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

Nguyen Thi Kim Dong, Nguyen Van Thu, Brian Ogle^* and T R Preston^**

Cantho University, Vietnam
ntkdong@ctu.edu.vn
* Department of Animal Nutrition and Management,
SwedishUniversity of Agricultural Sciences, Uppsala, Sweden
** UTA, TOSOLY, AA #48, Socorro, Santander, Colombia

Abstract

In the Mekong Delta water spinach (Ipomoea aquatica) stems are used for making pickles for human consumption, while the leaves (WSL) are normally discarded. This study was carried out to evaluate WSL as a potential feed for rabbits. Two experiments were carried out in the experimental farm of Cantho University to evaluate the effects of offering WSL at levels of 25 (WSL25), 50 (WSL50) and 75% (WSL75) of the intakes of a control diet (WSL0) (DM basis). Both experiments were complete randomized designs with four treatments, three replicates and two female rabbits per experimental unit. The first trial (Expt.1) was done on growing crossbred rabbits at 8 weeks of age to evaluate feed utilization, growth rate and economic returns. In the second experiment (Expt. 2) feed digestibility and nitrogen retention was determined in twelve week old rabbits.

In Expt.1, the dry matter (DM) intake was highest (P<0.001) on the diet with para grass only (WSL0). Crude protein (CP) intake was significantly higher (P<0.01) in the WSL50 and WSL75 diets. Organic matte (OM), NDF and ADF intakes significantly decreased (P<0.01) with increasing levels of WSL in the diet. The highest daily gain was found in WSL75 (P<0.01). Lower feed cost and higher income per rabbit, resulting in higher net benefit, were found for the WSL50 and WSL75 diets. In Expt. 2 the apparent digestibility (%) of DM was significantly higher (P<0.05) in all the diets that included WSL compared with WSL0, with the highest value in the WSL75 diet. The nitrogen retention tended to increase with increasing levels of WSL (P>0.05), and the highest value was found in the WSL75 diet.

It is concluded that the inclusion of WSL at levels of 50 and 75% in para grass based diets gave better daily gains and economic returns.

Key words: Digestibility, economic return, growing rabbits, para grass, water spinach, weight gain.

Introduction

In recent years, rabbit production in Vietnam has developed rapidly to meet an increased demand for fresh meat for human consumption to replace the shortfall in supplies of meat of chickens and ducks as a result of Avian Influenza. The rabbit (Orytotagus cuniculus) is a non-ruminant, plant eating animal that has the ability to consume many kinds of natural grasses, vegetables, leaves from trees, fruits and by-products from kitchens or markets. It is also a meat-producing animal which is especially well adapted to utilization of protein-rich foliages due to the nature of its digestive system in which enzymatic digestion precedes microbial fermentation in the caecum.

Water spinach (Ipomoea aquatica) is a vegetable with high potential to efficiently convert nitrogen from biodigester effluent into edible biomass with a high protein content (Sophea and Preston 2001). It is a valuable forage resource for rabbits, according to the reports of Phimmasan et al (2004) and Chat et al. (2005). In The Mekong Delta, water spinach is widely used as a human food, particularly the stems, which are pickled for home consumption. This results in large amounts of water spinach leaves (WSL), which are cheap and high in protein being available for animals. Para grass (Brachiaria mutica) has a high fibre content, and the correct combination between WSL and para grass could be ideal for feeding rabbits. However, optimum levels of WSL in roughage-based diets have not yet been determined.

This study aims to determine the optimum level of water spinach leaves as replacement for para grass in the diet on the feed utilization, growth rate and economic benefits of growing crossbred rabbits.

Materials and methods

Experiment 1. Effect of different levels of water spinach leaf 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 Cantho 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 females were housed in a wire mesh and wood cage, as an experimental unit. The dietary treatments were water spinach leaf supplementation to the diets at levels of 0 (WSL0), 25 (WSL25), 50 (WSL50) and 75% (WSL75) (DM basis), while para grass was fed ad libitum. The diets were adjusted every week by increasing the allowance by 5% (DM basis). Paddy rice was supplied at the same level of 30g per day per rabbit for all treatments to provide energy. The experimental period lasted 8 weeks.

