Two breeds of ducks (Improved Common ducks and local Muscovy) were fattened with local feed resources in two sequential trials at the Duck Experimental Farm of Cantho University. The basal diet was broken rice given ad libitum (BRAL) or restricted to 80 (BR80) or 60 (BR60) g/day. On each of these treatments the ducks had free access to freshly harvested duckweed from ponds fertilized with biodigester effluent and containing 39% protein in the dry matter. There were 54 ducks of mixed sexes of the Common type and they were fed from 28 to 63 days of age on the experimental diets. A group of six ducks was allocated to each of 9 pens giving 3 replicates of each dietary treatment. The local Muscovy ducks (45) were all females. They were allocated 5 per pen to give the same number of replicates as for the Common ducks and fattened from 28 to 70 days.

Common ducks grew at the same rate as the Muscovy but ate twice as much duckweed. Their feed conversion was poorer. Decreasing the offer level of the broken rice had no effect on growth and carcass traits of the Common ducks but slight depressed growth rate and reduced weights of chest and thigh muscle in the carcass. When rice levels were restricted the Muscly ducks appeared less able to compensate by eating more duckweed as compared with the Common ducks.

A feeding system of restricted levels of broken rice (60 to 80 g/day) with free access to fresh duckweed appears to be appropriate for the Common type of duck typically used for foraging on rice fields throughout Vietnam. More research is needed in order to ascertain if the apparent capacity of Common ducks to eat large quantities of duckweed really is a comparative advantage and, if so, how this can best be used to improve the economic benefits to small scale poor farmers.

In order to enhance economical development of farmer households in the rural areas, government policies have emphasized animal production as playing a very important role in the development strategy of agriculture, and that pigs and ducks need to be a priority in the Mekong Delta. In fact, duck production has been developed for a long time in the Mekong Delta because there are many good conditions for raising ducks such as: feed sources (rice, animal feeds, water plants), water source, climate, and plentiful labour. There are several methods applied to raise ducks, but the methods of herding immediately after transplanting and harvesting of rice, and scavenging in backyards are common with the raisers.

The Mekong Delta with 2.7 million hectares planted with rice produced 12 million tonnes of rice in 1995, but the price of rice and its byproducts fluctuates and has affected the expansion of animal production, while many kinds of water plants and water surfaces are wasted.

The research to be described in this paper is part of a series of studies aimed to drive maximum benefit from water plants as a supplementary feed for ducks (Bui Xuan Men 1995, 1996).

Duckweed (Lemna spp) is common in the Delta and grows very well on waste water surfaces in the region. It has high biomass production and with appropriate fertilization (Rodriguez and Preston 1996) can be rich in protein, vitamins and minerals. Duckweed has been used to feed fish and poultry by several farmers and it gave good results. Use of labour in households to grow duckweed to feed ducks for increase of income will contribute to an improved standard of living of the farmers.

There are indications that local breeds of pigs (eg: the Mong Cai) consume duckweed more readily than do imported exotic types such as the Yorkshire (Rodriguez and Preston 1996).

The hypothesis to be examined in the present study is that there may be differences between breeds of ducks in their capacity to consume duckweed when it is a major component of the diet.

Two breeds of ducks were compared: Improved Common ducks and local Muscovy. The basal diet was broken rice given ad libitum (BRAL) or restricted to 80 (BR80) or 60 (BR60) g/day. On each of these treatments the ducks had free access to freshly harvested duckweed. There were 54 ducks of mixed sexes of the Common type and they were fed from 28 to 63 days of age on the experimental diets. A group of six ducks was allocated to each of 9 pens giving 3 replicates of each dietary treatment. The local Muscovy ducks (45) were all females. They were allocated 5 per pen to give the same number of replicates as for the Common ducks and fed from 28 to 70 days.

The duckweed was grown on ponds fertilized with effluent from biodigesters. The chemical composition of the duckweed and the broken rice is given in Table 1.

Table 1: Mean values for composition of duckweed and broken rice used during the experiment
	Duckweed	Broken rice
Dry matter, %	4.93	86.89
As % of DM
N x 6.25	39.3	9.56
Ether extract	6.19	1.65
NFE	10.7	86.4
Fibre	16.6	0.84
Ash	17.4	0.98
Ca	1.003	-
P	1.52	0.14

Liveweights and feed intakes were recorded from 28 days to 63 days for both types of ducks. Common ducks were slaughtered at 63d and Muscovy ducks at 70 days according to traditional practice.

The data presented here for the two types of ducks should be viewed as preliminary as the experiment was not designed to make a direct comparison of the breeds. The Common ducks were mixed sexes and the Muscovy were all females; and the experiments were done sequentially not concurrently. However, all other factors were the same: housing, feeds and management.

Data on feed intakes for the two types of ducks are given in Table 2. In general, the daily intake of duckweed increased as the broken rice was restricted but the effects were more marked for the Common ducks, which ate 870 g/day of duckweed compared with the 323 g/day for the Muscovy ducks, on the maximum restriction of rice of 60 g/day. Some groups of common ducks were observed to consume as much as 1100 g/day which was the same as their liveweight at 6 weeks of age.

Table 2: Mean values for intake of dietary ingredients
	BRAL	BR80	BR60	SE/Prob
Feed intake, g/d
Broken rice
Common	101	78	60	1.42/0.001
Muscovy	102	76	56	1.38/0.001
Duckweed
Common	765	817	869	18.7/0.001
Muscovy	309	315	323	9.43/0.564
Total DM
Common	125	107	94	1.73/0.001
Muscovy	104	82	65	1.32/0.001

The contrasts are more easily appreciated in Figure 1. As a proportion of total diet dry matter the Common ducks ate twice as much duckweed as the Muscovy ducks at all levels of feeding of the broken rice.

