SOYBEAN AS LOCAL FISHFEED STUFF FOR LOCAL FISH FEED.

With regard to its composition,
soyabean meal appears to be a
reasonably good feed component
for aquaculture diets. It contains
about 47-50 percent protein, 5-6
percent ash, 1 percent lipid and about 40 percent carbohydrates. It
has a lysine content that
approaches that of fishmeal, but
for most aquaculture diets it is
deficient in the sulphur-containing
amino acids and in tryptophan. Because of these amino acid
deficiencies, soyabean meal cannot
be used as the only source of
protein and is generally
compounded with other feed-stuffs
when it is used in aquaculture rations. Although the inclusion of
soyabean meal in fish diets
presents no manufacturing
problems, problems relating to
palatability and availability of
nutrients have been encountered. There is abundant literature
discussing the deficiencies and
merits of this feedstuff. Several
investigators have demonstrated
that heating soyabean meal rather
severely not only increases its acceptability to fish, but also
improves the availability of
nutrients. This is first accomplished
by deactivating trypsin inhibitors,
and second by denaturing the
protein and thus making it more digestible, and third by detoxifying
natural toxicants. Several studies,
however, indicate that when
soyabean meal is heated, its
performance is still below the
expected considering its amino acid composition. Recent studies with
catfish, for example, show that
even when soyabean meal was
fortified with lysine, cystine, and
methionine, growth was
significantly lower than when menhaden meal was used as the
source of protein. It was concluded,
among other things, that soyabean
meal must contain some anti-
growth factors or that catfish do
not utilize free amino acids. Other workers, however, have shown
that the latter conclusion might not
be true, since the carp and salmon
can utilize free amino acids. Work done at the Northwest and
Alaska Fisheries Center has shown
that when soyabean meal
substituted for fishmeal in Oregon
moist pellet (OMP) type diets,
growth is significantly reduced when these diets are fed to
rainbow trout. It is believed that
part of the problem is due to the
phytate content of soyabeans. The
chemical structure of phytic acid is
shown in Figure 1. Chemically, phytic acid is the
hexametaphosphate of
myoinositol. In plant material it is supposed to occur as the Ca-Mg salt
of the hexametaphosphate. The
Merck Index states that there are
five calcium (Ca) ions and one
magnesium (Mg) ion per mole. This
may vary with plant species as well as with the maturity of the plant
seed. Its chemical structure is still
open to question because: (a) theoretically, it can have
several isometric forms (i.e., nine
stereoisomers and two
mesoforms), (b) it can form crosslinks between
phosphate groups, (c) the mechanism for Ca and Mg
binding remains uncertain, and (d) its ability to bind other ions
depends on how much Ca is
present. In plants, phytic acid occurs as the
Ca-Mg salt of the acid. Even after all
of the phosphate groups have
reacted with Ca and Mg, the
phytate is still capable of chelating
iron, zinc, and copper; its ability to chelate is enhanced in the presence
of excess calcium. Phytate has been
experimentally removed from
soyabean meal or other vegetable-
based ingredients by reaction with
the enzyme phytase (Figures 2 and 3). Growth studies with rainbow
trout using OMP-type diets in which
50 percent of the fishmeal was
substituted with dephytinized
soyabean meal are shown in Figure
4. Feeding untreated soyabean meal resulted in 25 percent
reduction in growth when
compared to the OMP control.
Dephytinized soyabean meal gave
growth values of about 8-10
percent better than untreated soyabean meal. Total substitution
with untreated soyabean meal
produced high mortality in the fish
after about 90 days. When the zinc,
iron, and copper contents of the
blood of the dead fish were determined, it was found that Zn
and Fe levels were significantly
lower than normal (as shown in
Table 3). Furthermore, feeding
purified (casein-based) diets
containing 0.5 percent of either Ca or Na phytate to rainbow trout
resulted in a 10 percent reduction
in growth. No other obvious
physiological defects were noted in
the diets containing phytates.
Compounds other than phytates present in soyabean meal are also
capable of binding trace nutrients.
Recent works by other
investigators strongly suggest that
diets containing proteins of
vegetable origin will perform more satisfactorily if they are fortified
with Zn and Fe. Recent laboratory
data show, for example, that
heating soyabean meal also
reduces its iron-binding ability so
that the improvement in performance of heated soya is not
only related to inactivation of
trypsin inhibitors and the
denaturing of protein but also to
the increased bio-availability of
micro-nutrients. The problems of soyabean meal, particularly those
concerning phytates, are
associated more or less with all
cereal-based components. The
absence of phytase in the
soyabean, however, may make the problem with soyabean meal more
severe than with other cereal-
based feedstuffs such as wheat
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