Livestock Research for Rural Development 9 (2) 1997

Citation of this paper

Evaluation of protein quality in duckweed (Lemna spp.) using a duckling growth assay


Nguyen Duc Anh and T R Preston

University of Tropical Agriculture, Ho Chi Minh City, Vietnam



The trials were done during a one-month period, from 18 Dec 1996 to 18 Jan 1997 at the University of Tropical Agriculture, outside Ho Chi Minh City, Vietnam. Two trials using 5 and 10-day-old ducklings, were carried out using a method designed to evaluate quality of protein sources. A duckling growth assay is an attractive biological test because results are obtained within two weeks after giving the test protein. Energy sources free of protein were used to ensure that the growth response reflected the quality of the test protein.

The first trial aimed to test the method with a standard protein source which was soya bean meal given at levels of 10, 15, 20, 25 and 30% (approximately 7, 9, 11, 13 and 15% crude protein in feed dry matter) in the basal diet. A protein-free basal diet as the major energy source was provided by brown sugar and cassava root meal (50/50 mixture). In the second trial, the treatments were 100% replacement of the protein from soya bean meal for assaying the protein quality of fresh duckweed. There were five levels of a ratio (fresh weight basis) between duckweed and basal diet (brown sugar/cassava root meal = 50/50) of 1:1, 2:1, 3:1, 4:1 and 5:1 (approximately 3.3, 5.2, 6.8, 8.3 and 9.5% crude protein in feed dry matter). Growth rate of the ducks was positively correlated with the protein level (r=0.99 and 0.93 for the soya bean and duckweed, respectively).

It is concluded that: (i) growth rate of the duckling can be used to assay quality of protein resources; (ii) duckweed protein appears to have equivalent biological value to the protein of soya bean meal.

Key words: Duckling growth assay, duckweed, non-conventional protein sources, protein-free basal diet



Finding a local alternative for the increasingly expensive conventional protein sources such as soya bean meal and groundnut meal in pig and fowl production in the poor areas of Vietnam requires research on non-conventional protein sources.

Duckweed (Lemna spp.) has received research attention because of its high nutritive value, especially the high protein content. Duckweed species are small floating aquatic plants found worldwide. They are monocotyledons of the botanical family Lemnaceae.

In trials with sewage water in Australia, the protein content of the duckweed increased from 20-25 to 35-40% in dry matter when N in the water increased from <5 to 15 mg/litre (Leng et al 1995). Yields of duckweed dry matter were in the range of 10 to 30 tonnes/ha/yr , equivalent to protein yields of duckweed of as high as 10 tonnes/ha/yr.

Conventional methods to evaluate nutritional quality of feeds are based on Weende and Van Soest analyzes, which aim to determine the principal nutrients present in feeds. The utilization of these fractions by the animal is assessed by measuring their digestibility which is done indirectly by subtracting nutrients excreted from those ingested.

Biological methods seem more appropriate because in most developing countries it is easier and cheaper to acquire animals and feeds than laboratory equipment and chemicals both of which also require foreign exchange. Furthermore, biological response data are more useful, and more easily understood, as a basis of making recommendations to farmers. Sarria et al (1995) used early growth in chicks fed basal diets of sugar/palm oil to assay the protein in soya bean meal but the results were unsatisfactory. Addition of cassava starch (21%) improved the intakes in an assay of the protein in Trichantera gigantea and Azolla but the growth responses to protein supply were not consistent. In this paper, preliminary data are reported using a duckling growth assay to evaluate non-conventional protein source derived from duckweed.



The objectives of the study were first to test the assay using a standard protein (soya bean meal) and secondly to apply the method to evaluate the protein in duck weed.


Materials and methods

Experimental design and animals

In trial 1, twenty five ten-day old ducklings with average initial weight of 200g were divided into five groups. There were five ducklings per group and they were fed the trial diets from 10 to 26 days of age. They were allocated to treatments in a completely randomized design. In trial 2, the experimental design and animals were similar to those in trial 1, except that the ducklings were five-days of age at the start with initial weight of 73g and the feeding period was from 5 to 17 days of age.

