This feeding trial was carried out to determine the effect of feeding Duckweed ( Lemna sp) as partial replacement of the protein in boiled soya beans given as protein supplement to sugar cane juice for meat ducks. The experimental design for the trials was five treatments, four replications (pens) and 10 birds per pen. Diets, containing five levels of crude protein, which were formulated from boiled soya beans and Duckweed (Lemna sp), with sugar cane juice ad libitum as the energy source, were fed to 200 Cherry Valley Hybrid growing ducks. The rations were fed from one month of age until the birds reached 70 days of age, when two ducks (one male and one female) per pen were slaughtered for carcass evaluation. Daily intakes of dry matter and crude protein decreased significantly with decreasing soya bean supply, as there were no corresponding increases in Duckweed intake. Daily gains decreased linearly and feed conversion ratio increased (P<0.001) with decreasing soya bean supply. Carcass traits were not affected by treatment.

Duckweed (Lemna sp) is a simple tiny green flowering plant that grows on pond surfaces and is often called pond scum in popular parlance. The plant grows well in different climates, and is fast growing, with a high protein content, and can efficiently absorb nitrogen and phosphorus as well as heavy metals.

The possibility of using duckweed in poultry feeding as a source of protein and energy in place of conventional ingredients appears to be of potential importance. There are four common genera and more than 40 different species, all growing in dense clusters, forming blankets on the surface of nutrient-laden, open fresh water (Hillman 1961).

Dried duckweed meal, which can contain up to 40% protein, compares favourably with soya bean as a source of plant protein (Porath et al 1979). The protein content of duckweed, however, responds quickly to the availability of nutrients in the aquatic environment (Leng et al 1995). Consequently, duckweed grows slowly in clear, low nutrient waters and has a higher content of fibre, ash and carbohydrates, but contains relatively less protein (Muztar et al 1976). In contrast duckweed grown on sewage lagoons grows rapidly and has a high protein content (Leng et al 1995).

The experiment was conducted at the University of Cantho during the rainy season, between May and July, 1993. A total of 200 F1 Cherry Valley hybrids was used, with initial average live weight between 910 and 940 g. The birds were weighed individually at the start and before slaughter but by group once per week during the four weeks of the trial. Each treatment had 10 birds per group, with five different treatments with four replications per treatment. The experimental period, which started at 28 days of age was preceded by a preliminary period of one week. The birds were fed restricted amounts of boiled whole soya beans and duckweed ad libitum and were provided with reconstituted sugar cane juice ad libitum as a source of energy.

The ducks were housed in pens constructed from bamboo, with thatched roofs and rice straw-covered packed-earth floors. Each pen accommodated 10 ducks and had 1.7 m² floor area. During the daytime the birds also had access to outside sand yards (one duck/m²), and for approximately six hours per day to a fresh water swimming area (five ducks/m²).

Feeders and drinkers used during the experiment were round plastic basins (40 cm diameter, 10 cm depth). Natural light was used during the day and electric bulbs (4 w/m² of floor) at night from hatching to 60 days. The temperature during the experiment averaged 30°C.

The ducklings were given a commercial type starter diet between hatching and 21 days of age, and were then allocated at random to one of the experimental diets. When they were 28 days of age the experiment was started using the test diets based on boiled whole soya bean, duckweed and sugar cane juice. The treatments were planned to give ratios of soya bean protein to duckweed protein of 100:0, 85:15, 75:25, 65:35 and 55:45.

Dry soya beans were weighed daily and boiled each morning for half an hour in order to destroy the antinutritional factors in the grain. After boiling the grain was put into a bamboo basket to drain and cool before feeding. The soya beans were offered twice a day in the morning and in the afternoon and were restricted to the equivalent of 25, 21, 19, 16 and 14 g protein/day for the five treatment groups, respectively.

The duckweed was harvested during the afternoons every day during the experimental period. After collection, the duckweed was brought to the farm and was put into a bamboo basket to drain for one hour before being fed to the ducks. The duckweed was offered ad-libitum in amounts of about 1 kg, and given three, four or five times per day. The feeding frequency was increased according to the growth of the birds, to ensure that there was minimum wastage of duckweed. Refusals were collected and weighed the following morning.

Due to technical problems concerned with the purchase and storage of sugar cane, reconstituted raw sugar solutions were used. Raw sugar was dissolved in cold water (approximately 2.2 kg raw sugar to 5 kg water), and then mixed and measured as Bohmer degrees, to give a value of 11 Bohmer, equal to 20 ?Brix (dissolved solids). This was offered to the ducks in plastic basins covered with a grille to minimize spillage of the juice.

Sugar cane juice was offered ad libitum three, four or five times per day, with 800 ml/pen in each feeding, the times increasing with the age of the ducks, to ensure that there were minimum losses. Juice remaining in the basins each morning was collected and weighed to calculate amounts consumed.

Sample preparation and analyses of the dietary ingredients for dry matter (DM), crude protein (CP), crude fibre (CF), ether extractive (EE), ash, Calcium, Phosphorus and carotenes were carried out at Cantho University.

