Digestibility indices and N balance in growing rabbits fed a basal diet of water spinach (Ipomoea aquatica) supplemented with broken rice

Pok Samkol, T R Preston¹ and J Ly²

Center for Livestock and Agriculture Development, (CelAgrid-UTA Cambodia), Cambodia
samkol@mekarn.org   ,   samkolpok@yahoo.com
¹UTA (Colombia), Socorro, Santander, Colombia
trpreston@mekarn.org
²Swine Research Institute, PO Box 1, Punta Brava, Havana City, Cuba
jlyca@yahoo.com   ,   iip@enet.cu

Abstract

Four Local and 4 New Zealand White male rabbits with an initial live weight of 1.75 ± 0.10 kg were fed water spinach (Ipomoea aquatica) ad libitum supplemented with broken rice (0, 4, 8 and 12g/day) according to a duplicate 4*4 Latin Square design.

There were no significant differences for the interaction genotype x diet in any of the indices examined. There was a significant decrease in faecal pH with increasing levels of broken rice. However, there was no significant effect among treatments on the DM, NH₃ and SCFA concentrations in the faeces. Faecal DM content was higher for local than for NZ White rabbits.

There were no significant differences in digestibility indices between levels of broken rice. There was a trend (P<0.10) for DM and organic matter digestibility indices to be higher in the local than in the NZ genotype. This trend was significant (P<0.05) for N digestibility favouring the local breed. There was no significant effect on N retention due to genotype or the level of broken rice.

It is concluded that water spinach is highly digestible for growing rabbits.

Key words: Broken rice, digestibility, N balance, rabbits, water spinach (Ipomoea aquatica)

Introduction

Water spinach (Ipomoea aquatica) appears to have a high potential as a forage source for rabbits according to the preliminary report of Hongthong Phimmasan et al (2004). It is a vegetable that is consumed by people and animals. It has a short growth period, is resistant to common insect pests and can be cultivated either in dry or flooded soils. Moreover, it has been found that water spinach is a vegetable with a high potential to efficiently convert nitrogen from biodigester effluent into edible biomass with high protein content (Kean Sophea and Preston 2001).

Water spinach was reported to have a high in vitro N digestibility (68.8%) and a high concentration of water extractable N (52.0%) (Ly and Preston 2001). In the preliminary trial conducted by Hongthong Phimmasan et al (2004), using the indirect "insoluble ash" method (Van Keulen and Young 1977), it was found that water spinach had high digestibility indices of DM, ash, organic matter and crude protein (84.7, 76.7, 88.4, and 79.6%, respectively) for growing rabbits.

Broken rice is commonly used as an energy source for pigs and poultry in Cambodia, and can be found easily in local markets. Moreover, it does not compete with human food, due to a current surplus of rice grain production in the country.

The aim of this experiment was to confirm the observations of  Hongthong Phimmasan et al (2004), concerning the nutritive value of water spinach in growing rabbits, but using the direct method for determining digestibility.

Materials and Methods

Location and climate

The experiment was conducted in the ecological farm of the Center for Livestock and Agriculture Development (CelAgrid-UTA Cambodia), located in Rolous village, Rolous commune, Kandal Stoeung district, Kandal province, about 26 km from Phnom Penh City, Cambodia. During the trial (10 July to 19 August 2004), the average ambient temperature was 25.9 ± 0.96 ^oC in the morning at 6: 00 am, 31.7 ± 1.59 ^oC in the middle of the day (12:00 am) and 28.3 ± 1.90 ^oC in the afternoon at 6: 00 pm.

Experimental design

Four local male rabbits (1.77 ± 0.10 kg live weight) and four New Zealand White males (1.74 ± 0.10 kg) were allocated to 4 experimental treatments according to a duplicate 4*4 Latin square arrangement (Table 1).

The treatments were:

• Br0: water spinach ad libitum
• Br4: same as Br0 but with 4 g/day of broken rice
• Br8: same as Br0 but with 8 g/day of broken rice
• Br12: same as Br0 but with 12 g/day of broken rice

The rabbits were housed in metabolism cages during the trial. The metabolism cages (50 x 55cm) were built to allow the quantitative collection of faeces and urine (Photo 1), and were installed in an open stable.

Each experimental period consisted of five preliminary days when the rabbits were adapted to the diets followed by another five days for collection of faeces, urine and feed refusals. The rabbits were weighed at the beginning of the trial and at the end of each period.

Feeds and feeding system

The water spinach was collected along the canal of CelAgrid-UTA Cambodia. The whole plant (combined stems and leaves) was offered as bunches hanging from the side of the cage (Photo 1). The proportions of stems and leaves were determined in samples taken at random on four occasions during the trial (Table 2). The level offered was approximately 50% greater than the recorded intake. Fresh water spinach was offered three times per day: in the morning at 7.30 am, at the middle of the day at 12:00 am and in the afternoon at 4:00 pm. On the basis of previous observations (Miech Phalla, personal communication), additional water was not provided as the fresh plant contains almost 90% water thus providing a ratio of moisture to dry matter of close to 9:1.

The broken rice was purchased in a local market, about 5 km from CelAgrid-UTA Cambodia. It was offered in a metal bowl anchored by wire to the side of the cage to avoid spillage. The broken rice was offered two times per day, in the morning at 7: 30 am and in the afternoon at 4:00 pm, in order to decrease the speed of eating by the rabbits.

Data collection

Feed refusals and faeces were collected every day and were 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, NH₃, Short Chain Fatty Acids (SCFA), DM, N, crude fiber and ash content. Organic matter concentration was calculated as 100 minus % ash in dry basis. Urine was collected in a plastic bucket to which 40% (w/v) sulphuric acid was added to maintain the pH below 4.0. At the end of each period the volume was measured and a sample analysed for N.

