Duckweed (Lemna minor) and water spinach (Ipomoea aquatica) as protein supplements for ducks fed broken rice as the basal diet

A feeding trial was carried out to evaluate the protein quality in water spinach or duckweed alone or mixed when included in a basal diet of broken rice for growing Muscovy ducks. There were three treatments, consisting in diets given ad libitum to 24 ducks: WS, water spinach plus broken rice (80:20 fresh basis); DW, duckweed plus broken rice (80:20 fresh basis) and MWD, water spinach and duckweed plus broken rice (35:45:20 fresh basis).

Total DM intake and total N*6.25 intake were highest for duckweed as the only supplement. Lowest values were with water spinach, with the mixed supplement (MWD) showing intermediate values. Average daily gain was highest for ducks fed diet DW (22.4 g/bird/day) and lowest when fed diet WS (6.2 g/bird/day). The growth rate of ducks was linearly related to total N*6.25 intake (R² = 0.89). Feed conversion was poorer for diet WS and MWD compared with DW (9.1, 4.3 and 3.8 g DM/gain for diet WS, MWD and DW respectively).

It can be concluded that growth rate of ducks can be used to assay the protein quality from vegetables. Total DM intake, total N*6.25 intake, live weight gain and feed conversion were better when the ducks were fed duckweed rather than water spinach as a supplement to broken rice.

Key words: Broken rice, ducks, duckweed, protein quality, water spinach

Introduction

Usually, conventional feed such as soya bean meal has been used as a source of protein in diets for pigs and poultry. However, soya bean meal is expensive, which results in high costs of production. The use of local feed resources can be a way to raise the income and to improve the standard of living of smallholder farmers. In this connection, many researchers reported that duckweed and water spinach have potential as a protein sources when combined with energy-rich feeds which are low in fiber such as broken rice, cassava root meal, sugar cane juice (Becerra et al 1995; Bui Xuan Men et al 1995, 1996; Nguyen Duc Anh and Preston 1997; Du Thanh Hang 1998; Le Thi Men et al 1997; Le Thi Men 1999; Ly et al 2002).

Duckweed (Lemna minor) is a simple tiny water plant that grows very well on pond surfaces. It can tolerate high nutrient stress and appears to be more resistant to pests and diseases than other aquatic plants. Moreover, it has high protein and carotene contents (Bui Xuan Men et al 1995). The protein content of duckweed responds quickly to the availability of nutrients in a water environment (Leng et al 1995).

Water spinach (Ipomoea aquatica) 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 convert efficiently the nitrogen in biodigester effluent into edible biomass with high protein content (Kean Sophea and Preston 2001).

Biological methods are rather easier and cheaper than chemical methods to determine the nutritive value of feeds. Furthermore, biological response data are more useful, and more easily understood, as a basis of making recommendations to farmers. Nguyen Duc Anh and Preston (1997) showed that growth rate of the duckling can be used to assay quality of protein resources.

Objective

The objective of the study was to evaluate the protein quality in water spinach and duckweed fed alone, or mixed, as the only supplement to a basal diet of broken rice for growing ducks.

Materials and methods

Location, climate and period

The experiment was conducted at the An Giang University, Vietnam, during the rainy season, between 31 August and14 September, 2003. Environmental temperature during the experiment averaged 27.3 °C, 31.8 °C and 27.7 °C at 6 am, 12 am. and 6 pm, respectively.

Experimental design and treatments

The experimental design was a Completely Randomized Design with two replicates per treatment. There were three treatments consisting of diets where broken rice was employed as the energy source, and different sources of vegetative protein as follows:

Experimental animals, housing and management

The animals were 24 Muscovy ducks of about 3 months of age. The ducks were raised in pens with thatched roofs and rice husk as a floor cover. Each pen accommodated 4 ducks and had 1.2 m² floor area. Feeders and drinkers used during the experiment were round plastic basins (28 cm diameter, 5 cm depth for feeders and 16 cm diameter, 3 cm depth for drinkers). Natural light was used during the day and electric bulbs at night.

Feed and feeding

The diets were prepared with 20% (on fresh basis) from broken rice and 80% (on fresh basis) from water spinach or duckweed or a mixture (35:45 on fresh basis) of both water spinach and duckweed. The broken rice, water spinach and duckweed were bought from the market and the prices were 2400, 1500 and 1000 VND, kg respectively. The water spinach was chopped finely before mixing with broken rice. Duckweed was mixed with broken rice without chopping. The ducks were fed ad libitum on each of the treatments. Feeds were offered twice daily at 07:00 am and 17:00 pm; water was freely available.

Chemical composition of feeds

The content of crude protein was highest and of DM lowest in the duckweed (Table 1). The level of 31.3% crude protein in DM of the duckweed indicated that it was of good quality. Keansombath Lampheuy (2003) showed that the range of crude protein in unfertilized and fertilized (200 kg N/ha from biodigester effluent) duckweed was from 16.7 to 34.5% in DM.

