Farm Animal Metabolism and Nutrition

trichothecenes possess a basic tetracyclic
sesquiterpene structure with a six-
membered oxygen-containing ring and an
epoxide group. The synthesis of the two
types of mycotoxins is characteristic for
a particular Fusarium species. However,
a common feature of the secondary
metabolism of these fungi is their ability to
synthesize ZEN, a phenolic resorcyclic
lactone which occurs as a co-contaminant
with certain trichothecenes. The
fumonisins are synthesized by another
distinct group of Fusarium species. These
mycotoxins are long-chain polyhydroxyl
alkylamines containing two propane
tricarballylic acid moieties attached to two
adjacent carbon atoms. Three members of
this group (fumonisin B 1 , B 2 and B 3 ) may
occur together in maize.

Endophyte alkaloids

The endophytic fungus Acremonium
coenophialum occurs in close association
with perennial tall fescue, while another
related fungus, A. lolii, may be present in
perennial ryegrass (Porter, 1997). The fungi
provide the grass with defensive secondary
compounds while the plant serves as a
source of essential nutrients for the fungus.
These secondary metabolites are also
toxigenic towards animals. Ergopeptine
alkaloids, mainly ergovaline, occur in A.
coenophialum-infected tall fescue, while
the indole isoprenoid lolitrem alkaloids,
particularly lolitrem B, are found in A. lolii-
infected perennial ryegrass.

Phomopsins

In Australia, lupin stubble is valued as
fodder for sheep, but infection with the
fungus Phomopsis leptostromiformis is a
major limiting factor due to toxicity arising
from the production of phomopsins by the
fungus. Mature or senescing parts of the
plant, including stems, pods and seeds, are
particularly prone to infection. Of these
mycotoxins, phomopsin A is considered to
be the primary toxic agent. Phomopsin A is

a hexapeptide with the sequence Phe-Val-

Ile-Pro-Ile-Asp, modified by oxidation,

chlorination and methylation.

Sporidesmin

Pithomyces chartarum is an ubiquitous

saprophyte of pastures with the capacity to

synthesize sporidesmin A which represents

an important group of diketopiperazine

derivatives arising from the formation of a

cyclic anhydride between two amino acids.

A sulphur-containing ring imparts biological

activity to the molecule (Flaoyen and

Froslie, 1997).

Metabolism

Many ANFs and mycotoxins are meta-

bolized in the digestive system and in the

tissues of animals, resulting in detoxification

or in promotion of adverse effects. A

number of instances have been selected here

to illustrate the diversity of metabolism and

consequences for farm animal productivity.

Amino acids

The toxicity of SMCO in cattle and sheep

arises after its fermentation by rumen

bacteria to dimethyl disulphide oxide which

is then reduced to dimethyl disulphide. It is

the latter metabolite which precipitates the

typical symptoms of haemolytic or kale

anaemia in ruminants (D’Mello, 1994). On

the other hand, metabolism of mimosine

by ruminants depends upon geographical

differences in rumen microbial ecology.

During this metabolism, 3-hydroxy-4(1H)-

pyridone (3,4-DHP) is synthesized; this itself

is endowed with deleterious properties, but

in addition another isomer, 2,3-DHP may be

produced in the rumen. Some rumen bac-

teria are capable of detoxifying both forms of

DHP to as yet unidentified compounds

(D’Mello, 1991a). Despite these reactions,

considerable quantities of mimosine and

3,4-DHP may escape metabolism to appear

in the faeces. Furthermore, conjugated forms

388 J.P.F. D’Mello