Study on digestibility and feeding systems of duckweed in growing ducks

J Khanum, A Chwalibog^* and K S Huque^**

Department of Livestock Services, Krishi Khamar Sarak, Farmgate, Dhaka-1215, Bangladesh
^*Department of Animal Science and Animal Health, The Royal Veterinary and Agricultural University,
Bulowsvej 13, 1870 Frederiksberg C., Copenhagen, Denmark,
^**Animal Production Research Division , Bangladesh Livestock Research Institute,
Savar, Dhaka, Bangladesh
kjahanara@hotmail.com

Abstract

The experiment was conducted at Bangladesh Livestock Research Institute (BLRI) to determine the intake and digestibility of fresh duckweed in growing ducks and to evaluate the growth performance and carcass characteristics of ducks fed on fresh duckweeds, either in confinement or through grazing. The intake and digestibility trials were performed in two phases at 9-10 and 18-19 weeks of age. A control diet was a commercial feed compound and the treatment diets were: 50% control diet + ad libitum fresh and harvested duckweed (DWH), 50% control diet + ducks were allowed to graze on duckweed in a lagoon (DWG), and the last diet was foraging fresh duckweed by the ducks in the lagoon only (DWO).

All the birds of DWO group died during the first 3 weeks of the trial, which indicated that duck could not be rear by feeding duckweed only. The intake of fresh biomass, nitrogen and crude fibre were significantly higher in duckweed than in the control diet in both phases, but the intakes of dry matter and organic matter were significantly lower. In both phases, digestibilities of dry matter, organic matter and crude fibre in duckweed diets were significantly lower than in the control diet. However, the apparent and true protein digestibilities of duckweed were significantly higher than in the control diet in the first phase but lower in the second phase. The final live weight and average daily body weight gain were not different between duckweed diets but significantly lower than in the control diet.

Key words: Duckweed, digestibility, feed intake, growth performance, feeding system

Introduction

Both quality and cost of feeds and their availability affect duck production, especially in the dry period. In the scavenging period, ducks forage on different types of flora and fauna, the nutritional values of which are still unknown to farmers.

Duckweed, which has a fast growth rate with fluctuating levels of protein (15 to 45% on dry matter basis) and of biomass availability throughout the year, was evaluated as a dietary supplement for chickens or ducks (Muzafonou 1968; Abdulayef 1969; Truax et al 1972; Haustein et al 1992a, 1992b, 1994). It has also been reported that duckweed can replace 50% of the fishmeal in a conventional diet for ducklings (Hamid et al 1993). Feeding duckweed containing 38.6% crude protein (CP) to ducks as a replacement for roasted soybeans showed that duckweed can totally replace soybeans as a protein source for a duck fattening system based on broken rice (Men Bui Xuan et al 1995). Feeding fresh Lemna to layer ducks up to 24% of the diet as a replacement of rice bran and oil cake, improved egg production and yolk color (Hossain 1998).

Nevertheless, high water content (90 to 93% of the biomass) and the bulkiness of duckweed often make it difficult for the farmers to minimize the costs of harvesting and processing. Hossain (1998) found that a large duck may daily eat about 1300 to 1600 g of fresh duckweed under confinement, and the production, collection and supply of a huge amount of biomass are laborious and costly. Huque and Chowdhury (1998) developed a system of producing duckweed all the year round, using farm wastewater, and suggested to allow ducks to forage fresh duckweed from duckweed lagoons. Rural farmers usually allow their ducks to forage on duckweed that grows on different types of wastewater. This may cause problems because of heavy metal contents. Therefore, it is necessary to produce the duckweed in nutrient-rich water with a minimum level of heavy metals and subsequently to evaluate its nutritional value to ducks.

A high fresh biomass intake (Hossain 1998) may help reducing feed costs without affecting egg and meat production. Thus, the present work was undertaken with the following objectives:

• To determine the intake and digestibility of fresh duckweed in ducks

• To determine the growth performance and carcass characteristics of growing ducks fed on fresh duckweeds either in confinement or through grazing.

