The technical literature shows that duckweeds (Lemnaceae)
have a strong potential as primary feedstocks for production of livestock
and fish. These plants grow rapidly and are high in protein and dietary minerals
and low in fiber. They do not produce toxic alkaloids and are palatable to
a wide variety of domestic animals and fish. Feeding trials show that they
are suitable for animal production, and that the meat is tasty and nutritious
for human consumption. Growth of Lemnaceae requires shallow ponds, but their
culture reduces evaporation from the water surface and decreases growth of
undesirable algae. Duckweeds are adapted to many climatic zones around the
world, and hence are widely suitable for primary food production.
Duckweeds are the smallest flowering plants. They grow as small colonies of plants floating on the surfaces of quiet bodies of water. Growing vegetatively, their multiplication can be extremely rapid, given the proper conditions. These plants are almost all leaf, having essentially no stem tissue, and only one or a few, very fine roots. In nature, duckweeds serve as food for many species of fish and aquatic birds. They can tolerate and grow under a wide range of conditions, including on water polluted with high concentrations of bacteria and some agricultural wastes. These characteristics have brought the duckweeds to the attention of environmental engineers and agriculturists alike.
A delightful, nontechnical discussion of duckweeds, has recently appeared in Pacific Discovery (Armstrong, 1989), while an article in Smithsonian (Stewart, 1989) mentions their role in advanced biological sewage treatment. A thorough technical discussion of all aspects of their biology has been published (in English) by Professor Elias Landolt of the Swiss Geobotanical Institute (Landolt and Kandeler, 1987).
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Productivity of Duckweeds:
Culley et al. (1981) and Landolt and Kandeler (1987, pp. 371-373) summarize many earlier studies demonstrating the unusually high productivity of this aquatic plant. Dry weight increases of 10-20 tons/ha/yr are the norm. Doubling times in the range of 24 hr have been observed on many occasions, a rate of increase results in 64 g/g dry weight/week, or 73 tons/ha/yr (Landolt and Kandeler, 1987, p. 371). As Landolt and Kandeler point out, the observed values are therefore about 1/3-2/3 the theoretical value. Under summer conditions in Louisiana with heavy fertilization, up to 44 tons/ha/yr have been obtained (Said et al. 1979).
Needless to say, these rates of increase will be negatively affected by diminished rates of fertilization or cold weather in the temperate zone. However, it is clear that the species of this group are capable of a level of productivity closer to that obtained with microorganisms than with other higher plants.
The high levels of fertility required to obtain this massive growth of duckweeds might seem to be an obstacle, but in fact are economically feasible, if the use of nitrogenous runoff wastes from feedlots or other ready sources are foreseen.
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Nutritional Value of Duckweeds:
Much reliable analytic data is available to support the usefulness of the duckweeds as valuable food sources (Landolt and Kandeler, 1987). Most species have protein contents in the range of 15-45%, depending on the nitrogen supply, and the amino acid balance is favorable, with only TRP and MET generally limiting (Landolt and Kandeler, 1987, pp 375-377. The yearly protein yield/ha is up to tenfold higher with Lemna than with soybeans, and nearly as much better than for alfalfa (Said et al. 1979).
Recent work with Lemna paucicostata in Nigeria (Mibagwu, and Adeniji, 1988) indicates an especially high nutritional value. Their analyses of plants from three locations in the Kainji lake area showed a crude protein ranging from 26.3-45.5% of dry weight:
"The amino acid content compared favourably with that of blood, soyabean and cottonseed meals and considerably exceeded that of groundnut meal. The levels of the essential amino acids surpassed the FAO reference pattern, except for methionine which met 61.4% of the recommended value. The levels of minerals were high but should not pose any toxicity problems if incorporated into animal feeds. The levels of nitrogen in the plant are comparable to those in commercial fertilizers. The plant could be a good dietary supplement and nutrient source for humans, livestock and fish...." "With an average standing crop of 309 kg dry mass/ha and doubling time of 1.2 d, 129 kg DM/ha of dry duckweed are obtainable daily in the Kainji Lake area, which could ensure a daily supply of 59 kg of high-quality protein for poultry and fish feed formulation...."
