334 Organic waste recycling: technology and management
ash than emergent plants. However, the amino acid composition of protein in
aquatic weeds is similar to many forage crops (section 7.3b).
Figure 7.9 compares the crude protein content of fresh and dried aquatic
weeds to that of fresh and dried alfafa hay. Aquatic weeds compare favorably on
a dry mass basis, but not on a fresh mass basis.
Even though nutritional value of aquatic weeds compare favorably (when
dried) with alfafa, the cost of artificial drying, grinding, formulating with other
feed to improve palatability and pelleting makes the cost of feed from aquatic
weeds expensive (Frank 1976; NAS 1976). Nevertheless, water hyacinth can
support the growth of livestock, if it is partially dried and properly
supplemented, and if the animals are accustomed to it.
None of the feeding tests reported in the literature produced evidence of
toxins in aquatic weeds. Potentially toxic substances such as nitrates, cyanides,
oxalates, tannins and discoumarins are all present in aquatic weeds, but they also
occur in many terrestrial types of forage, so that in general aquatic plants are no
more hazardous to livestock than conventional forages (NAS 1976). Boyd
(1974), however, reported a concentration of tannins of 10% or more of the dry
weight in some species of aquatic weeds, which would greatly impair the
digestibility of their protein.
Silage
A promising technique to eliminate the expense of artificially drying aquatic
weeds is to convert them to silage (Frank 1976; NAS 1976). Ensiling aquatic
weeds could become very important in the humid tropics where it is difficult to
sun-dry plants to make hay. According to NAS (1976), water hyacinth silage
can be made with 85-90% moisture content since the fiber retains water well
and thus the material does not putrefy. But Bagnall et al. (1974) found that
chopped water hyacinth alone could not be made into silage since it putrefied
and that 50% or more of the water had to be pressed from the hyacinth before it
could be made into acceptable silage. The aquatic weeds could be wilted in the
shade for 48 hours, or chopped and pressed to remove some of the water. Also,
since silage is bulky, the silos should be located near the animals and supply of
plants.