Feeds, feeding and management

Para grass was collected daily in the areas surrounding Cantho University. WSL were bought from farmers who plant and sell only WS stems for human consumption. The animals were fed three times a day at 8:00h, 15:00h and 19:00h. Para grass was offered ad libitum for all dietary treatments, and WSL was given at levels of zero (WSL0; control diet), 25 (WSL25), 50 (WSL50) and 75% (WSL75) of the amount of the control diet consumed, and adjusted daily. Paddy rice was supplied once per day at 11:00h with the amount of 30g / rabbit / day. Fresh water was available at all times. The refusals and spillage were collected and weighed daily in the morning to calculate the feed intake. The animals were vaccinated to prevent some diseases, especially rabbit hemorrhagic diarrhea and parasite diseases.

Measurements

The feeds and refusals were taken for analyses of DM, OM, CP, EE, NDF, ADF, and Ash following the procedures of AOAC (1990) and Van Soest et al (1991). At the beginning of the experiment two rabbits per experimental unit were weighed individually and thereafter weekly. Daily feed intakes, growth rate, and feed conversion ratios were measured and calculated. An economic analysis was also done for each treatment.

Experiment 2. Effect of different levels of water spinach leaf supplementation on nutrient digestibility and nitrogen retention

Animals and experimental design

The experimental design was similar to Experiment. 1, except that 12-week old rabbits were used. One week was allowed for adaptation and then faeces and urine were collected for 6 days. Feeds and refusals were measured daily. Dry matter, CP, EE, NDF and ADF digestibilities were calculated according Mc Donald (2002).

Statistical analysis

The data from both experiments were analyzed by analysis of variance using the ANOVA of General Linear Model of Minitab Reference Manual Release 13.20 (Minitab, 2000). Economic analyses were done using current prices in Vietnamese Dong (VND) to compare net incomes and feeds cost in the different treatments.

Results and discussion

Experiment 1: Feeding trial

Feed characteristics

Feed and nutrient intakes

Daily intake of para grass (PG) decreased significantly as the WSL supply increased (Table 2). However, the total DM intake in rabbits fed PG only (control diet, WSL0) was significantly higher (P< 0.001) than for those fed the diets that included WSL, possibly due to the low DM content in WSL, although total DM intake was unaffected by the inclusion level of WSL in the offered feed, and was similar for WSL25, WSL50 and WSL75. The DM intakes were higher than the values of 49.7- 74.6g DM/day and 65-75.5g DM /day reported in previous studies in which rabbits were fed water spinach-based diets (Hue and Preston, 2006 and Samkol et al., 2006, respectively). CP intake was least on the PG only (WSL0) and WSL25 diets, and increased with increasing WSL intakes, being highest on the WSL75 diet, as a result of high CP content in WSL. The CP intakes in the current study are considerably higher than those reported by Phimmasan et al. (2004) (8.8 - 11.3g/day). The intakes of OM, NDF, ADF and ME in the WSL supplemented diets were lower than those in the control diet.

Growth rate, feed conversion ratio and economic analysis

Significantly higher final live weight and daily gains were found for rabbits in the WSL50 and WSL75 treatments as compared to those fed PG only and 25% WSL, with the highest values  for rabbits supplemented with 75% WSL (Table 3).  The difference was probably due to the higher CP intakes from WSL, as intakes of DM were higher for the WSL0 treatment. The performance results in the present study are similar to those in a study by Ha et al. (1996), who reported that the weight gains of pure New Zealand rabbits fed grasses, concentrate and legume leaves were 15.5g and 20.6g per day. The performance of the rabbits supplemented with WSL in this study were also within the range reported by Samkol et al. (2006), and were slightly lower than those found by Phimmasan et al. (2004) and Hue and Preston (2006). Feed conversion ratio was poorest for animals fed PG alone (WSL0), and improved (P<0.01) with inclusion level of WSL, as a result of better daily gain and lower DM intakes compared to the control treatment. Feed conversion ratios were acceptable and similar to the values of 4.9 - 6.0, and 7.0, reported by Akinfala et al. (2003) and Suc et al. (2000), respectively.

