning and thereafter once a week until the end of the experiment to calculate the average daily gain (ADG) as the slope of the linear regression of live weight on time. Total feed and feces collections were made over 7 consecutive days in the middle of the experiment. The feeds offered and refusals were collected and weighed daily in the morning. Representative samples of feces (10%) were collected daily from the total feces collected and stored at -25°C. At the end of the 7 days, the samples were bulked according to individual animals. Feed samples were taken over the same period.

Feed and feces samples were analyzed for DM according to AOAC (1990).  Feed conversion ratio (FCR) was calculated as a ratio of DM intake/live weight gain. Apparent total tract DM digestibility was computed as DM digestibility (%) = (A-B/A)*100, where A and B are total DM intake and total DM in feces, respectively.

Data were subjected to analyses of variance (ANOVA) for a 5x2 factorial model with interactions, using the General Linear Model (GLM) of Minitab 16 (2010). Pair-wise comparisons of means were done using the Tukey method. Regression analyses were made using Microsoft Excel (2007).

Results and discussion

Intake of the basal diet decreased as the offer level of paddy rice was increased (Table 1) but there were no differences in total DM intake. DM intake on sweet potato vines was 15% higher than on water spinach.

The higher the level of paddy rice that was offered, the more it was consumed (Table 1; Figures 1 and 2); however, at the same time, intake of the basal diet was reduced accordingly, so that total DM intake was not affected (Figures 3 and 4).

Figure 1. Relationship between level of paddy rice supplementation and feed intake in rabbits fed a basal diet of sweet potato vines	Figure 2. Relationship between level of paddy rice supplementation and feed intake in rabbits fed a basal diet of water spinach

Figure 3: Effect of paddy rice intake on forage intake and total feed intake in rabbits fed a basal diet of sweet potato vines	Figure 4: Effect of paddy rice intake on forage intake and total feed intake in rabbits fed a basal diet of water spinach

Growth rates on both sources of forage were increased by supplementation with paddy rice (Table 2; Figure 5) but the effects were relatively small (on average about 20 g paddy rice were needed to increase growth rate by 1 g/day). There were no differences in growth rate between the two forages, although there was an indication that the response to paddy rice was less with water spinach than with sweet potato vines (Figure 5).

Figure 5. Effect of paddy rice intake on growth rates of rabbits fed basal diets of sweet potato vines or water spinach

Figure 6: Relationship between proportion of paddy rice in the diet and DM digestibility in rabbits fed basal diets of sweet potato vines (٠) or water spinach (٠)

To find the mechanism behind the positive effect of paddy rice supplementation on live weight gain of rabbits fed a basal diet of protein-rich vegetables, improvement in digestibility might have been expected. However, the relationships between the proportion of paddy rice in the diet and DM digestibility showed the opposing effect (Table 3;  Figure 6), with no differences between the two sources of forage. By contrast, the DM feed conversion improved with paddy rice supplementation (Table 3). Feed conversion was better with water spinach than with sweet potato vines. One explanation for these apparently opposing findings could be the energy cost of excreting the excess protein present in both water spinach and sweet potato vines when these are fed as the sole  diet. This could explain both the greater weight gain and the better feed conversion. However, these benefits have only been apparent when the low protein-high carbohydrate supplement was paddy rice. This effect was not observed with rice grain as the supplement (Hongthong Phimmasan et al 2004; Huyen et al 2013).

The implications from these results and others reported in the literature (Nguyen Huu Tam et al 2008; Inthapanya and Preston 2009;  Luyen and Preston 2012; Chhay Ty et al 2013; Hang et al 2013; Huyen et al 2013; Phuong et al 2013; Tam et al 2013) is further confirmation that paddy rice does improve growth rate and feed conversion when the basal diet is composed of forages. However, when the forages are high in crude protein and are highly digestible the improvements in performance of growing rabbits attributable tot he paddy rice will rarely be economical. Thus, assuming the  sale value of a live rabbit is VND 40 000/kg live weight and that paddy rice has a cost of VND4000/kg then even at the point of maximum response to paddy rice supplementation (between 0 and 1% of LW), the value of the increased growth rate (1.2 g/day = 48 VND is considerably less that the cost of the supplement (20.4 g/day = VND 81). Thus under most circumstances the most economical feeding system is likely to be either water spinach or sweet potato vines fed as the sole diet.

Conclusions

• Growth rate and feed conversion in rabbits fed basal diets of water spinach or sweet potato vines was improved by supplementation with paddy rice, even though overall DM digestibility of the diet was reduced. However, under most circumstances, the improvement in performance is unlikely to compensate for the added cost of the supplement.

Acknowledgements

The authors are grateful to the Swedish International Development Agency, through the regional MEKARN Project, for the financial support of this study.

References

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Received 28 April 2013; Accepted 9 May 2013; Published 2 June 2013

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