Materials and methods

The experiment was conducted in Bangladesh Livestock Research Institute (BLRI), Savar, Dhaka, Bangladesh. The duckweed production technology developed by BLRI (BLRI 1998) was used for continuous production of duckweed in one of the three farm waste recycling lagoons.

Housing and sanitation

Small houses of 8 x 5 sq. ft. in size were built on the bank of the duckweed lagoon and ducks of the individual treatment groups were housed in 4 different houses of the same size. All the houses with raised floors were built using bamboo materials, and feed and water troughs were collected from the local market. Strict sanitary measures were taken during the experimental period. Every morning the experimental houses and cages were washed and cleaned with fresh water and disinfectants were also used. Feeders and drinkers were cleaned and dried daily before use. All the birds were vaccinated against Duck Plague and Duck Cholera as per the recommendation from vaccine manufacturers.

Duckweed production

The duckweed production technique developed by BLRI was used for continuous supply of duckweed of a uniform quality. Initially continuous nutrient loading created problems for duckweed production, and the problem was minimized through draining nutrient-rich water and adding fresh underground water. This delayed the start of the growth trial for about two weeks. Irrigation of fresh water was done at certain intervals to maintain nutrient concentration at the recommended levels. Other management factors were followed as per recommendation. Lemna minor was cultivated, but Spirodela polyrrhiza was mixed to some extent with Lemna. Spirodela polyrrhiza was removed irregularly  by using nets. The 600 x 50 sq. ft duckweed lagoon was divided into 4 parts using metal netting to prevent  mixing ducks of different groups. Each part of the lagoon was further sub-divided into small compartments using bamboos to protect the duckweed from the wind. During the growth trial the water in the duckweed lagoon had a pH of 6.85 and contained 27.6 mg/litre NH₄⁺N and 0.06% of DM. The temperature of surface water was 21^º C, and the bottom water 19^º C. Three lagoons were situated in parallel and each was connected with an animal shed of BLRI through a wide drain, by which waste was supplied in a controlled way.

Feeding duckweed

Experimental rations (starter and grower finisher) were formulated using locally available feed ingredients. The chemical composition of the feed was determined and shown in Table 2. Initially the fresh duckweed was collected and mixed with the control diet of the three treatment groups. The ducks were gradually adapted to duckweed. Problems occurred with the third and fourth groups of birds when they should be adapted to foraging duckweed in the lagoon. The ducks were reluctant to remain in the lagoon and to forage duckweed. They swam for a while and foraged and then returned to their house. They were controlled by a caretaker, who guided them to forage on the duckweed.

Intake and digestibility

The intake and digestibility trial was conducted on ducks of 9 weeks of age, and it was repeated with ducks of 18 weeks of age. In both phases, the feed ingredients of the control diet were the same, but the composition of feed was changed when the ducks were 11 weeks old. Duckweed, on the other hand, was collected from the same lagoon.

Twenty-five ducks were housed in individual cages and divided into three groups, consisting of ten in each of the first two groups and five in the third group. The first two groups were fed with the control diet and fresh duckweed diets separately to determine intake and digestibility of the two diets. The ducks were adjusted to the diets for 10 days and then feed refusals and droppings were collected individually for five days. In order to determine endogenous nitrogen losses the third group of 5 ducks was kept off feed for 36 hours and then fed with 50 g glucose, and droppings were collected for 72 hours.

Calculation

True protein digestibility (TPD) was calculated by the following formula:  

TDP =CP intake with feed –[(CP excreted in faeces - CP in endogenous substances) / CP intake with feed]

However, the calculation of TPD is not a strict measure of digestibility as urine is not separated from faeces, and the total difference between intake and content in droppings represents the amount, that is not only digested but also includes urinary loss, e.g. the amounts that are metabolized.

True metabolizable energy (ME) was calculated by using the following formula:

True ME (kcal/kg DM)= 3951+54.4 EE - 88.7 CF - 40.8 Ash (Wiseman 1987)

Growth trial

The growth trial was conducted on growing Xinding ducks of 9 weeks of age. The ducks were collected at 4 weeks of age from the government duck farms. All the ducks were reared in the farm under confinement and fed with a mixed diet. After moving the ducks into BLRI they were divided into four groups consisting of 20 ducks in each. The birds were fed in groups and daily feed was supplied according to dry matter (DM) requirements suggested by Leeson and Summer (1997). All the ducks were fed with a similar diet and fresh duckweed was gradually introduced in the treatment diets. The four treatments were:

• Concentrate feed (Control)

• 50% control diet + ad libitum fresh duckweed, collected and supplied to ducks (DWH),

• 50% control diet + ducks were allowed to forage on duckweed lagoon (DWG).

