This trial was carried on a small farm in the Mecong delta close to Cantho city. Local Muscovy ducks (120) were used as these are preferred by many farmers in the area. The basal diet was broken rice and the protein supplement came from roasted soya beans to supply 100 (control), 40 (DW60) or 0 (DW100) protein on the three treatments. Duckweed was available ad libitum on the experimental treatments DW60 and DW100. Males were managed separately from females. The birds were kept in groups of 5 so there were four replications of each treatment/sex combination.

The intakes of duckweed increased as the soya beans were restricted and, on average, reached 38% of liveweight (mean for both sexes) on the diets without soya beans. Growth rates were highest for males fed the control diet (36.1 g/day). There were no differences between the DW60 and D100 diets for both males and females (range from 21.7 to 22.5 for females and 27.6 to 28.3 for males). Feed dry matter conversion was poorer (4.06 to 4.17 for females and 4.12 to 4.23 for males) on all duckweed diets compared with the controls (3.76 and 3.24 for females and males respectively).

There was a slight indication (P=0.1 for females and 0.34 for males) that carcass yield was reduced on the duckweed treatments but differences were small. There were no differences in weights of chest and thigh muscle nor in heart and liver weights.

For farmers growing the duckweed there were economic benefits on all duckweed diets with best results from the complete substitution of the soya beans.

In an experiment done "on-station" at the Cantho University it was shown that duckweed (Lemna spp) could completely replace roasted soya beans in a fattening diet for improved ducks (Peking x Cherry Valley crosses) in which the energy source was broken rice (Bui Xuan Men et al 1995).

The aim of the following trial was to evaluate the same basic treatments (replacement of roasted soya beans with duckweed in a diet of ad libitum broken rice) but under "on-farm" conditions with local Muscovy ducks which are commonly used by farmers in the Mekong Delta, accounting for about 20% of the total duck population (Phuoc et al 1994).

The experiment was carried out on a small farm in the suburbs of Cantho town where Muscovy ducks are commonly raised. The experiment was conducted between May and July 1995, in the early rainy season. One hundred and twenty 4-week-old local Muscovy ducklings ( sixty females and sixty males) were allocated in groups of 5 to three treatments with equal numbers of males and females (in separate pens) on each treatment. The treatments were:

• Control: Broken rice balanced with protein from roasted soya beans to supply 100% of requirements and duckweed ad libitum
• D60: The same as the control but with soya restricted to 40% of that in the control diet
• D100: The same as D60 but with no soya beans

The Muscovy ducklings were selected at one day of age from clusters of small breeding flocks scavenging in the backyards and gardens of small holdings. They were brooded in a shed built in a garden and fed conventional diets ad libitum from 1 to 28 days of age. The birds were identified and then individually weighed at the beginning and end of the trial and at weekly intervals. Females were slaughtered at 70 days of age and males at 84 days. All ducks were given the duck plague vaccine and a dose of antibiotics on the seventh and twenty-fifth day of age to prevent diseases.

The energy source was broken rice fed ad libitum in feeders providing constant access to the ducks. The amounts of protein from soya beans for the control group were based on the recommendations of Leclercq (1985) and Yeong and Azizah (1989). This corresponded to 35 g/day of ground soya beans for males and 27 g for females.They were roasted on a stove to destroy anti-nutritional factors, stored whole and milled into a meal once weekly. They were fed twice daily at 08.00-08.30 and 16.00-16.30. Salt and a mineral-vitamin premix (0.25% of the diet) were added to the control diet but not to the DW60 and DW100 diets.

The duckweed used in the experiments was grown on ponds enriched with nutrients from effluent from biodigesters and home wastewater. It was collected twice daily: in the morning and in the afternoon. Before feeding it was washed with tap water and put in a big bamboo basket for one hour to drain the excess water from the duckweed to reduce bulk. The nutrient content of the feeds is shown in Table 1.

Table 1: Mean values for composition of duckweed, broken rice and soya beans used during the experiment
Composition	Duck	Broken	Soya
	weed	rice	bean meal
DM, %	4.7	86.8	87.0
As % of DM
N x 6.25	38.6	9.5	44.0
Ether extract	9.82	1.42	21.1
NFE	8.58	80.4	16.1
Fibre	18.7	2.01	9.76
Ash	19.0	1.15	5.63
Ca	0.71	0.0057	0.18
P (total)	0.62	0.2	0.92
K	4.29
Na	0.14
Fe	0.27
As mg/kgDM
Mn	1723
Zn	75
Cu	20
Carotene	1025

Data for feed intake are given in Table 2. The duckweed was consumed readily and, as expected, increased as the soya bean component was restricted. Males ate more than females. The intakes of duckweed as a percentage of liveweight on the diets without soya beans were slightly lower (38% mean of both sexes) than was recorded in the earlier study (Bui Xuan Men et al 1995) with mixed sex common ducks (43%) under the same feeding conditions.