Chemical analyses

Chemical analyses of diets and faeces were undertaken following the methods of AOAC (1990) for ash, N, and crude fiber. The DM content was determined using the microwave method of Undersander et al (1993). Fresh faeces were analysed for pH by a digital meter with glass electrode. Faecal short chain fatty acids (SCFA) and ammonia were determined by titration of the sample recovered after steam distillation of filtered faecal slurry (1:5 by weight of fresh faeces and water) as outlined by Ly et al (2001). The N content of urine was determined by the AOAC (1990) procedure.

Statistical analyses

The data were subjected to analyses of variance according to the general linear model of the Minitab software (Minitab release 13.31, 2000). When the "F" test was significant (P<0.05), the means were separated using the Tukey comparison option in the Minitab software. The model used was the following:

Yhijk ₌ m + Bh + Ti + Pj +Ak + ehijk

where

Yhijk = Dependent variable,
m = overall mean,
Bh breed effect,
Ti = treatment effect,
Pj = period effect,
Ak = animal effect,
BhTi interaction of breed and treatment,
ehijk = random error

Results and discussion

Animal status

No signals of discomfort were apparent during the conduct of the trial, and the animals showed good health and gained in live weight.

Feed characteristics

There was a low content of ash, N and crude protein in the broken rice (Table 3). Crude protein concentration was higher in leaves than in stems of water spinach. Crude fiber content was higher in the stems than in leaves.

Feed intake

The rabbits selected more leaves than stems (Figure 1).

There were no significant differences among treatments in total DM intake (Table 4).

However, DM intake from stems plus leaves and the intake of crude protein, declined as the level of broken rice increased (P<0.01). The introduction of increasing levels of broken rice in the ration provoked a significant (P<0.01) decrease in total crude protein intake by the animals.

Rabbits of the local breed selected more leaves than stems and had higher intakes of DM and crude protein, compared with the NZ rabbits. DM intake per unit live weight was higher with 4 and 12 g broken rice compared with zero and 8 g/day (P<0.05) and higher for the local compared with the NZ breed (P<0.01).

The range of intakes (40 to 42 g DM/kg live weight) are slightly lower than was reported by Hongthong Phimmasan et al (2004) on similar diets (48 to 66 g DM/kg live weight).

Faecal characteristics and output

There was a significant (P<0.05) decrease in faecal pH when broken rice levels were increased (Table 5).

This was related with a significant (R² = 0.999) curvilinear increase in SCFA concentration (Figure 3).

The increase of SCFA in the faecal output is in accordance with the findings of Hongthong Phimmasan et al (2004), who reported that the increase of SCFA was probably due to the fermentation in the caecum and large intestine of the starch in the broken rice. Bellier et al (1995) described the relationship between pH and SCFA, with the caecal pH varying inversely to the increase in SCFA concentration. Gidenne and Perez (1993) showed that faecal losses of starch were very low in most cases, although as levels of starch increased in the diet, those losses were significantly greater. Starch digestion normally occurs in the small intestine; however, De Blas and Gidenne (1998) reported that it may also be degraded to some extent in other parts of the digestive tract such as the stomach and the large intestine.

Digestibility indices and N balance

There were no significant differences in digestibility indices due to feeding of supplementary broken rice. However, there was a tendency for digestibility of DM (P=0.066) and organic matter (P=0.091) to be higher in the Local breed as compared to the New Zealand genotype. The difference was significant for crude protein (P=0.028) in favour of the local breed. These differences between the breeds can be explained by the greater selection for leaves, compared with stems, by the local breed (Table 3), and the fact that digestibility of DM and especially of crude protein were related to the proportion of leaves consumed (Figures 4 and 5).

There were no differences in digestibility of crude fiber between the two breeds; however, the fiber intake by the local breed would be less because there is much less fiber in the leaves than in the stems (Table 3).

The digestibility coefficients in the present study were in the same range as those reported by Hongthong Phimmasan et al (2004) (84 to 89% for DM and 76 to 80% for crude protein). They are higher than has been observed with commercial pelleted diets fed to local rabbits in Mauritius (69.2% for DM and 74% for crude protein) (Ramchun et al 2000a), or a combination of Star grass with mash (68.8% for DM and 81.3% for crude protein) (Ramchun et al 2000b). Bamikole and Ezenwa (1999) in Nigeria reported digestibilities of 50.2% for DM and 61.4% for crude protein for a diet of Verano stylo and 52.8% for DM and 61.8% for crude protein with Guinea grass. Cunha et al (2004) in Portugal offered alfalfa as a source of fiber and recorded 61.4% digestibility for DM and 73.2% for crude protein.

There were no differences in any of the measurements of N retention due to broken rice supplementation and no differences between breeds (Table 6).

Conclusions

• Fresh water spinach has a high nutritive value for rabbits as shown by the high voluntary intake, and high apparent digestibility of DM and crude protein.
  
  
• There were no nutritional advantages from providing additional digestible energy in the form of broken rice.
  
  
• With an offer level of about 50% over recorded intakes, the rabbits selected strongly for leaves rather than stems.
  
  
• Water spinach appears to be a complete diet for rabbits providing water as well as all required nutrients.

Acknowledgements

This paper is derived from the thesis submitted by the Senior Author as partial requirement for the MSc degree at SLU, Uppsala, Sweden. The authors would like to thank the Swedish Agency for Research Cooperation with Developing Countries (SAREC) for funding this study through the regional MEKARN project. In addition, thanks are given to Mr. Von Vyreak and Miss Chuon Vasna for their assistance during the conduct of the experiment. We address our gratitude to Mr. Chhay Ty for help with analyses in the laboratory.

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