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Feeding Trials with Duckweeds:
Duckweeds can be grown separately and then provided to the fish, or produced in the same pond. Production of Lemna in the same pond is not likely to work efficiently, however. Vigorous aeration of the water, as is practiced in catfish-culture, will disturb the growth of the plant. The photosynthetic activities of the plant do not oxygenate the water, in fact the covering plants reduce gas-exchange with the atmosphere (Landolt and Kandeler, 1987, p 387).
Grass carp seem particularly adapted to feeding on Lemna, and there is a large literature devoted to this application (summarized by Landolt and Kandeler, 1987, pp. 387-388.). As reviewed by Landolt and Kandeler, channel catfish have been successfully raised on duckweeds, but no commercial application of these findings seems to have been developed.
The growth of hybrid carp were studied by Cassani and Caton (1983) to determine feeding preference and feed consumption. The hybrid was grass carp, Ctenopharyngodon idella (Val.) X bighead carp, Hypophthalmichthys (Aristichthys) nobilis Rich, 12 to 18 months old. Their conclusion was, "The most preferred plant was Lemna gibba when in combination with six other species." Moreover, at the same order of preference was exhibited at two different growth temperatures (12-15 C vs. 25-28 C). This preference was the same, based either on the time to complete consumption or the relative quantity consumed. Mean daily consumption (g) fish at 25-31 C for Lemna gibba tested separately was 178.
Gaigher, et al. (1984) compared the growth of hybrid tilapia fish (Oreochromis niloticus X O. aureus) on commercial pellets vs. duckweed. The fish were cultured at high densities in an experimental recirculating unit for 89 days with duckweed (Lemna gibba) or a combination of duckweed and commercial pellets. They conclude that a combination of pellets and Lemna gave the best performance:
When fed on duckweed alone, intake rate was low, feed conversion ratio good (1:1) and relative growth rate poor (0.67% of bodyweight daily). Sixty-five percent of the duckweed consumed was assimilated and 26% converted to fish. When the fish were fed on pellets in addition to duckweed the rate of duckweed consumption decreased and growth rate of the fish doubled with feed conversion ratios between 1.2 and 1.8. Seventy percent of the mixed diet was assimilated but only 21% converted. Fish grown on the mixed diet performed similarly to fish grown on pellets but had a better feed conversion ratio.
Porath, et al. (1985) attempted to recycle the solid wastes of these fish as a fertilizer for Lemna. The duckweed (Lemna gibba) was grown in shallow ponds containing mineral nutrients. However, the tilapia waste was poor in free NH3 and ammonium compounds. When separated and incubated at 38 C to allow anaerobic digestion to release mineral nutrients, prolonged digestion was necessary before it supported growth of the plant.
Tilapia were given duckweed as food as young fish from the larval to the fingerling stages (Moreau, et al., 1986). A comparison was made of three types of food: Lemna minor (duckweed, produced in shallow ponds or year-round in greenhouses), Chlorella (phytoplankton) and Daphnia (zooplankton). These authors studied both the nutritional value of these foods and the resulting growth rates of the fish.
Crayfish are often released in irrigated rice fields in rice- growth areas of the United States to control weeds (often duckweeds), according to Landolt and Kandeler. It is not known if the deliberate growth of Lemnaceae would be an efficient means of production of these crustaceans.
2. Warm-blooded animals.
Porath, et al. (1985) reported preliminary tests using duckweed to substitute for animal protein-rich feed in diets of young lambs and Awasi sheep. Landolt and Kandeler (1987) summarize many other studies of this kind. It need not be stressed that many waterfowl normally feed upon this plant. Chickens have also been prime candidates for this food source, since their high efficiency of feed conversion would further contribute to its economic viability. Work with laying hens has been particularly encouraging (Haustein et al., 1990).
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Numerous demonstration projects prove the usefulness of duckweeds as the primary food source for production of fish and livestock. These studies show that duckweeds are highly productive and economical to grow, and that the plants provide nutritious and palatable food for meat animals. Based on this farming system, and with no additional basic research, highly profitable agricultural ventures can be developed.
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