The economic analysis showed that the cost of feed decreased with increasing levels of WSL. Furthermore, due to the increased growth rates with increasing supply of WSL the net benefits (feed costs minus income) were highest for the WSL50 and WSL75 treatments.

Experiment 2: Digestibility trial

The chemical composition of the feedstuffs used (Table 4) was similar to those in Experiment 1, except for a slightly lower CP content of the WSL. Feed intakes ranged from 67 to 88g DM / day, and tended to be lower for the diets including WSL,  although the differences were not significant.

The apparent DM digestibility coefficients were significantly higher (P<0.05) in the diets with inclusion of WSL than that in the control diet (Table 5). However, no significant difference in CP digestibility was found among the treatments. Digestibility of OM and EE was lower in the control diet (WSL0) than in the diets with WSL (P<0.05). The probable explanation is the high fibre content in PG, and Gidenne et al (1998) and Perez et al (1996) stated that a high level of fibre in the diet leads to a decrease of retention time and an increase of caecotrope production because of increasing bacterial fibrolytic activity, which in turn results in a reduction of diet digestibility (De Blas et al 1999). The DM and NDF digestibilities are similar to those in a study with rabbits reported by Samkol et al. (2006) in which water spinach based diets had a DM digestibility of 73.5 -78.3% and NDF a digestibility of 48.2- 56.3%. However, the CP digestibility values in the present study are superior to the values of 59.9 -70.9% reported by Pok Samkol et al (2006). Our results are also in agreement with the lower digestibility indices of DM, CP and CF that were found in diets including high fibre supplements (Hue and Preston 2006).

Table 5. Effect of level of offer of water spinach leaves (WSL) on digestibility of nutrients and nitrogen retention
Item	WSL0	WSL25	WSL50	WSL75	SE/P
Feed intake (g DM/day)	88.0	67.0	73.5	74.5	12.1/0.18
Apparent digestibility (%)
DM	62.7a	70.5b	71.7b	73.0b	2.29/0.05
OM	62.4a	70.1b	74.9bc	76.0c	1.14/0.001
CP	82.0	84.5	83.5	83.0	2.24/0.90
EE	72.7a	78.5b	80.0b	81.0b	0.86/0.01
NDF	42.7	52.0	53.3	57.5	4.53/0.25
ADF	33.0	40.0	46.7	51.1	3.76/0.08
Nitrogen balance (g/kgW0.75)
Nitrogen intake	1.21a	1.28a	1.48b	1.60b	0.04/0.001
Nitrogen retention	0.76	0.84	0.88	1.07	0.09/0.15
Means with different letters within the same row are significantly different at the 5% level.

Both the nitrogen intake and nitrogen retention increased corresponding with increasing levels of WSL in the diet, but differences were significant only for the nitrogen intake (P<0.001). The values in the present study are in a similar range to those reported by Thu and Dong (2005), and superior to those of Samkol et al (2006).

Conclusions and implications

• Water spinach leaves have a high nutritive value for rabbits, as confirmed by high protein content and the improved apparent digestibility of most nutrients in the diets including water spinach leaves.

• Increasing levels of water spinach leaves in the diets of growing crossbred rabbits increases feed utilization and growth rate.

• Levels of from 50 to 75% of water spinach leaves supplemented to para grass diets give higher economic benefits for small-holder farmers.

• Comprehensive studies on water spinach leaves as a supplement for rabbits are necessary.

Acknowledgements

Financial support of this work from SIDA-SAREC through the MEKARN project is gratefully acknowledged. The authors would like to thank the Department of Animal Science, Faculty of Agriculture and Applied Biology of Cantho University for infrastructure support.

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