At the end of the experiment the ducks were weighed individually and two ducks per pen, (one male and one female) were selected at random for carcass analysis. The birds were slaughtered at 72 days of age, eviscerated and plucked, and the carcass weight recorded.

The data were analysed by analysis of variance using GLM (General Linear Model) procedure of MINITABt version 8.2 program statistical statistical software (Minitab Inc., 3081 Enterprise Drive, State College, PA, 1680-3008, 1991 USA). The balanced design of the trial permitted the use of GLM with initial live weight as a covariant. A similar method was used to analyse the carcass traits.

An economic analysis of the dietary treatments was carried out. Feed costs/day and per kg of daily gain were calculated, and profit over feed costs.

The chemical composition of the whole boiled soya bean and duckweed are shown in Table 1. The duckweed had on a dry matter basis a mean N x 6.25 content of 26.3%, dry matter 4.5%, crude fibre 11.0%, ether extract 3.2%, calcium 1.1%, and phosphorus 0.5%. The protein content of the duckweed was at the low end of the range reported by Leng et al (1995).

Table 1: Chemical composition of the cane juice and the protein sources: boiled soybean and Duckweed
	Boiled	Duckweed	Sugar cane
	soybean		juice
Analysed composition (%)
Dry matter	91.8	4.5	20
Analysed composition (% in DM)
N x 6.25	38.8	26.3	1
Crude fibre	8.4	11.0	--
Ether extract	20.0	3.2	--
Ash	5.1	15.9	--
Ca	0.4	1.1	--
P	0.5	0.5	--
Carotenes (mg/kg DM)	--	535	NA

NA Not analysed

The raw sugar used in the experiment was collected from a small sugar refining factory in the area, and is the syrup from sugar cane juice after evaporation. The sample used in this experiment had the following analysed chemical composition: ?Brix (total dissolved solids) 71; dry matter 71%; and (% of dry matter) N x 6.25 1.0; sucrose 59.8; reducing sugars 7.1.

The growth data in Table 2 show that there were significant differences between the treatments for live weight gain (P<0.001), with a linear decrease as the soya bean levels were restricted which in turn led to a decrease in intake of total N x 8.25. The regression equation for the effect of N x 6.25 intake on daily gain was:

Y = -40.5 - 0.078X + 4.60X²
r² = 0.55
X = N x 6.2, g/day
Y = liveweight gain, g/day

It was intended that 15, 25, 35, and 45% of the total N intake from whole soya beans should be replaced by N from duckweed, but in fact the ducks appeared to be unable to consume more than 450 g fresh duckweed daily. Thus the net effect was that total intake of N x 6.25 decreased from 26 to 20 g/day as soya bean levels were restricted. Only on the L15 diet was the planned rate of substitution achieved without decrease in N intake; but in this case also the liveweight gain was significantly less than on the control indicating that the protein in the duckweed was inferior in digestibility, or in amino acid balance, than that in whole soya beans.

Table 2: Effect of partial replacement of soybeans with duckweed in sugar cane juice based diets on feed intake and performance of growing ducks
	Ctl	L15	L25	L35	L45	SE/prob
Liveweight, g
Initial	926	922	917	933	91
Final	2165	1985	194	1852	1782
Daily gain	29.5	25.5	24.4	21.6	20.8	0.72/0.001
Feed intake, g/d
Soya bean	70.0	60.0	52.5	45.5	38.5
Duckweed	--	434	447	437	450	5.0/0.001
Cane juice	413	399	406	420	413	11.3/0.721
Total DM	147	154	149	145	138	2.3/0.005
Total N*6.25	25.7	27.3	24.8	22.2	19.8	0.06/0.001
N from duckweed
Per day (g)	--	5.2	5.3	5.2	5.3	0.06/0.001
Of total N (%)	--	19.0	21.4	23.4	27.0
Feed conversion
g DM/g LW gain	4.9	6.0	6.1	6.7	6.6	0.20/0.001

Total dry matter intake was similar for the control and L15 to L35 diets but was significantly less for the L50 diet for which soya bean intake was only half that of the control. Feed conversion was, however, poorer on all duckweed diets than on the control indicating that probably one limiting factor was the low digestibility of the duckweed used on this experiment. Evidence for this belief can be seen in some preliminary data reported by Vo Than Hai et al (1995) for pigs where protein digestibility on a diet of sugar cane juice and duckweed was only 50%.

It can be concluded therefore that the reduction in daily gains and deterioration in feed conversion were probably a result of both decreasing intakes of digestible energy and of total and digestible protein. The sample of duckweed used in this experiment was at the low end of the range reported by Leng et al (1995). It can be speculated that the results might have been better if a duckweed of a higher protein content had been used. Certainly, samples from duckweed grown for sale, and for feeding to ducks, in a village in the Thu Duc region (close to Ho Chi Minh city) had on average 35% of N x 6.25 in the dry matter, indicating that the villagers were aware of the need to manage the duckweed for optimizing its nutritive value.

