Use of duckweed (Lemna spp) as replacement for soya bean meal in a basal diet of broken rice for fattening ducks

An experiment was conducted at Cantho University to determine the effects of feeding duckweed (Lemna sp.) replacing the roasted soya beans in diets based on broken rice for crossbred meat ducks. The trial included five treatments, with four replicates and 10 ducks per replicate (pen). The five diets were based on broken rice ad libitum supplemented with 27 g/day roasted soya beans (control D0) or 19, 15, 12 or zero g/day soya beans with ad libitum fresh duckweed (D30, D45, D60 and D100). These diets were fed to growing crossbred ducks (Czechoslovak x Cherry Valley hybrids) from 28 to 63 days of age, when two birds (one male and one female) per pen were slaughtered for carcass evaluation.

Daily feed intakes were 95, 108, 108, 105 and 107g DM and daily live weight gains were 26.1, 29.1, 28.3, 27.1, 27.6g for control, D30, D45, D60 and D100, respectively. Corresponding feed conversion ratios were 3.7, 4.2, 4.2, 4.1, and 4.2.There were no significant differences in the carcass traits between treatments. The diet with 100% replacement of soya beans by duckweed was the most profitable for the farmer.

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. In other systems the ducks are raised in the backyard or in the gardens of households or kept in 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 farmer cannot herd their ducks in the fields as there is no feed. On the other hand, the law in the country now prohibits the farmers from keeping their ducks in the canals and rivers so as to eliminate environmental pollution so the population of ducks has decreased and this results in fluctuation in the supply of meat and eggs. In order to meet the increasing demand of the consumers, there is increasing interest in confinement of the ducks making use of locally available feeds.

Duckweed, 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 and carotene that are necessary for growing animals. Duckweed has been used traditionally 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 objective of the experiment was to determine the optimum level of duckweed as a replacement for soya beans in diets for fattening ducks based on broken rice grain.

200 crossbred ducklings were fed from 28 to 63 days of age in the Experimental Duck Farm, Cantho University. The trial included five dietary treatments and four replicate as follows:

D0: Broken rice grains supplemented with 27 g/day of whole roasted soya beans (control)

The broken rice was given ad libitum on all diets as was the duckweed in diets D30 to D100. The whole soya bean seeds were roasted and ground and had the following chemical composition: DM 87%, N x 6.25 38.3%, ether extract 18.3%, NFE 98%, crude fibre 8.5%. A premix containing trace minerals, vitamins and salt was mixed (0.25%) with the control diet but not in the other diets.

The duckweed was grown on ponds fertilized with effluent from a biodigester. The chemical composition of a representative samples of the feeds used in the trial is given in Table 1.

Table 1: Mean values for composition of duckweed used during the Experiment

Composition	Duckweed	B. rice	Soya 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

The duckweed was consumed readily on all treatments from D30 to D100. Intake of broken rice was depressed slightly on the diet with 70% of the control level of soya beans (D30) and increased when no soya beans were given (D100). The intake of duckweed increased as the soya beans were restricted reaching an average of 560 g/day. These levels of intake are higher than those (434 to 450 g/day) recorded at our laboratory by Becerra et al (1995) for ducks in a similar trial but fed with reconstituted sugar cane juice instead of broken rice. This may be a reflection of the use in our experiment of duckweed grown in ponds fertilized with digester effluent which resulted in a higher protein content (35% in dry matter compared with 23%). Total intake of protein was highest on the 70% soya bean level (D30) and least on the control and D100 diets.

Table 2: Mean values for intake of dietary ingredients

Feed intake, g/d	D0	D30	D45	D60	D100	SE/Prob

Broken rice	82	78	83	82	92	1.31/0.001
Roasted SBM	27	19	15	12	0
Duckweed	0	496	499	505	566	4.88/0.001
Premix + salt	0.25	0	0	0	0
Total DM	94.7	108	108	105	107	1.11/0.001
Total N x 6.25	17.1	22.7	21.6	20.6	17.9	0.16/0.001

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 soya beans (D100) supported slightly better growth than on the control diet. Feed conversion was best on the control diet and did not differ among 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.

Table 3: Mean values for live weights, growth and conversion rates of ducks fed duckweed as replacement for soya beans in basal diets of broken rice

	D0	D30	D45	D60	D100	SE/ Prob.

Live weight, g
Initial	859	851	830	859	842	11.9/ 0.37
Final	1771	1869	1822	1807	1806	22.6/ 0.05
Daily gain	26.1	29.1	28.3	27.1	27.6	0.57/ 0.003
Conv (DM)	3.70	4.21	4.23	4.11	4.17	0.04/ 0.001

Table 4: Mean values for carcass trait of ducks given duckweed as replacement for soya beans in diets based on broken rice

	D0	D30	D45	D60	D100	SE/Prob.

Live weight, g	1870	1865	1851	1819	1821	28.1/ 0.58
Carcass wt, g	1253	1219	1211	1175	1198	25.8/ 0.31
Carcass yld, %	73.5	72.5	72.6	72.2	72.8	0.79/ 0.83
Chest muscle, g	203	166	183	164	175	10.1/ 0.07
Thigh muscle, g	162	156	141	156	153	6.85/ 0.25
Heart, g	14	12	14	12	14	1.01/ 0.49
Liver, g	61	61	56	59	61	2.8/ 0.71
Gizzard, g	55	52	52	57	56	2.26/ 0.51
Small int, cm	188	186	186	190	191	4.49/ 0.88
Large int, cm	12	13	13	12	13	0.58/ 0.36
Caecum, cm	34	36	34	36	34	1.16/ 0.46

There would appear to be marked economic benefits to the farmer from using duckweed to replace soya beans in broken rice diets for fattening ducks in situations where the duckweed is grown on the farm and managed and harvested by household labour (Table 5). This emphasises the importance of an integrated farming system as a means of reducing costs and improving the economic competivity of the small scale farmer.

Table 5: Estimates of feed costs assuming situations of purchase or farm-based production of duckweed (in VND; about 11,000VND=1US$)

Feed cost/ kg gain	D0	D30	D45	D60	D100

Duckweed purchased*	11,589	13,364	12,866	12,267	10,923
Duckweed grown by farmer**	11,589	9,492	8,952	8,310	6,498

* Based on prices per kg for roasted soya beans 5,400, broken rice 1,800, fresh duckweed 200, premix 36,000 and salt 1,000
** Assumes no cost of duckweed as opportunity cost of family household labour (women and children) is usually zero

From the results of the experiment it can be seen that fresh duckweed can completely replace roasted soya beans and a vitamin- mineral premix in broken rice based diets for fattening ducks without reduction in growth performance or carcass traits. The poorer feed conversion on the diets containing duckweed has no economic significance as seen in Table 4 since duckweed can be grown easily on the farm whereas soya beans 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 soya bean protein is another advantage for the integrated farming system.

This research forms part of my programme of study 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. Development of the basic experimental facilities were facilitated by my grant from the International Foundation for Science (IFS), Grant No B/208-1.