DUCKWEED AS ALTERNATVE TO MANY FISHFEED INGREDIENTS

Duckweed as a Primary
Feedstock for Aquaculture. A Summary of its Potential
Advantages. Cross Introduction Productivity Nutritional Value Feeding Trials Conclusions References Summary: 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. Introduction: 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, non technical
discussion of duckweeds, has
recently appeared in Pacific Discovery, 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). [ top of page ] 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 micro organisms
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 feed lots or other ready sources are
foreseen. [ top of page ] Nutritional Value of Duckweeds: Much reliable analytic data is
available to support the use fulness
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 thre e
locations in the Kainji lake are a
showed a crude protein ranging
from 26.3-45.5% of dry weight: "The amino acid content compared
favourably with that of blood,
soybean and cottonseed meals
and considerably exceeded that of
groundnut meal. The levels of the
se esential amino acids surpassed the FAO reference pattern, except for
methionine which met 61.4% of the
re commended 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...." [ top of page ] Feeding Trials with Duckweeds: 1. Fish. 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 body weight 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 the sea
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
are as 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 lamb s
and Awasi sheep. Landolt and Kandeler (1987) summarize many other studies of this kind. It need
not be stressed that many
water fowl 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). [ top of page ] Conclusions: 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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