• Foraging fresh duckweed by ducks in the lagoon only (DWO)_.

The ingredient composition of the control diets is shown in Table 1.  Feed 1 was used from 9^th to 10^th week and feed 2 was used from 11^th week and onwards. These two feeds and duckweed 1 and duckweed 2 were also used in the first and second phase for the determination of digestibility.

The control diet was formulated to meet the metabolizable energy (ME) and crude protein (CP) requirements of the birds according to Leeson and Summer (1997). The growth trial was started after 8 weeks of age and continued up to an age of 17 weeks. Live weight was taken weekly up to 17 weeks of age.

Carcass characteristics

To study the carcass characteristics of the growing ducks under different feeding schemes two ducks from each group were slaughtered at the end of the growth trial and data of live weight, dressed carcass weight, breast meat weight, thigh weight, drumstick weight and wings weight were collected.

Chemical analysis of samples

The feed and dropping samples collected during the experimental period were analyzed for proximate components according to AOAC (1990) procedures.

Statistical analysis

Results were evaluated by analysis of variance (ANOVA) using the General Linear Models procedure of Statistical Analysis System (SAS, version 8.2). Where significant effects were detected, differences between treatment means were tested using the Duncan's (1955) multiple range test (where there are more than two treatments). A 5% significance level was used.

Results and discussion

Chemical composition of diets

The crude protein of the duckweed (Table 2) was high, according to the range reported by Leng (1995).

Differences in fresh and nutrient intakes of the two diets between the two phases

The intake of fresh duckweed (1083 to 1600 g/day) was in the range reported by Hossain (1998) for Xinding ducks (640 to 1600 g / duck).

The daily DM and OM intakes of the control diet by ducks were found higher than the duckweed diet in both phases. The differences in DM intake / duck per day were significant. When DM intake was calculated per metabolic live weight (kg^0.75 ) the differences were reduced, but still a higher intake was found in the control diet.

The differences in OM intake, g / duck per day were, significant . A similar trend to that of DM intake was found when the OM intake was expressed on metabolic live weight basis (Tables 2 and 3).

The daily nitrogen intakes, on the other hand, calculated both as g /duck or mg /kg^0.75 were significantly (P<0.01) higher in duckweed than in the control diets. Similar to nitrogen, the daily crude fibre intakes were significantly (P<0.01) higher in the duckweed diets in both the phases. The higher crude protein and crude fibre contents of the duckweed diet than of the control diet resulted in higher intakes by ducks fed on duckweed in spite of higher DM intake of the control diets.

Men Bui Xuan et al (1996) reported 38.6% CP, 9.8% crude fat and 18.7% crude fibre contents of duckweed grown on waste water, which is similar to the duckweed composition as presented in Table 2. The authors reported that crossbred ducklings of 830 to 860 g live weight had a variation in total DM intakes of mixed diets of broken rice, roasted soybean and fresh duckweed from 94.7 g to 108 g/d per duck. The figures were almost the same as in the present investigation, being 870 to 1100 g live weight and daily DM intake from 82.8 to 91.4 g /duck.

Digestibility of nutrients

The digestibility of DM, OM, CF and apparent and true protein digestibility in the first and second phase is presented in Tables 5 and 6. In both phases digestibility of DM and OM of duckweed was significantly (P<0.01) lower than that of the control diet. The apparent and true protein digestibility of duckweed, on the other hand, was significantly (P<0.01 or P<0.05) higher in duckweed in the first phase. Both apparent and true protein digestibility of duckweed were lower in the second phase and minimized their differences from the control diet. However, Table 2 shows that the chemical composition of duckweed fed to ducks in both phases was similar.

