1Department of Biological and Agricultural Engineering
2Department of Forestry,
North Carolina State University,
Raleigh, NC 27695, USA.
Duckweed species have been used for several years as a means for recovering nutrients from wastewater and at the same time producing feed for livestock, fish and poultry1.
We have grown the duckweed species Lemna gibba 8678 on anaerobic ally digested wastewater from a swine rearing facility at the Lake Wheeler Agricultural Research Center, Raleigh, North Carolina. The Lemna produced was dried and analyzed for dry matter, total protein, and fiber, amino acid and trace mineral composition. This composition was compared with that of soybean and fish meal and its potential value as a feed ingredient in fish and shrimp diets assessed based on this analysis. The practical value of wastewater-grown duckweed was examined based on its nutritional value in formulated diets.
Duckweed species have been used for several years as a means for recovering nutrients from wastewater and at the same time producing feed for livestock, fish and poultry (Skillicorn et al 1993). The four main genera of Lemnaceae are Lemna, Spirodela, Wolffia and Wolffiella. Species of the first three genera can all grow on wastewater and are the ones most likely to be considered as novel plant protein sources. Duckweed species of the family Lemnaceae are among the fastest growing land plants and grow worldwide in a wide variety of climatic and ecological conditions. Normally found floating on standing water, duckweed species are easily harvested from surface waters and once dried produce a meal that has high protein content (15 to 40%) with a low fiber content as well. This meal has been used to feed cattle, pigs, poultry, fish and crawfish with favorable results (Skillicorn et al 1993). Due to its rapid growth and ability to grow well on wastewater from agricultural and domestic sources the potential of duckweed meal to contribute to aquaculture should be investigated further.
Due to its high moisture content (up to 95% water) duckweed meal needs to be dried to preserve its nutritional value. Once dried this meal is fairly stable and it resembles alfalfa meal in appearance. It should be protected from sunlight and changes in humidity. Normally once dried no further treatment is necessary. Dried duckweed can be palletized using commercially available equipment without the need to add a binding agent.
The composition of wastewater grown L. gibba is given in table 1. The high protein (41% crude protein) content resembles that of soybean meal, although in our sample the fiber (31% ADF) content is higher than is typical for soybean meal and more closely resembles Leuceana leaf meal (Hertampf, J.W. and F. Piedad-Pascual 2000). Table 2 shows the amino acid composition of protein from the wastewater grown L. gibba. The protein content of duckweed species is one of the highest in the plant kingdom, but is dependent on growth conditions. Table 3 shows that among the essential amino acids leucine, arginine and valine are the most abundant while methionine; cysteine and tryptophan are the least abundant. Duckweed has very high concentrations of lysine and methionine for a plant derived protein and more closely resembles proteins of animal origin in this respect. The amino acid profile of Lemna and Spirodela compares favorably with that of soybean and peanut meal (Mbagwu and Adeniji 1988). Crude fat levels in the literature range from 1.8 to 9.2% of dried duckweed meal based on the conditions of growth.
The presence of oxalic acid and tannins as crystalline inclusions (idioblasts) could interfere with feeding value of duckweed to monograstic animals.
Fasakin et al (1999) found that duckweed meal (from Spirodela polyrrhiza) can replace up to 30% of the total diet of the blue tilapia (Oreochromis niloticus). Hasan and Edwards (1992) grew tilapia in static water concrete tanks and fed them L. perpusilla and S. polyrrhiza up to 75g duckweed per kg wet fish weight. They found that these fish slowly consumed Spirodela while Lemna was rapidly consumed. Robinson et al (1980) found that the inclusion of L. minor meal into channel catfish diets had no effect on the rate of feed conversion nor on the energy per gram of fish gain. He concluded that the inclusion of Lemna meal into commercial diets would not significantly affect feed quality and that duckweed meal may be a suitable protein source practical channel catfish diets.Wastewater grown duckweed was used as the sole source of feed for the polyculture of Chinese and Indian carps in Bangladesh (Skillicorn et al 1993). Duckweed therefore has great potential as a locally produced feedstuff in countries where imported plant proteins such as soybean meal are scarce or very expensive.
To my knowledge no articles have been published on the effect of incorporating duckweed meal into penaeid shrimp diets. However duckweed has been used as feed for the red claw crayfish (Cherax quadricarinatus), (Fletcher and Warburton, 1997). They found that decomposed Spirodela species supported crayfish growth as well as commercial pellets did.
The abundance of carotenoids and pigments can stimulate crustacean growth (Hertampf and Piedad-Pascual, 2000). This observation could contribute to the value of duckweed as a feed material in crustacean diets. Further work is clearly necessary to test the effectiveness of duckweed as a feed material in crustacean diets.
Table 2. Amino Acid composition of dried Lemna gibba (g amino acid/100g dry L. gibba)
Taurine 0.03 Methionine 0.64
Aspartic Acid 3.51 Isoleucine 1.66
Threonine 1.68 Leucine 2.89
Serine 1.39 Tyrosine 1.27
Glutamic Acid 3.67 Phenylalanine 1.75
Proline 1.42 Histidine 0.73
Glycine 1.93 Ornithine 0.05
Alanine 2.30 Lysine 1.85
Cysteine 0.44 Arginine 2.14
Valine 2.12 Tryptophan 0.40
Table 3. Essential Amino Acid Composition of Dried Lemna gibba meal (g amino acid/100g dry L. gibba)
1 Skillicorn, Paul, William Spira and William Journey. 1993. Duckweed Aquaculture. A new aquatic farming system for developing countries. The World Bank, Washington, DC.
2 Hertampf, J.W. and F. Piedad-Pascual. Handbook on Ingredients for Aquaculture Feeds. Kluwer Academic Publications, Dordrecht 2000.
3. Mbagwu, I.G. and H.A. Adeniji. 1988. The nutritional content of duckweed (Lemna paucicostata Hegelm.) in the Kainji Lake area, Nigeria. Aquatic Botany, 29:357-366.
4. Fasakin, E.A., A.M. Balogun and B.E. Fasuru. 1999. Use of duckweed, Spirodela polyrrhiza, L. Schleiden, as a protein feedstuff in practical diets for tilapia, Oreochromis niloticus L. Aquaculture Research 30:313-318.
5. Hasan, M.S, and Edwards, P. 1992. Evaluation of duckweed (L. perpusilla and S. polyrrhiza) as feed for Nile tilapia (Oreochromis niloticus). Aquaculture 104:315-326.
6. Robinson, H. R., M.W. Brunson and E.J. Day. 1980. Use of duckweed in diets of channel catfish. Proceedings of the Annual Conference of Southeastern Association of Fish and Wildlife Agencies. 34:108-114.
7. Fletcher, A. and K. Warburton. 1997. Consumption of fresh and decomposed duckweed Spirodela sp. By Redclaw crayfish, Cherax quadricarinatus,(von Martens). Aquaculture research 28:379-382.