Aquatic weeds and their utilization 3157.3.2 Protein content
For most species of aquatic weeds about 80% of the total-N is in the form of
protein. Aquatic weeds contain 8-30% of crude protein (on a dry-matter basis) -
a range similar to that found in terrestrial crops. The considerable variations in
crude protein content are due to both seasonality and environment. The crude
protein content of Typha latifolia decreased from 10.5% in April to 3.2% in July
(Boyd 1970) and that of Justicia americana from 22.8% in May to 12.5% in
September (Boyd 1974). In addition, the crude protein content of Typha latifolia
from different sites varied from 4.0-11.9% (Boyd 1970), and that of water
hyacinth grown on a stabilization pond was 14.8% compared to 11.3% in
samples from a lake (Bagnall et al. 1974). This indicates that the crude protein
content increases as the nutrient content of the water in which the plant is grown
increases. According to Wolverton and McDonald (1976b), the crude protein of
water hyacinth leaves grown in wastewater lagoons average 32.9% dry weight,
which is comparable to the protein content of soyabean and cotton seed meal.
This value is more than two times the maximum crude protein content of water
hyacinth reported by Bagnall et al. (1974). Duckweeds have a crude protein
content over 30% and have a better spectrum of amino acids as regards to lysine
and methionine than the other plants (Oron et al. 1986).
Although the protein content of various aquatic weeds differs greatly, the
amino acid composition of the protein is relatively constant, nutritionally
balanced, and similar to many forage crops. But the levels of methionine and
lysine - generally considered as the limiting amino acids in plant proteins - are
lower than in terrestrial crops (Boyd 1970). Additional information on protein
content of aquatic weeds is given in section 7.6.2.
7.3.3 Mineral content
The ash content of aquatic weeds varies with location and season. Sand, silt, and
encrusted insoluble carbonates from the water account for much of the mineral
content. Although silt can be washed off the plants, in practice it represents part
of the chemical composition of the harvest. The amount of minerals varies from
15-25% of the harvest (dry weight), depending on the waterway's chemical
content and turbidity (Table 7.2).
The amounts of phosphorus, magnesium, sodium, sulfur, manganese, copper
and zinc in aquatic weeds growing in nature are similar to those in terrestrial
plants. However, aquatic plants are often richer in iron, calcium and potassium
than land forages, and some species are known to concentrate such minerals.
Elements can be exceptionally high in aquatic plants grown in sewage or