Five experiments were carried out both on station and on small farms in the Mekong Delta, Vietnam to evaluate the use of duckweed as a replacement for soyabeans in broken rice based diets for growing ducks, and to determine the effects of feed supplementation of scavenging ducks. 

The two first experiments were carried out with crossbred common ducks on station, and with Muscovy ducks on a small farm. The ducks were fed broken rice diets, in which soyabeans were replaced partially or completely by fresh duckweed in the growing period. There were no significant differences in daily weight gains between the crossbred ducks fed the diet with 100% duckweed replacing soyabeans and the conventional diet. The gains were significantly higher (P<0.001) for the ducks fed 30 and 45% soyabeans replaced by duckweed. Feed conversion ratios (FCR) were significantly inferior in all diets with duckweed. There were no significant differences in the gains between female Muscovy ducks fed the diet with 56% and 100% duckweed replacing soyabeans and the conventional diet. However, the daily gains were significantly lower with 47% and complete replacement for the male Muscovy ducks. The FCRs tended to be inferior for the duckweed diets. There were no significant differences in carcass traits between treatments for the crossbred, and the female and male Muscovy ducks. 

Two experiments were conducted with improved common ducks and female Muscovy ducks at the Cantho University research station. The ducks were fed levels of 80 or 60 g/duck/day of broken rice and duckweed ad-libitum from 28 to 63 days of age for the common ducks and 28-70 days for the females. Daily gains were significantly lower with the restricted broken rice diets, except for the 80 g/day broken rice diet for common ducks. The FCRs were significantly improved in the restricted broken rice diets. Complete replacement by duckweed in the diet with 60 g/day broken rice had the lowest feed cost. 

An experiment was carried out using scavenging ducks on farms in the rural area. The ducks were given supplements consisting of a mixture of broken rice and dried fish meal, or broken rice and dried fish meal alone. These diets were fed to crossbred scavenging ducks from 28 to 70 days of age at night in the farmers' households. The liveweight gains were significantly higher for the fish meal and broken rice mixture than for the broken rice diets. However, the cost per kg gain for these diets were higher compared with no supplement. 

Key words: Crossbred ducks, Muscovy ducks, duckweed, soyabean, daily gain, intake, local resources. 

Introduction 

There are around 30 million ducks raised annually in Vietnam, of which some 65% are estimated to be in the Mekong Delta. In the countryside the ducks are raised by scavenging in the rice fields especially in the harvesting season and immediately after. In other systems the ducks are raised in the backyard or in the gardens of households or kept on the canals. Ducks are easy to raise and develop in the area because they can resist diseases and consume many kinds of different feeds to produce valuable products in a short time. Duck production has contributed a considerable amount to the income of households. However, nowadays many varieties of high yielding rice are planted and harvested in a short period with only a limited time available for the duck flocks to scavenge, so this system is becoming less feasible. Also, in the dry season the farmers cannot herd their ducks in the fields as there is no feed. Another constraint is that the law of the country now prohibits the farmers from keeping their ducks on the canals and rivers so as to eliminate environmental pollution. As a result, the population of ducks has decreased and this leads to fluctuations in the supply of meat and eggs. In order to meet the increasing demand of the consumers there is increasing interest in confinement of ducks making use of locally available feeds. 

The population of Muscovy ducks is about 20% of the total duck population raised in the Delta. The ducks are commonly allowed to roam around in the backyards and gardens or confined in simple enclosures around farmer households, and today raising has become especially popular around the cities. Muscovy ducks are easy to raise because they are more resistant to diseases and efficiently consume different feeds to produce valuable products that can be sold at high prices. In the integrated animals-pond-garden farming system Muscovy ducks are suitable to raised due to the fact that they are less active, not noisy and easy to breed and manage. 

Duckweed, (Lemna minor) which is common throughout the Delta, is a tiny water plant that grows very well on stagnant pond surfaces. It can tolerate high nutrient stress, and appears to be more resistant to pests and diseases than other aquatic plants in the area. It has a high content of nutrients, especially protein (26-40% of dry matter) and carotene, that are necessary for growing animals. Duckweed has been commonly used in Vietnam to feed fish and poultry. So, as part of the overall development strategy of the integrated farming system, duckweed can be a useful candidate to be developed as a feed resource for ducks so as to improve production all the year round. 

