Farm Animal Metabolism and Nutrition

Glucosinolates

Glucosinolates are thioglucosides which
are of particular significance in brassica
vegetables and forage crops. Over 100
different glucosinolates have been identified
and characterized (Duncan, 1991). They all
share a common structure, being composed
of a thioglucose group attached to an R-
group. A wide range of R-groups has been
identified. Removal of glucose from the
glucosinolates by plant or microbial
thioglucosidase (myrosinase) results in the
release of a diverse array of aglycone moi-
eties. The aglycone residues undergo fur-
ther degradation to yield a number of toxic
metabolites. The most common breakdown
products are isothiocyanates and nitriles
but, depending upon conditions such as
pH, temperature and metallic ion concen-
trations, a number of other metabolites may
also be produced.

Amino acids

In addition to the glucosinolates, forage
and root brassica crops contain a non-
protein amino acid in the form of S-
methylcysteine sulphoxide (SMCO). Its
structural resemblance with the indispens-
able amino acid, methionine, has been
alluded to by D’Mello (1991a). The aromatic
non-protein amino acid, mimosine, occurs
in the foliage and seeds of the tropical
legume ipil ipil or koa haole, commonly
referred to as Leucaena leucocephala.
Mimosine is widely regarded as an
analogue of the physiologically important
amino acid, tyrosine, and its neuro-
transmitter derivatives, dopamine and
norepinephrine, found in the brain
(D’Mello, 1994).

Phyto-oestrogens

The phyto-oestrogens are a diverse group
of isoflavonoid compounds found pri-
marily in legumes (Whitten et al., 1997).
Isoflavonoids have been classified into
seven groups based on structural features

including: isoflavone, isoflavan, isoflava-

none, coumestan, pterocarpan, rotenoid

and coumaronochromone (Whitten et al.,

1997). It is well recognized that soybeans

contain relatively high concentrations of

the glycosides of the isoflavones, daidzein,

genistein and glycitein. In subterranean

clover (Trifolium subterraneum) and in red

clover (T. pratense), an important isoflavone

is formononetin (Adams, 1995).

Other plant secondary compounds

A diverse array of other plant secondary

metabolites have been associated with

specific toxic effects in farm animals.

These include steroidal saponins which

occur as glycosides in certain pasture

plants (e.g. Brachiaria decumbens and

Panicum spp.), vasoactive lipids of western

yellow pine (Pinus ponderosa), gossypol

pigments of the cotton plant (Gossypium

spp.) and HCN present as glycosides in a

wide range of plants such as linseed

(Linum usitatissimum), cassava (Manihot

esculenta) and sorghum (Sorghum spp.).

Mycotoxins

The major toxigenic species of fungi and

their respective toxic metabolites are

presented in Table 18.2. These metabolites

are known as mycotoxins, while the

deleterious effects they precipitate are

referred to as mycotoxicosis. The synthesis

of a particular mycotoxin generally is

restricted to a relatively small number of

fungal species and may be species- or even

strain-specific. A significant feature now

emerging is the co-production of two or

more mycotoxins by the same species of

fungus which offers a different perspective

to the interpretation of well-known cases of

mycotoxicoses.

Aflatoxins

Over the past 30 years or so, attention has

focused predominantly on the aflatoxins

386 J.P.F. D’Mello