Housing and management

The ducklings were housed in a shed made from bamboo with a roof of palm leaves. The shed was divided into five pens each with dimensions of 0.48m2 area and 1.10m2 area of running yard. The feeders were round plastic basins 15 cm in diameter and 8 cm deep. The drinkers were round plastic basins 20 cm in diameter and 10 cm deep. Both drinkers and feeders were cleaned before feeding.

Diets and feeding

The dry matter, and crude protein in the dry matter, of the feedstuffs are shown in Table 1. A protein-free basal diet was formulated from brown sugar and cassava root meal (in proportions of 50:50) as the major energy source.

In trial 1, the ducklings were fed in groups according to treatment from 10 to 26 days of age. The protein source was soya bean meal given at levels of 10, 15, 20, 25 and 30% in the feed (approximately 7, 9, 11, 13, and 15% crude protein in feed dry matter). Duckweed was supplied at a level of 5% (dry basis) of the diet as a source of vitamins and minerals.

In trial 2, the ducklings were fed in groups of 5 from 5 to 17 days of age and given the same basal energy diet as in trial 1. Duckweed was supplied at ratios (fresh basis) of: 1:1, 2:1, 3:1, 4:1 and 5:1. The calculated contents of DM and crude protein in the feed are shown in Table 2. The basal diet and the duckweed were mixed together before feeding and offered ad-libitum.


The ducklings were weighed individually at the start and then every four days. Feed offered and refused was recorded daily. Samples of offered and refused feed (20%) were taken daily, bulked over 4 days and stored in a refrigerator prior to determinng nitrogen and dry matter.


Results and discussion

The growth responses of the ducklings to soya bean level (X) are shown in Figures 1 and 2 and were described by the following equations:

Y1 = 56.8 + 23.1X - 0.45X2 ; r2 = 0.995 (Y1 = final liveweight; Figure 1)

Y2 = -9.1 +1.55X - 0.03X2 ; r2 = 0.993 (Y2 = daily gain; Figure 2)

The results show that the growth rate of the ducklings was linear up to 25% of soya bean in the diet and then curvilinear from 25 to 30%. It could be that at the 30% level there was some nutrient imbalance which limited the growth rate.

In trial 2, liveweight at 17 days (Y3) and daily gain (Y4) of the ducklings were linearly related with crude protein from duckweed in feed dry matter (X) as follows:

Y(3) = 51.8 + 10.3X ; r2 = 0.927 (Figure 3)

Y(4) = -1.63 + 0.85X ; r2 = 0.93 (Figure 4)

The above results show that duckweed can be used as non-conventional protein source to replace completely the soya bean meal and be the sole source of protein in diets for ducks. Bui Xuan Men et al (1996) also reported that duckweed could replace completely soya bean protein in diets for fattening ducks although in that case the energy source (broken rice) contained more protein (8-9%) than the sugar and cassava root meal used in the present experiment.

The close relationship between protein supply and duck growth over such a short period (12 days) confirms that growth of ducklings using a protein-free energy source can be used as a biological assay to assess protein quality in a test diet.

In Figure 5 are shown the relationships between daily liveweight gain and the crude protein in the diet for the two sources of protein: duckweed and soya bean meal over the range of 3 to 13% protein in dry matter. The linear equations were:

Y(d) = -1.63 + 0.85X ; r2 = 0.93

Y(s) = -4.24 + 1.17X ; r2 = 0.928

The higher value of the "X" coefficient for soya bean indicates some limitation to the availability of the protein in the duckweed. As there is more fibre in duckweed (10% in dry matter) than in soya bean meal (about 5% fibre), it is likely that the digestibility of the protein in duckweed is the factor limiting its utilization.



It is concluded that:


Bui Xuan Men, Ogle R B and Preston T R 1996 Duckweed (Lemna spp.) as replacement for roasted soya beans in diets of broken rice based for fattening ducks on a small scale farm in the Mekong Delta. Livestock Research for Rural Development. Volume 8, Number 3:14-19

Leng R A, Stambolie J H and Bell R 1995 Duckweed-Potential high protein feed resource for domestic animal and fish. Livestock Research for Rural Development. Volume 7, Number 1:36Kb

Sarria Patricia and Preston T R 1995 A chick assay method for the evaluation of non-conventional protein sources derived from Nacedero (Trichanthera gigantea) and Azolla (Azolla filiculoides). Livestock Research for Rural Development. Volume 7, Number 3:32-38

Received 1 May 1997

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