The main objectives of these experiments therefore were to determine the optimum levels of duckweed as replacement for soyabeans in diets for fattening crossbred meat-type and Muscovy ducks based on broken rice fed ad-libitum, and evaluate effects on carcass quality. In addition, the economic benefits obtained from the use of duckweed were calculated. Feed supplementation for scavenging ducks was investigated in order to bring about improvements in the traditional duck-rice integrated farming system. 

Trials with duckweed in diets based on broken rice offered ad-libitum 

Methods and materials 

Experiment 1 was carried out at the Experimental Duck Farm, Cantho University in the Mekong Delta between March and May 1995, in the late dry and early rainy season. A total of two hundred four-week old crossbred ducklings [female Pekin (imported a long time ago) X male Cherry Valley (Czechoslovakia)] with initial average liveweights of between 830 and 860 g were used in the trial. The one day old ducklings were selected from scavenging breeding flocks and incubated at the traditional duck hatchery of Omon district, Cantho province, then brooded and fed a conventional diet ad-libitum from 1 to 28 days of age at the experimental farm. The birds were identified and then individually weighed initially, weekly and at slaughter. They were allocated at random to the five treatments with four replicates and ten birds balanced for sex per replicate. 

Experiment 2 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 ixty males) were used, with six treatments (three for each sex) and four replicates of five ducks per replicate. The one day old Muscovy ducklings were selected 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 experimental birds were identified and then individually weighed initially, weekly and at periodic slaughters of 70 days for the females and 84 days for the males. All ducks were given the duck plague vaccine and given a dosage of antibiotics on the seventh and twenty-fifth day of age to prevent diseases. 

The ducks in experiment 1 were fed in groups with 10 ducks per group from 28 to 63 days old on broken rice based diets offered ad-libitum as the major energy source. Based on the protein intake from broken rice, protein from roasted soyabeans was supplemented to balance the protein in the diets. Soyabeans in the experimental diets was replaced by fresh duckweed ad-libitum at levels of 30, 45, 60 or 100%, compared to no replacement (control). 

The Muscovy ducks in experiment 2 were kept in groups of 5 ducks separated from 28 to 70 days old for females and 28 - 84 days old for males and fed diets based on broken rice ad-libitum as the major energy source. Roasted soyabeans were supplemented to balance protein requirements of the control ducks. Based on the control diet, replacement of 60 or 100% soyabeans by duckweed ad-libitum in the experimental diets were fed to the females and males, respectively. 

A premix containing trace minerals and vitamins, and common salt was mixed (0.25%) with the control diet (but not in the other diets) for both experiments 1 and 2. 

Duckweed used in the experiments was grown on ponds enriched with nutrients from effluent from biodigesters and home wastewater. Duck-weed was collected twice daily in the morning and in the afternoon. Before feeding it was cleaned by tap water and put in a big bamboo basket for one hour to drain the excess water from the duckweed to reduce bulk. 

Results and discussion 

 Experiment 1 

The chemical compositions of the duckweed, broken rice and roasted whole soyabeans as analysed at Cantho University were as follows: DM content 4.7%, on DM basis: CP (N*6.25) 38.6%; EE, 9.8%; NFE, 8.58%; CF, 18.7%; ash, 19% and carotene 1025 mg/kg DM. The broken rice was collected from the rice mill of an identical lot and preserved during the experiment. The sample analysed had the following chemical composition: DM, 86.79%; CP, 8.25%; EE, 1.24%; NFE, 69.77%; CF, 1.75% and ash 1.00%.The whole soyabean seeds were roasted to eliminate the anti-nutritional factors, and ground and had the following chemical composition: DM, 87.01%; CP, 38.3%; EE, 18.3%; NFE, 14%; CF, 8.5% and ash 4.9%. The effects of dietary treatment on feed and protein intakes are shown in table 1. 
 