Farhat et al (1998) reported nutrient digestibility of different types of feed. The authors reported that DM and neutral detergent fibre (NDF) digestibility of a range of feeds in ducks varied from 90 to 63.5% and 89 to 19.2%, respectively, depending on the type and quality of feeds. The authors also reported that digestibility of feed ingredients was increased with the age of ducks from 3 to 6 weeks in Pekin ducks, but for Muscovy ducks, the was similar from the age of 7 weeks to 11 weeks.

From the above results it may be concluded that fresh duckweed may be characterized by its high intake and low digestibility of DM, OM, CF.

Growth performance of ducks

Initial live weight was not significantly different (p>0.05), being 772, 800, and 794 g in the control, DWH and DWG diets, respectively (Table 7; Figure 1). Within 3 weeks after the beginning of the growth trial all the birds from DWO group died. The final live weight after 63 days of growth period for the remaining groups was 1345, 1256 and 1189 g for control, DWH and DWG, respectively, and the differences in final live weight of treatment diets varied significantly (P< 0.01) from that of the control diet. There were significant differences (P<0.01) in daily weight gain between control and duckweed diets (Figure 1), but the differences between DWH and DWG were not significant (P>0.05).

Duckweed, despite having a high crude protein content, was found to be digested at a level of 27.0 to 33.5% DM in ducks. Its apparent and true protein digestibility varied from 40.0 to 56.0% and 45.9 to 63.8%. On the other hand, the DM digestibility of the control diet was found to be same (67.9%) in different age groups of ducks. The apparent and true protein digestibility varied from 42.0 to 46.0% and 51.3 to 58.4%.

The protein digestibility of the control and duckweed was almost similar, but DM digestibility of duckweed was comparatively lower.  Replacement of 50% of the control diet by feeding duckweed affected nutrient availability and resulted in reduced live weight gain in ducks of the treatment groups, and may also have caused the death of DWO group. The other important factor that may cause lower live weight gain, is the energy content in control diet and duckweed. The calculated energy content of the control diet fed to ducks at different age groups varied from 13.5 to 14.7 MJ/kg DM. But the energy content of duckweed was calculated to be 11.2 to 11.4 MJ/kg DM. The energy content of duckweed reported to be 10.1 MJ/ kg DM (Men Bui Xuan et al 2002,) No significant differences in live weight gain were found between the feeding systems (DWH vs DWG).

Carcass characteristics

Table 8 shows the carcass characteristics of growing Xinding ducks. The treatment effects on the carcass characteristics of ducks did not vary significantly (P>0.05), except for a significantly (P<0.05) lower live weight (1310 g) prior to slaughtering the ducks in control + fresh duckweed feeding under confinement (DWH). The carcass yield of ducks varied from 64.7 to 70.7% irrespective of treatment responses. The difference in meat to bone ratio in response to feeding duckweed was not significant (P>0.05). Men Bui Xuan et al (1996) reported a carcass yield of 72.2 to 73.5% of crossbred ducks fed on diets supplemented with fresh duckweed, which is similar to the present investigation. Ferdaus (1999) reported similar meat yield characteristics of ducks.

From this study, it was estimated that the feed cost per kg live weight gain for control, DWH and DWG group was  Euro 0.45 (Tk. 28.8), 0.24 (Tk. 15) and 0.23 (Tk. 14.4), respectively. The feed cost for the control (commercial feed compound) group was almost double that of the DWH and DWG groups. It is apparent that duck raising with duckweed is much cheaper and more economically viable, especially since feed cost contributes the major production cost in duck rearing.

Conclusion

• Duckweed has a high content of organic matter and protein but it has low digestibility. Ducks are, however, unable to receive the entire nutrient content from the duckweed.

• Low DM and crude protein digestibility resulted in poor growth performances and indicated that the ducks could not be reared when fed on duckweed only.

• Although the replacement of 50% feed by duckweed reduces performance parameters, the carcass characteristics are not influenced.

• Replacement of 50% of the control diet through feeding duckweed under confinement or through grazing fresh duckweed reduced the feed costs.

• Farmers may produce duckweed in water lagoons at their homesteads and/or may collect it from natural sources to supply as feed to their ducks. This will help to reduce feed costs by about 50%.

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