Very little research has been done on the effects of including duckweed in diets for growing poultry. However in experiments on duckweed as a source of protein for laying hens it was possible to replace up to 40% of the diet with duckweed without problems. Dried duckweed was included in the diets of two commercial strains of laying hens at 0, 15, 25, and 40% inclusion (Hausten et al 1990). Egg production and egg weights were compared with those of hens fed a standard isocaloric and isonitrogenous control diet. At all levels of duckweed, hens maintained egg production and had mean egg weights similar to layers fed a control diet. Eggs from Leghorn hens fed 15 and 25% duckweed had higher protein content than control eggs. Also the addition of duckweed to the diets significantly increased yolk pigmentation, an important commercial application for this plant (Hausten et al 1990)

No vitamin/mineral premix was included in the diets of the groups that received duckweed and yet no health problems or deficiency symptoms were observed. The ability of duckweed to supply vitamins and minerals is an important advantage in rural areas where premixes may not be available or expensive. However, it should be mentioned that the ducks in this trial were given commercial starter diets, which contained vitamins and minerals, up to one month of age, and some of these would have been stored in the body.

Another interesting observation was the deeper yellow colour of the legs, beaks and skin of ducks fed with duckweed compared to the control group. This is appreciated in the market, and leads to higher retail prices. Duckweed samples were analysed for carotene content, which was found to be 535 mg/kg on a dry matter basis.

The effects of the treatments on carcass parameters are shown in Table 3. Carcass quality was not affected by dietary treatment (P>0.05). The yield of all edible parts did not differ between the treatments.

Table 3: Effect of partial replacement of soybeans with duckweed in sugar cane juice based diets on carcass composition of growing ducks
Item	Ctl	L15	L25	L35	L45	SE/prob
Liveweight, g	2265	2125	2020	1850	1845
Carcass weight, g	1656	1461	1227	1261	1282	82.9/0.7
Yield, % of LW	73	69	61	68	70	2.7/0.1
Weight organs, g
Heart	12.7	11.0	12.4	12.4	12.7	0.15/0.04
Liver	47.0	54.5	55.0	57.2	52.8	4.4/0.66
Gizzard	60.8	67.0	62.5	61.7	65.5	4.9/0.8
Weight cuts,g
Breast	363	326	318	316	312	17.0/0.23
Breast muscle	199	191	180	191	176	18.8/0.25
Breast fat+skin	97.9	74.1	70.0	66.3	67.8	13.9/0.51
Breast bone	71.6	59.3	62.5	59.5	63.3	6.76/0.9
Carcass measurements, cm
Length	22.8	22.0	23.4	22.6	22.5	0.55/0.60
Small int.	190	181	212	211	189	13.9/0.82
Large int.	13.7	13.0	13.3	14.4	11.6	0.97/0.66

The results are shown in Table 4. As can be seen losses were noted for all the diets in the experiment due mainly to the high cost of raw sugar. Cost of feed/kg gain, and net losses were lowest for the control group.

Table 4: Economic analysis of replacing soybeans with duckweed in diets for fattening ducks (1US$ = 10,500 VND)
	Ctrl	L15	L25	L35	L45
Daily intakes, g
Soya beans	70.0	60.0	52.5	45.5	38.5
Fresh duckweed		434	447	437	450
Cane juice	413	399	406	420	413
Premix, salt	0.26	-	-	-	-
Cost of the diet/day:
Soya beans	231	198	173	150	127
Fresh duckweed	-	43.4	44.6	43.7	45.0
Cane juice	74.8	72.1	73.4	76.0	74.8
Premix, salt	4.8	-	-	-	-
Total	310	313	291	269	247
Cost for total expt	13045	13168	12230	11334	10365
Cost feed/kg gain	10529	12382	11918	12329	11916
Profit/bird	-1894	-3596	-2994	-3060	-2536

From the results of this trial it can be concluded that fresh duckweed of relatively low protein content (<25% in DM) can only be used in limited amounts to substitute for conventional protein in the diets of fattening ducks. There were no adverse effects on health, but the reductions in growth rate and in feed conversion efficiency were considerable when duckweed replaced more than 20% of the soya bean protein.

However, it should be emphasised that the results obtained in the present trials relate to the use of duckweed of relatively low protein content (23% N x 6.25 in DM). There is an urgent need to assess the potential advantages of using duckweed grown in nutrient rich water which it is known leads to production of material of higher protein content (35-40% in DM) (Leng et al 1995).

We would like to thank the Swedish International Development Authority (SIDA) for its financial and material support of this project.

My study was made possible through my supervisors, Dr. Brian Ogle and Dr. Thomas R. Preston, and I gratefully extend my sincere thanks to them.

I also wish to thank the University of Cantho for allowing me to use their experimental facilities, and in particular Mrs. Kim Dong for her help in field work, Mrs. Xuan Dung for her special co-operation in the laboratory, Mr. Sanh my assistant in the field work and Mr. Men.

(Received 1 August 1995)