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

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

Hanoi University of Agriculture, Vietnam
*Finca Ecologica TOSOLY, AA48 Socorro, Colombia
 

Abstract

An experiment was conducted to determine effects of graded levels of paddy rice supplemented to water spinach (Ipomoea aquatica) or sweet potato (Ipomoea batatas) vines as basal diets for growing New Zealand White rabbits on feed intake, digestibility, average daily gain (ADG), and feed conversion ratio (FCR). 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 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, 4% of live weight (LW). The rabbits were housed and fed in individual cages. The experiment lasted 10 weeks following 7 days of adaptation.

It was found that level of paddy rice supplementation had a highly significant effect on the intake of forage either in an absolute term (g/head/day) (P<0.001) or as percentage of LW (P<0.001) but without affecting total feed intake (P>0.05). The more paddy rice was supplemented the less forage was consumed. Paddy rice supplementation reduced digestibility (P<0.01) but improved feed utilization/lowered FCR (P,001), and increased ADG (P<0.001). Significant differences between water spinach and sweet potato vines were found for basal diet intake (P<0.001) and FCR (P<0.05). Compared to sweet potato vines, water spinach was less consumed but better converted to weight gain with similar (P>0.05) digestibility. As a result, no significant effect of the type of basal diet on ADG was found (P>0.05). No significant (P>0.05) interactions between level of paddy rice supplementation and type of forage (basal diet) were found for any observed parameters.

Due to the opposing effects of paddy rice supplementation on live weight gain and digestibility in rabbits fed a basal diet of protein-rich vegetables, further investigations are warranted to find the underlying mechanisms.

Key words: New Zealand White rabbits, paddy rice, supplementation, sweet potato vines, water spinach.

Introduction

Rabbits efficiently utilize fibrous feed by courtesy of their feeding and digestive strategies (Leng 2006). Rabbits can get energy from forages; they always have done in the wild state and their digestive system is developed for this purpose. Among other forages, water spinach (Ipomoea aquatica) and sweet potato (Ipomoea batatas) vines are common vegetables used for rabbit feeding in Vietnam as the basal and even only diet. Water spinach has high biomass yield and is rich in crude protein content (280 g/kg DM), with low crude fiber (120 g/kg DM)  (Ly Thi Luyen 2003) and has proved to be a valuable basal diet for rabbits (Hongthong Phimmmasan et al 2004, Nguyen Thi Kim Dong et al 2006a,b), Pok Samkol et al 2006a,b,c) and Doan Thi Giang et al 2006). Sweet potato vines also have high protein content, from 25 to 29% in DM, while the protein content in stem is from 12 to 14% in dry matter (Hong 2003).

To improve the nutritional balance in diets based on such protein-rich vegetables, different supplements rich in digestible carbohydrates or fiber, or both, have been carried out (Hongthong Phimmmasan et al 2004; Khuc Thi Hue and Preston 2006; Doan Thi Giang et al 2006; Nguyen Huu Tam et al 2008); However, results obtained from those studies have been questionable. Paddy rice  seems to be an appropriate supplement to protein-rich vegetables fed as the basal diet. Nevertheless, so far, it has not been clear what is the pattern of response in terms of growth rate to level of paddy rice supplementation. This is a question of not only nutrition but also economics. It is needed to know what are the mechanism underlining each effect and what are nutritional and economic advantages of supplementing paddy rice to protein-rich vegetables.

The present study was aimed to test the hypothesis that responses of growing rabbits in terms of feed intake, digestibility and growth rate to level of paddy rice supplemented to water spinach or sweet potato vines fed as a basal diet are positive following non-linear curves.

Materials and Methods

Experimental design and procedures

Treatments and animals

A 5 x 2 factorial experiment was conducted to determine effects of 5 graded 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 2 basal diets for growing New Zealand White rabbits on feed intake, digestibility, average daily gain (ADG), and feed conversion ratio (FCR).

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 were divided into 5 sub-groups (treatments) of 4 rabbits each to receive one of the 5 graded levels of paddy rice.

Feeding regime and management

The experiment period lasted for 10 weeks following 7 days of adaptation. Before the experiment began the rabbits were vaccinated against hemorrhagic diarrhea and drenched against intestinal parasites.

The  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 in individual cages. Paddy rice was fed once per day at 11:00am. Drinking water was made available at all times.

 Data collection and measurements

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 taken 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 time period.

Feed conversion ratio (FCR) was calculated as a ratio of DM intake/live weight gain.