The rate of liveweight gain was significantly higher on the D30 and D45 diets than on the control diet. The diet with complete replacement of the soyabeans (D100) supported slightly better (ns) growth than the control diet. Feed conversion was best on the control diet and did not differ between the diets containing duckweed. 

Mean values for carcass traits are given in table 3. The weights of chest and thigh muscle tended to be higher on the control diet. There were no differences in weights of the components of the digestive tract nor of the heart and liver. 
 

There would appear to be marked economic benefits to the farmer from using duckweed to replace soyabeans in broken rice diets for fattening ducks in situations where the duckweed is grown on the farm and man-aged and harvested by household labour (table 4). This emphasises the importance of an integrated farming system as a means of reducing costs and improving the economic competitiveness of the small scale farmer. 

Experiment 2  

Data for feed intake are given in table 5. The duckweed was consumed readily on the Df0, Df60, Dm60 and Dm100 diets. The intakes of duckweed were higher on the Df100 and Dm100 diets than those of Df60 and Dm60, respectively, and the males consumed more than females. Intake was con-siderably increased as the soyabean was completely replaced by duckweed (297g to 323g for the females and 490 to 540 g for males), but the levels of intakes were lower than those of common ducks (540 vs 560) reported by Men et al. (unpublished) under the same feeding conditions. From the data it can be seen that common ducks, especially the improved local breed when herded, consumed higher levels of duckweed than Muscovies. The intake of broken rice decreased on the Df60 and Dm60 diets, with 40% of the control level of soybeans, and increased when no soybeans were given (Df100 and Dm100). Like for duckweed, the intakes of broken rice were higher for the males than for the females. 
 

The rate of weight gain was slightly higher on the Df0 diet than on Df60 and Df100 diets, but there was no significant difference for females (23.4 vs 22.5 and 21.7), whereas the gain was significantly higher on the Dm0 diet than on the Dm60 and Dm100 diets for males. The results also show that use of high amounts of duckweed to replace most or all soyabeans in broken rice based diets for fattening male Muscovies, which have a high potential growth rate, decreased weight gains. So whether the high duckweed diets can be fed male Muscovies for breeding needs to be investigated. 
 

Mean values for carcass traits are given in table 7. The carcass yield and thigh muscle tended to be higher on the control diets for both the females and males. There were no significant differences in weights of the components of the digestive tract except the hearts of the males. 

From the results of experiment 1 it can be seen that fresh duckweed can completely replace roasted soyabeans and a vitamin-mineral premix in broken rice based diets for fattening ducks without any reduction in growth performance or carcass traits. The poorer feed conversion on the diets containing duckweed has no economic significance, as shown in table 4 since duckweed can be grown easily on the farm whereas soyabeans usually have to be purchased. The fact that protein yields of duckweed can be as high as 10 tonnes/ha/year (Preston, 1995) compared with less than one tonne/ha/year for soyabean protein is another advantage for the integrated farming system. 

The results of experiment 2 for small farms show that farmers can grow duckweed in an integrated farming system and use it to replace soyabeans and mineral-vitamin premixes completely in broken rice based diets for fattening female Muscovy ducks without any reduction in growth performance and carcass quality. However, when feeding male muscovies with high genetic potential for growth, the complete replacement of duckweed in broken rice based diets reduced weight gain, but the gain was still higher than that of the traditional methods. Replacement with duckweed can be applied in feeding muscovies in the early developer stage because normal growth is possible and inputs can be reduced in the long term. 

Trials with duckweed in restricted broken rice based diets  

Experiments 3 & 4 

Methods and materials  

Two similar experiments were carried out with 54 common ducks fed duckweed from 28 to 63 days of age and 45 local female Muscovy ducks from 28 to 70 days of age on the experimental farm of Cantho University. The trials included three treatments and three replicates. The diets are shown in table 9. 
 

The broken rice analysed had DM 86.9% and CP 9.6% of DM and the duckweed had DM 4.9% and CP 39.3% of DM. 

Results and discussion  

Data for feed intakes are given in table 10. 
 