Table 2: Intakes of dietary ingredients, dry matter (DM) and crude protein (CP).
	Control	DW60	DW100	SE/Prob
Feed intake (28-70d), g/d
Broken rice
Females	77	78	86
Males	106	91	105	1.43/0.001
Roasted SB
Females	25	11	0
Males	29	16	0
Duckweed
Females	0	297	323
Males	0	490	540
Premix + salt
Females	0.25	0	0
Males	0.30	0	0
Total dry matter
Females	88	91	90
Males	117	115	116	1.34/0.001
Total N x 6.25
Females	16	16	13
Males	20	22	18	1.19/0.001

Data on liveweight change and feed conversion are in Table 3. Liveweights at 70 days for males and females on the different diets are compared in Figure 1. Liveweights at 70 days and growth rates from 28 to 70 days were highest for males fed the control diet (36.1 g/day). There were no differences in growth rates between the DW60 and D100 diets for both males and females (range from 21.7 to 22.5 for females and 27.6 to 28.3 for males). Feed dry matter conversion was poorer (4.06 to 4.17 for females and 4.12 to 4.23 for males) on all duckweed diets compared with the controls (3.76 and 3.24 for females and males respectively).

Table 3: Live weights, daily gains and feed conversion rates of Muscovy ducks fed broken rice and soya beans replaced by duckweed
	Control	DW60	DW100	SE/Prob
Live weight, g
Initial
Females	657	619	650
Males	787	758	775	23.09/0.001
70 days of age
Females	1638	1563	1563
Males	2303	1946	1935	36.71/0.001
84 days of age
Males	2713	2391	2393	43.54/0.001
Daily gain, g
28-70days
Females	23.4	22.5	21.7
Males	36.1	28.3	27.6	0.83/0.001
28-84days
Males	34.4	29.2	28.9	0.75/0.001
Feed conversion DM
28-70days
Females	3.76	4.06	4.17
Males	3.24	4.12	4.23	0.16/0.003
28-84days
Males	3.64	4.27	4.21	0.11/0.004

Data on carcass composition are in Annex table 1. There was a slight indication (P=0.1 for females and 0.34 for males) that carcass yield was reduced on the duckweed treatments but differences were small. There were no differences in weights of chest and thigh muscle nor in heart and liver weights.

In general the results were similar to those obtained at the experiment station (Bui Xuan Men et al 1995). There were some indications that the Muscovy breed used in this trial were not able to consume duckweed as well as the common ducks used by Bui Xuan Men et al (1995). However, there were many confounding factors and a more precise experiment is required in order to explore this hypothesis.

The economic analysis is in Table 4. For farmers growing the duckweed the feed costs per kg liveweight gain were reduced substantially by feeding duckweed with the greatest benefit on the diet that contained no soya beans.

Table 4: Estimates of feed costs assuming situations of purchase or farm based production of duckweed (about 11,000VND=1US$)
	Control	DW60	DW100
Feed cost/ kg gain
Duckweed purchased*
Females	12,077	11,520	10,111
Males	10,491	12,489	10,727
Duckweed grown by farmers**
Females	12,077	8,880	7,134
Males	10,491	8,914	6,789

* Based on prices per kg for roasted soya beans VND 5,400, broken rice 1,800, fresh duckweed 200, premix 3,600 and salt 1,000
** Assumes no cost for the duckweed as opportunity cost of family household labour (women or children) is usually zero

The results show that if the farmers use the family household labour and the waste water surfaces to grow duckweed for the substitution of soya beans in broken rice based diets for feeding Muscovies, they can get clear economic benefits. This is very important for improving living standards and opening up the possibility of an efficient solution for the use of the enormous surplus labour force in the rural areas, especially in the case of the poor farmers.

The duckweed feeding system undoubtedly confers a competitive advantage to those farmers who have the resources (biodigester to supply the effluent: a sufficient area of pond surface; and the family labour to harvest the duckweed) to grow a good quality (high protein) product.

This research was part of the programme of study of the Senior Author for the Masters Degree in "Livestock-based Integrated Farming Systems for Sustainable Use of Renewable Natural Resources", at the Swedish University for Agricultural Sciences, Uppsala and was supported by SAREC (Swedish Agency for Research Cooperation with Developing Countries) and the International Foundation for Science (IFS), Grant No B/208-1.

Received 21 August 1995

Annex Table 1: Mean values for carcass traits of Muscovy ducks given duckweed as replacement for soya beans in broken rice based diets
	Control	DW60	DW100	SE/Prob
Females at 70 days of age
Live weight, g	1641	1548	1572	25.12/0.066
Carcass yield, %	73.1	70.1	71.1	0.91/0.109
Chest muscle, g	168	167	168	6.25/0.984
Thigh muscle, g	164	148	156	9.37/0.489
Heart, g	9.5	9.6	9.9	0.33/0.721
Liver, g	38	42	43	3.14/0.505
Males at 84 days of age
Live weight, g	2758	2451	2495	88.06/0.074
Carcass yield, %	70.0	70.6	68.1	1.17/0.336
Chest muscle, g	277	290	275	13.40/0.701
Thigh muscle, g	272	243	246	9.52/0.102
Heart, g	15.0	13.4	12.6	0.54/0.033
Liver, g	72.0	76.4	78.4	3.82/0.140