in ruminants. From their studies with sheep,
Wang et al. (1994) concluded that CTs are
able markedly to increase transulphuration
of methionine to cysteine for body
synthetic reactions. This may be important,
since cysteine is a major component of
wool protein. CTs have also been
implicated in bloat suppression in cattle.
Bloat is believed to be due primarily to the
formation of a stable foam in which
fermentation gases are entrapped. The
active foaming agent is the soluble F1
fraction of leaf protein. Bloat in grazing
ruminants is often associated with low-
tannin pasture legumes such as lucerne
and clover, but sainfoin, lotus and tropical
browse legumes are considered to confer
protection by virtue of their content of CTs.
Using in vitro methods, it has been demon-
strated that CTs from a variety of legumes
reduced the compressive strength of
protein foams in a dose-dependent manner,
irrespective of differences in the chemical
structures of these tannins.
Another attribute of high-tannin
legumes has emerged from recent
investigations with sheep infested with
nematode parasites (Niezen et al., 1995). In
view of consumer apprehension about
chemical residues in animal products, the
potential use of such legumes as a substi-
tute for conventional drugs would be con-
sistent with efforts to promote sustainable
practice in animal production. Preliminary
indications are that the legume, sulla
(Hedysarum coronarium) may confer
beneficial effects in terms of performance
of lambs infested with the nematode
Trichostrongylus colubriformis, whereas
lucerne was inferior in this respect. The
effect is attributed to a direct action of CTs
or some other component in sulla on the
establishment or persistence of the parasite.
However, it is conceded that the CTs in
sulla may also have increased the post-
ruminal supply of protein through protec-
tion in the rumen, and that this effect may
have contributed to the improved perform-
ance of parasitized lambs.
A further benefit of plant secondary
compounds may arise through an anti-
protozoal action. It is maintained that more
microbial and dietary protein becomes
available to the ruminant when protozoa
are excluded from the rumen. A number of
tropical forages are endowed with defauna-
tion properties, possibly arising from the
secondary compounds they contribute.
Although Enterolobium species are parti-
cularly effective, a number of Acacia
species also show potential in this respect
(Table 18.6).Haemolytic anaemia
The adverse effects of SMCO in ruminants
arises after its fermentation by rumen
bacteria to dimethyl disulphide. The latter
is a highly reactive metabolite which is
believed to inactivate key proteins through
blockage of sulphydryl groups. A severe
haemolytic anaemia develops within 1–3
weeks in animals fed mainly or solely on
Brassica forage. Early signs include loss of
appetite, reduced milk production, the
appearance of refractile, stainable granules
(Heinz–Ehrlich bodies) within the erythro-
cytes and a decline in blood haemoglobin
levels. Organ damage generally accom-
panies this condition. Spontaneous but
incomplete recovery is observed in
survivors continuing to graze the forage, but
further fluctuations in blood haemoglobin
concentrations may ensue. If the forage is
withdrawn, blood chemistry is restored to
normal within 3–4 weeks.Hepatotoxicity
There is ample evidence that liver damage
in farm animals may arise from intake of
several ANFs and mycotoxins. Thus,
haemolytic anaemia in sheep fed Brassica
forage is attended by extensive liver
damage in which the organ becomes
swollen, pale and necrotic. Many hepato-
genous photosensitization disorders are
linked with the consumption of plants
containing steroidal saponins (Flaoyen and
Froslie, 1997). Plants such as Narthecium
ossifragum, Brachiaria decumbens and
various Panicum species have been
implicated in these disorders. A common
observation is the accretion of birefringent
crystals in hepatocytes and in association
with the bile duct. The crystals are394 J.P.F. D’Mello