Figure 1: Relative intakes of duckweed by Common and Muscovy ducks fed different amounts of broken rice

Mean value for liveweight change are in Table 3. Initial weight at 28 days and weights at 63 days were similar for both types of ducks. Liveweights at 63 days were significantly less for the 60 g/day level of broken rice but were similar for the ad libitum and the 80 g/day level.

Table 3: Mean values for live weights and growth rates from 28 to 63 days of age of common ducks and Muscovy ducks fed broken rice and duckweed
	BRAL	BR80	BR60	SE/Prob
Live weight, g
Initial (28d)
Common	694	686	674	13.5/0.585
Muscovy	639	646	644	19.4/0.969
Final (63d)
Common	1595	1537	1369	18.3/0.001
Muscovy	1562	1422	1292	37.0/0.001
Daily gain (28-63d)
Common	25.8	24.3	19.9	0.68/0.001
Muscovy	26.4	22.2	18.5	0.92/0.001

Mean values for carcass traits are in Annex Table 1. The ages at slaughter of the two types of ducks were different (the Muscovy were slaughtered at 70 days and the Common ducks at 63 days) thus the only valid comparisons are between levels of broken rice. The muscle weights of the Common ducks were not affected by the restriction in the rice intake; however, there were indications that for the Muscovy type, chest and thigh muscle weights decreased as rice offer level was reduced.

Despite the higher intakes of duckweed by the Common ducks their liveweights at 63 days were only slightly greater than those of the Muscovy. As a result their dry matter feed conversion was poorer despite the fact that this was measured over a shorter time period (28 to 63 days for Common and 28 to 70 days for Muscovy duck). The confounding in time and sex of the two sets of data makes it impossible to draw firm conclusions as to whether these are real differences between the two types of ducks or due to chance. Direct observations on behavior showed that the Common ducks were much more eager than the Muscovy to eat the duckweed. The fact that these differences in intake were not reflected in more rapid growth rate may be related to the nature of the nutrients provided by the duckweed -- mainly protein and cell wall carbohydrate. The extra protein consumed by the Common ducks may have been surplus to requirements and in this case there would have been an energy cost to eliminate the excess nitrogen.

Future experiments with energy sources of lower protein content than broken rice should clarify this aspect. The attempt of Becerra et al (1995) to address this issue, by using sugar cane juice as the energy source, was frustrated by the fact that the duckweed used was of much lower protein content (only 23% in dry matter compared with the 36% in the present study) and intakes of duckweed were relatively low. At that time the importance of fertilizing the duckweed to attain a high protein content (Leng et al 1995; Rodriguez and Preston 1996) was not appreciated. Furthermore, the ducks used by Becerra et al (1995) were of the "improved" Cherry Valley type and perhaps were not well adapted to eating the duckweed.

As in the earlier studies there were marked economic benefits from using the combination of restricted levels of broken rice and ad libitum duckweed (Table 4).

Table 4: Mean values for feed conversion and feed cost from 28 to 63 days of age of common ducks and from 28 to 70 days for Muscovy ducks fed broken rice and duckweed
	BRAL	BR80	BR60	SE/Prob
Conversion (DM)
Common	4.87	4.44	4.77	0.08/0.015
Muscovy	4.30	4.20	3.70	0.09/0.08
Feed cost/kg gain (VND)
Duckweed purchased*
Common	12977	12502	14161
Muscovy	10113	10221	9559
Duckweed grown**
Common	7548	6965	5796
Muscovy	7047	5778	5427

* Based on price per kg for broken rice 1,800 VND, fresh duckweed 200 VND (11000 VND=1USD)
** Assumes no cost of duckweed as opportunity cost of family household labour is usually zero

A feeding system of restricted levels of broken rice (60 to 80 g/day) with free access to fresh duckweed appears to be appropriate for the Common type of duck typically used for foraging on rice fields throughout Vietnam.

Muscovy ducks ate less duckweed on a daily basis and this feed accounted for a much smaller proportion of their total dry matter intake than was the case for Common ducks. They also had poorer growth rates and slightly inferior carcass traits when rice levels were restricted and appeared less able to compensate by eating more duckweed.

Feed conversion rates were better for Muscovy than for Common ducks at all levels of broken rice feeding.

More research is needed in order to ascertain if the apparent capacity of Common ducks to eat large quantities of duckweed really is a comparative advantage and, if so, how this can best be used to improve the economic benefits to small scale poor farmers.

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 Common (70 days of age) and Muscovy ducks (84 days of age) fed broken rice and duckweed
	BRAL	BR80	BR60	SE/Prob
Live weight, g
Common	1543	1533	1403	50.11/0.169
Muscovy	1743	1600	1490	36.26/0.008
Carcass weight, g
Common	977	1030	923	33.11/0.154
Muscovy	1212	1103	1053	21.79/0.006
Carcass yield, %
Common	63.4	67.1	65.8	1.06/0.110
Muscovy	69.5	69.0	71.1	2.75/0.856
Chest muscle, g
Common	102	118	97	9.13/0.288
Muscovy	187	172	143	7.33/0.015
Thigh muscle, g
Common	120	127	123	9.23/0.880
Muscovy	145	143	124	12.50/0.459
Heart, g
Common	9.1	9.1	9.3	0.53/0.964
Muscovy	10.0	9.7	9.2	0.27/0.174
Liver, g
Common	77	52	40	3.21/0.001
Muscovy	48	46	37	3.48/0.118