The daily intake of duckweed increased as the broken rice was restricted reaching a maximum of 1100 g at 6 weeks of age, which was equivalent to the ducks' own live weights. These levels of intake are higher than those (870 to 560 g) recorded in the report by Men et al. (unpublished) for ducks in a similar feeding trial, but of a different crossbred line. Total intake of protein was slightly decreased on the B60 diet. Total intake of DM was highest on the ad-libitum broken rice diets and least on the B60 diet. The daily intake of duckweed by the female Muscovy ducks increased slightly as the broken rice was restricted, reaching an average of 333g. The level of intake is lower than that (333 vs 870 g) recorded in trial 1 with common ducks. Total intake of DM on the Bm60 diet was lowest (66 vs 106 g for Bma), and was lower than that of common ducks (66 vs 94g). The intake of broken rice was lower than the amount offered because in the first week of the experiment the ducks consumed less feed than was expected. 

The daily gain males was gradually decreased as the levels of broken rice were reduced, although there was no significant difference between Ba and B80. The gain was lowest for the B60 diet and significantly different when compared with Ba and B80. Feed conversion was best for the B80 diet, but no difference was found between Ba and B60 diets. 

With female Muscovy ducks, the daily gains significantly decreased on the restricted broken rice diets compared with the Bma (control). The gains on the Bma diet is equivalent with that on Ba diet in trial 1. The efficiency of feed conversion of female Muscovies is better than that of common ducks on the same diets. 

The results of experiment 3 show that when the price of broken rice is high, providing 80g daily of broken rice for improved ducks given duckweed ad-libitum gives good growth and carcass traits. Growing duckweed for ducks saves capital and can help to solve unemployment in the area. The results of experiment 4 show that levels of 80g broken rice per duck per day and duckweed ad-libitum for fattening female Muscovies decreased their weight gains because the ability of muscovies to consume fresh duckweed is lower than that of common ducks. The Bm80 diet can be used for developers because in this period breeding ducks are not grown so rapidly, but the effects on reproduction need to be investigated. Effect of energy and protein supplementation of scavenging ducks raised in paddy rice fields 

Methods and materials  

A total of 800 crossbred Cherry Valley ducklings were brooded and trained to scavenge from 1-28 days of age in the farmer households of a hamlet of Cantho province. After brooding the ducks were divided into 4 flocks with 190 ducks in each and were herded and managed by 4 farmer households. The ducks were let loose in rice fields post harvest from 6.00 h to 17.30 h daily. There were 3 supplemental diets: 50 g/day of a mixture of broken rice and dried fish meal (BFM) for flock 1, 50 g/day broken rice (BR) for flock 2 and 20 g dried fish meal (FM) for flock 3 and compared to no supplementary feed (S0) for flock 4 (control). These diets were given to the ducks from 28 to 70 days of age and given in the evening. At the end of the trial, four ducks (2 males and 2 females) from each flock were slaughtered for carcass evaluation. 

Results and discussions  

Data from table 12 show that, although having the lowest initial live weight, weight gains of ducks given BFM supplementary feeds were higher that of the unsupplemented flock, and were highest from flock 1 to flock 3. Weight gains were significantly higher for flock 1 and for flock 2 than for flock 3 and flock 4. The feed conversions were higher for flock 1 and flock 2, and the costs per kg gain are highest for flock 1, followed by flock 2 and 3. The flock 1 ducks began moulting at 63 days of age, while flock 4 still had the original plumage at 70 days of age. Ducks with full plumage sold at a higher price (12%) than those in flock 4. However, all four flocks were in much better condition than flocks fed and managed by the traditional methods of the farmers in the hamlet. Over the same time period and with the same breed reared, several local farmers finished their ducks at 75 days of age, with average live weights from only 1.2 to 1.3 kg and with poor quality carcases. 

Mean values for carcass traits are given in table 13. Live weights at slaughter, carcass weights and yields, and chest muscle tended to be higher for flocks 1 and 2. There were no significant differences in the weights and the lengths of the digestive tract components, except the liver. 