Chemical analyses

Feed and feces samples were analyzed for DM according to AOAC (1990).

Daily feed intake was calculated as g DM/head/day and as percentage of live weight.

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.

Location and weather

The experiment was carried out 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. The mean daily temperature and relative humidity in the area during the time of the experiment are given in Table 1.

Statistical analyses

Analyses of variance (ANOVA) were made 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

Table 2 shows relationships between levels of paddy rice supplementation and feed intake in rabbits fed a basal diet of sweet potato vines or water spinach vines. The level of paddy rice supplementation had a highly significant effect on the intake of the basal diet (P<0.001) but not on the total feed intake either in an absolute term (g/head/day) or as percentage of live weight (P>0.05). Significant differences in the observed parameters were also found between water spinach and sweet potato vines as basal diets (P<0.001). No interactions between level of paddy rice supplementation and type of forage (basal diet) were found significant (P>0.05).

As can be seen in Table 2, the more paddy rice was supplemented, the more it was consumed; however, at the same time, the intake of the basal diet was reduced accordingly. As a result, the total feed intake remained almost unaffected.

Concerning effects of paddy rice supplementation on growth of rabbits, Table 3 and Table 4 show relationships between the average daily gain (ADG) of rabbits and level of paddy rice supplementation and type of forage (sweet potato vines vs. water spinach vines) in the basal diet. The effect of level of paddy rice supplementation on ADG was found highly significant (P<0.001), but no significant difference in ADG was found between water spinach and sweet potato vines as basal diets (P>0.05). And, there was no significant interaction between level of paddy rice supplementation and type of forage (basal diet) for ADG (P>0.05). The higher amount of paddy rice was supplemented, the more it was consumed and, as a result, the higher the ADG was.

From Tables 3 and 4, it can also be seen that the rabbits had higher ADG for the first 5 weeks compared to those calculated for the whole 10 weeks of experiment. It was definitely due to the unfavorable changes in the weather conditions as shown in Table 1. It was getting hotter and wetter, and, as a result, the ADG was becoming lower and lower toward the end of the experiment. It was too hot for the exotic rabbits during the last weeks of the experiment.

It is clear from the above tables that the more paddy rice was offered, the more it was consumed with lesser intake of forage, and, as a result, the proportion of paddy rice in the diet increased accordingly. Figure 1 and Figure 2 show relationships between proportion of paddy rice in the diet and ADG in rabbits fed a basal diet of sweet potato vines or water spinach vines. There were strong positive relationships between the proportion of paddy rice in the diet and live weight gain of rabbits. This positive effect of paddy rice on growth may be explained by means of energy concentration of the diet. However, a previous study by Hongthong Phimmmasan et al (2004), supplementation with highly digestible carbohydrate (high energy density) in the form of broken rice was not successful. So, on the other hand, the positive effect might be due to the fibrous husk of paddy, as in other experiments (Khuc Thi Hue and Preston 2006; Doan Thi Giang et al 2006), supplements rich in fibre gave positive results with rabbits fed diets based on water spinach . It might also be due to combined effects of both fibre (from the husk) and starch of paddy rice supplemented to diets based on protein-rich vegetables. The results of this study confirmed the results from the experiment by Nguyen Huu Tam et al 2008 that rabbits fed on water spinach had higher feed intake and live weight gain when they were supplemented with  paddy rice.

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, relationships between proportion of paddy rice in the diet and DM digestibility in rabbits fed a basal diet of sweet potato or water spinach vines in the present study, as reflected in Table 5 and Figures 72 and 3, show the opposing effect. The more paddy rice was consumed, and the higher proportion of it in the diet as a result, the lower the digestibility was; whereas feed utilization (FCR) tended to be better. This should be questions for further investigation. Where was the effect inserted? Was it because of low digestibility of the husk, which lowered digestibility of the total diet? Is there something in the husk or somewhere in paddy rice, not in broken rice, that can improve post-digestive metabolism in favor of live weight gain? Or was it due to other reasons?

It should be also noted that type of forage (water spinach vs. sweet potato vines) had no significant effect on digestibility of the total diet DM (P>0.05). However, water spinach had a better (lower) FCR compared to sweet potato vines (P<0.05). No interactions between level of paddy rice supplementation and type of forage (basal diet) on  digestibility and FCR were found significant (P>0.05).

Acknowledgements

The authors are very grateful to the Swedish International Development Cooperation Agency, Department for Research Cooperation (Sida-SAREC) through the regional MEKARN Project, for the financial support of this study.  

References

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