rendering them unavailable. The ability of
phytic acid to bind metal ions is lost when the
phosphate groups are hydrolyzed through
the action of enzyme phytase. Although
phytase activity has been shown to be
present in ruminants, animals with a simple
stomach such as fish lack this enzyme in their
gastrointestinal tracts and hence cannot utilize
the phytate bound phosphorus or other metal
ions. Therefore, feeds are often supplemented
with phosphorus in the form of mono or di-
calcium phosphate. Phosphorus and calcium
requirements are interdependent. Addition
of microbial phytase in the diet of Nile tilapia
significantly improved the growth of fish.
Variations in the quantitative values reported
in literature can also be expected due to
differences in dietary ingredients used.
Table 1: Essential amino acid requirements
of young O. mossambicus and O. niloticus.
Percent of dietary protein
Amino acid
O. mossambicus O. niloticus
Arginine
2.82
4.20
Histidine
1.05
1.72
Isoleucine
2.01
3.11
Leucine
3.40
3.39
Lysine
3.78
5.12
Methionine
0.99
2.68
Phenylalanine
2.50
3.75
Threonine
2.93
3.75
Tryptophane
0.43
1.00
Valine
2.20
2.80
Duckweed:
Duckweeds are small (1-15 cm) free-floating
aquatic plants with worldwide distribution.
They are monocotyledons belonging to the
family Lemnaceae (which is derived from the
Greek word ‘Limne’, meaning pond) and are
AgriKultuur |AgriCulture
classified as higher plants or macrophytes,
although they are often mistaken for algae
and some taxonomists consider them as being
members of the Araceae. Duckweeds serve
as nutrient pumps, reduce eutrophication
effects and provide oxygen from their
photosynthesising activity. Duckweeds are
often seen growing in thick blanket-like mats
on still nutrientrich fresh and slightly brackish
waters. They do not survive in fast moving
water (> 0.3 m/sec) or water unsheltered from
the wind. They grow at water temperatures
between 6 and 33 °C.
Each frond of duckweed absorbs nutrients
through the whole plant, not through a central
root system, directly assimilating organic
molecules such as simple carbohydrates
and various amino acids. The entire body is
composed of non-structural, metabolically
active tissue; most photosynthesis is devoted
to the production of protein and nucleic acids,
making duckweeds very high in nutritional
value. The nutritional content of duckweed
is probably more dependent on the mineral
concentrations of the growth medium than on
the species or their geographic location. Water
low in nutrients generally results in reduced
nutritional content in duckweed. Crude fibre
content is generally lower (varying between
7-10 percent) for duckweed grown in nutrient-
rich water than that grown in nutrient-poor
water (11-17 percent). Compared with most
plants, duckweed leaves have little fibre (5
percent in cultured plants) as they do not need
to support upright structures. Crude fibre
content was generally lower, varying between
7-10 percent, for duckweed grown in nutrient-
rich water as opposed to 11-17 percent for
duckweed grown in nutrient-poor water.
In general, the ash content ranges between
12-18 percent. Duckweeds are known to
10