From the results of the experiment it is clear that supplements of protein-energy feeds to growing crossbred meat ducks improved the daily liveweight gains and carcass quality, and shortened the raising time. Supplementation is simple to carry out, and an improvement of the traditional scavenging system, and the method meets the increasing demand of the consumers for better quality products. 

Discussion and conclusion  

From the results of the four experiments it can be seen that fresh duckweed, a locally available unconventional feed source, can completely replace soyabeans in broken rice based diets for fattening crossbred Common ducks and female Muscovy ducks without any problems regarding daily live weight gains, carcass quality and marketing. Replacement decreased the investment by using available wastewater and household labour for managing and collecting duckweed, and increased the economic benefits for the farmers. Carcasses of ducks given duckweed are an attractive yellow colour due to storage in the body of carotenes. However, complete replacement of duckweed in diets for male Muscovy ducks with a genetic potential for high growth rate tended to decrease the weight gain. The high protein content of cultivated duckweed is important because the ducks need to consume large amounts due to the low DM content of duckweed. In fact, growing duckweed is easy for farmers in the Mekong Delta, because the region is an alluvial lowland plain with an inexhaustible water supply and large amounts of decaying organic matter and human wastewater. These conditions make growing duckweed in integrated farming systems very profitable not only for animal feed but also for the treatment of wastewater from animals and humans. Also, this contributes to solving one of the environmental problems that have concerned the government. 

An interesting and important observation is the relative capacity of the Common duck breed, compared with the Muscovy duck, to consume duck weed. A strict comparison of the two breeds is not possible as they were not included in the same experiment. The data in table 14 shows the comparison between the breeds on the two main feeding systems: ad libitum or restricted broken rice (confounded with sex in the first case and with location in the second). When the broken rice was restricted, the Common ducks ate almost twice the amount of duck weed as the Muscovy ducks (P=0.001), and even when the rice was given ad libitum the intake still tended (P=0.14) to be higher for the Common ducks. The economy of liveweight gain (feed cost/unit LW gain) was best on the restricted rice system, although the rate of gain was slightly lower, and in each case the Common ducks were better than the Muscovy breed. Differences between pig breeds were also observed by Rodriguez (1996) when duckweed was the major protein source; the local Mong Cai breed appeared to be able to consume more of this feed than the exotic (Large White) breed. As stated earlier, it was not the aim of the experiments to make a strict comparison of the breeds, hence the confounding with location and/or sex. Nevertheless, the observations are interesting and provide a strong justification for future research when the aim is to maximise use of local feeds such as duckweed. The most effective way of maintaining biodiversity is by showing that local (indigenous) breeds may have distinct comparative advantages when local feed resources are used. 

Supplementary feeding for scavenging ducks is an improvement of traditional production to improve growth rates and quality of meat ducks. It is a reasonable method to shorten raising time and keep the duck population stable, as with modern rice cultivation opportunities for duck flocks to scavenge are limited. Supplementing 50 g of a broken rice and dried fish meal mixture, or 50 g broken rice per bird per night for herded duck flocks showed that daily weight gains could be improved and also rearing time could be reduced. All supplements resulted in high weight gains and carcass weights when compared with other traditionally reared flocks. The 
ducks formed their adult plumage 2 weeks earlier than flocks raised by traditional methods. It is clear that if fed a balanced feed of energy and protein (16.8% CP and 11.74 MJ ME/kg DM) ducks grow faster than those fed broken rice or dried fish alone. However, the supplemental cost for the protein-energy diet was highest because of the high price of broken rice and dried fish meal. The farmer can overcome the problem by using duckweed in an integrated farming system as shown in experiment 1, or by gathering shellfish available in the canals nearby the home for the supplement. 

Acknowledgements  

This research forms part of the programme of study of the senior author for the Master of Science Degree in "Livestock-Based Integrated Farming Systems for Sustainable Use of Renewable Natural Resources", at the Swedish University of Agricultural Sciences, Uppsala. Development of the basic experimental facilities were facilitated by a grant from the International Foundation for Science (IFS). I would like to thank Dr Brian Ogle, Dr T. R. Preston and Mr Frands Dolberg for useful guidances and their advisory roles that made this study possible. 

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