Diet characteristics

The calculated contents of DM and N*6.25 in dry basis of the diets ranged from 21.4 to 26.6% (for DM) and 12.7 to 13.1 for N*6.25 in DM (Table 3).

Table 2. Characteristics of the experimental diets
		Treatments#
	WS	DW	MWD
Ingredients, % fresh basis
Water spinach	80	-	35
Duckweed	-	80	45
Broken rice	20	20	20
Chemical analysis
DM, %	26.6	21.4	23.7
N*6.25, % in DM	13.1	12.7	12.9
# WS, DW and MWD are water spinach, duckweed and mixed water spinach and duckweed, respectively

Measurements and chemical analysis

The ducks were weighed at the start and afterwards at 5-day intervals. Feed offered and refused was recorded daily. Samples of feed offered and refused were taken every 5 days to determine N following procedures of AOAC (1990) and DM by micro-wave radiation (Undersander et al 1993).

Statistical analysis

Growth rates of the ducks were calculated as the linear regression of live weight (Y = g) on time (X = days). The data were analyzed by the General Linear Model (GLM) option of the ANOVA software of Minitab (version 13.31). Treatment means were compared using Least Significant Difference (LSD). The statistical model was

where µ is the overall mean, t _i is the treatment effect and e_ij is the experimental error

Results and discussion

Feed intake and growth rate

Total DM intake and total N*6.25 intake were highest for duckweed as the only supplement (Table 3). Lowest values were with water spinach, with the mixed supplement (MWD) showing intermediate values.

Table 3. Effect of water spinach or duckweed fed alone, or mixed, as supplements to broken rice for growing Muscovy ducks
	Treatments#			SEM	Prob.
	DW	MWD	WS	SEM	Prob.
Feed intake, g DM/day
Water spinach	-	12.4^a	20.2^b	0.59	0.001
Duckweed	17.3^a	7.3^b	-	0.48	0.001
Broken rice	66.6^a	50.2^b	35.7^c	2.29	0.001
Total DM	83.9^a	69.9^ab	55.9^b	3.09	0.001
Total crude protein	10.9^a	9.1^ab	7.4^b	0.38	0.001
Live weight, g
Initial	430	438	492	18.4	0.174
Final	771^a	691^a	583^b	18.0	0.012
Daily gain	22.4^a	16.7^a	6.2^b	1.21	0.006
Feed conversion, g DM/g gain	3.8^a	4.3^a	9.1^b	0.34	0.003
Feed cost/kg gain, VND##	22410	25791	57394	-	-
# WS, DW and MWD are water spinach, duckweed and mixed water spinach and duckweed respectively
## 15500 VND = 1 US dollar
^a,b,c Means without common superscripts in the same row are different at P<0.01

Average daily gain was highest for ducks fed duckweed as the only supplement and lowest for water spinach (Table 3). There was a linear growth response of the ducks to intake of total crude protein (Figure 1).

The results obtained in the current study clearly show that the growth rate of ducks was linear to total crude protein intake either derived from duckweed or from water spinach alone or both mixed, when added to a basal diet of broken rice as source of energy. Nguyen Duc Anh and Preston (1997) also reported that the growth response of ducklings was linearly related with crude protein from duckweed or soya bean meal. Feed conversion was poorer for water spinach alone or mixed with duckweed (9.1 and 4.3 respectively), as compared to duckweed (3.8), which resulted in feed cost being lowest for the duckweed treaatment.

Figure 1. Relationship between intake of crude protein and growth rate of Muscovy ducks

Conclusions

Acknowledgments

We are grateful to the MEKARN Program, financed by Sida-SAREC, for giving the senior author the opportunity to undertake this experiment. Sincere thanks are expressed to Dr. Julio Ly from the Swine Research Institute (Havana), who provided knowledge and valuable suggestions during the conduct of the trial. Special thanks are expressed to Mr. San Thy and Mr. Chhay Ty and to the classmates of the MEKARN MSc. Course, for helping in the field work and the laboratory. We also wish to thank the An Giang University and, in particular, the Faculty of Agriculture and Natural Resources for use of their experimental facilities.

References

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Becerra M, PrestonT R and Ogle R B 1995 Effect of replacing whole boiled soya beans with azolla in the diets of growing ducks. Livestock Research for Rural Development, 7(3): http://www.cipav.org.co/lrrd/lrrd7/3/7.htm

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Table 1. Chemical composition of the feeds
	DM, %	N, % in DM	N*6.25, % in DM
Duckweed	5.5	5.0	31.3
Water spinach	11.9	3.6	22.3
Broken rice	85.1	1.3	7.9