The extensive morphological lesions which
occur in the small intestine of sensitized
animals significantly impair digestive and
absorptive function by virtue of markedly
reduced surface area in the gut, through
abnormalities in the maturation of entero-
cytes and through reductions in the
secretion of key enzymes such as lactase.
For example, xylose absorption, an indicator
of absorptive competence, is substantially
higher in milk-fed than in soybean-fed
calves, with these differences diverging
with age. Similar effects are seen in piglets
with soybean hypersensitivity, except that
absorptive competence is restored rela-
tively early in the post-weaning period.
Direct measurements of nutrient uptake
confirm the abnormalities implied by the
xylose tests. For example, sensitized calves
show a decline in net nitrogen absorption
efficiency to 0.25 as compared with 0.57
for unsensitized and 0.85 for casein-fed
calves. Calves given a series of liquid feeds
containing heated soybean products
readily develop abnormalities in movement
of digesta leading to diarrhoea.
The digestive tract is the primary focal
point for the action of CTs. The ability of
animals to tolerate CTs has received much
attention following the findings that certain
animals are capable of secreting proline-
rich proteins (PRPs) in saliva, which may
constitute a first line of defence against
ingested CTs (see D’Mello, 1992). The open
structure of these proteins induces a high
affinity for CTs, and there are indications
that the complex so formed is resistant to
digestive attack. There are suggestions that
the goat produces PRPs in significant
amounts, whereas grazing ruminants lack
these salivary proteins. Domestic goats are
well recognized as mixed feeders, consum-
ing appreciable quantities of CT-rich
browse. If the synthesis of PRPs by goats is
confirmed, then this may account for the
higher digestibility of fibre in goats com-
pared with sheep when both are offered
leaves of Acacia pendula or Prosopis
cineraria, whereas no species differences
were observed when lucerne (Medicago
saliva) represented the main item of feed
(Kumar and D’Mello, 1995). The two
browse legumes, A. pendula and P. cineraria
are known to contain CTs.
Consistent with the above account,
cattle and sheep would be expected to be
sensitive to CTs. Adverse effects may be
seen in sheep when CTs such as those in
lotus (Lotus pedunculatus) or in browse
legumes such as Acacia species comprise a
significant part of their diets. The evidence
is now unequivocal (Table 18.5). Primary
manifestations include impaired rumen
function, resulting in depressed intake,
wool growth and liveweight gain. The
deleterious effects on rumen function have
been ascribed to complexing of CTs with
microbial extracellular enzymes. However,
an overall deficit of rumen-degradable N in
ruminants fed high-tannin forages and
browse may also occur. This undersupply
may reduce the digestibility of structural
carbohydrates.
A number of mycotoxins adversely
affect digestive morphology and rumen
function (Table 18.4). At 4 mg kg^1 diet,
DON may induce greater corrugations in
the fundic region of the stomach of the pig
and, at higher levels (19 mg kg^1 ), the
mycotoxin may cause erosions in the
gastric mucosa. T-2 toxin also induces
lesions in pigs, specifically on the mucosa
of the pars oesophageal region, the
incidence being dose-related. Chronic
exposure to aflatoxin affects ruminal func-
tion by decreasing cellulose fermentation,
volatile fatty acid synthesis, protein
degradation and motility.Effects on nutrient partitioning and utilization
DELETERIOUS EFFECTS. The deleterious effects
on nutrient utilization are best exemplified
by the action of the non-protein amino
acid, canavanine, present in the jack bean
(C. ensiformis). Canavanine is structurally
similar to arginine, an essential amino acid
for poultry. When jack beans form a sub-
stantial proportion of a broiler diet, the
resulting depression in growth of chicks
can be explained on the basis of a
canavanine–arginine antagonism. Such an
interaction involves increased degradation
of arginine and possibly synthesis of
aberrant tissue proteins resulting in392 J.P.F. D’Mello