Agriculture Reference
In-Depth Information
produce microbial protein. However, when supply of
energy is low, microbial use of ammonia for protein syn-
thesis will be reduced. When ammonia accumulates in the
rumen, this increases ruminal pH. An elevated pH increases
the amount of ammonia in the un-ionized, absorbable form
exacerbating ammonia absorption. The liver can detoxify
ammonia by converting it to urea, but some ammonia may
bypass the liver through uptake into the lymph system and
enter the blood directly. Ammonia toxicity is associated
with systemic alkalosis that, probably through altering
calcium and magnesium status, intoxicates the central
nervous system. Preventive measures include avoiding
high urea diets or feeding readily fermented carbohydrates
when feeding urea or when animals graze lush, rapidly
growing, high protein pastures. Ruminal bacteria adapted
to urea form ammonia less rapidly, so adaptation is recom-
mended. Finally, chemical inhibitors of urease or slow
release urea complexes can slow the rate of ammonia pro-
duction and will help avoid toxicity. Treatments include
administering VFA (vinegar) to lower rumen pH and
reduce the rate of absorption of ammonia. In severe cases,
rumen evacuation may be necessary.
Organic Nitro Compounds
Glycoside compounds found in various
Astragalus
species
such as crown vetch (Coronilla varia L.) and timber
milkvetch (Astragalus miser varia T.) are rapidly hydro-
lyzed in the rumen producing toxic nitro compounds,
that is, 3-nitropropionic acid (NPA) and 3-nitroproponal
(NPOH). These compounds can be metabolized in the
rumen or absorbed. NPA is toxic while NPOH is converted
to NPA by the liver. Rumen microbes may partially metab-
olize these compounds by reduction of aliphatic nitro
groups to their corresponding amines, 3-aminopropanol,
and
-alanine, respectively. This explains why ruminants
have greater tolerance to these compounds than nonrumi-
nants do.
β
Mimosine Toxicity
Mimosine is an alkaloid,
- 3 - hydroxy - 4 pyridone amino
acid, found in genus Leucaena and few other Mimosa
species. Leucaena is a legume shrub/tree that provides a
high quality feed in certain areas of the tropical and sub-
tropical regions of the world. However, when these feeds
comprise more than 30% of the diet, they may cause severe
toxicity. Goats may lose hair when fed more than 50%
leucaena in the diet. Rumen microbes convert mimosine
to 3,4 dihydroxy pyridone (3,4 DHP), a toxic goitrogenic
intermediate. However, certain strains of bacteria found in
tropical ruminants can degrade mimosine to nontoxic prod-
ucts. Toxicosis has been reported in Australia, Papua New
Guinea, Africa, and Florida. Pure colonies of bacteria that
can degrade 3,4 DHP were isolated (
Synergestes jonesii
)
and have been inoculated successfully into the rumen,
increasing tolerance to mimosine (Hammond, 1995).
β
Nitrite - Nitrate Toxicity
Nitrite/nitrate toxicity occurs when ruminants consume
forage from stressed, often overfertilized grass pastures
and crops. Such plants accumulate nitrate as its potassium
salt. Accumulation by plants is enhanced by drought and,
with cool-season grasses, by low temperature and cloudi-
ness (low light). Nitrate itself is not toxic, but during
normal rumen fermentation, nitrate is reduced to nitrite (a
toxic intermediate). Nitrite can be converted further into
ammonia that in turn can be used to produce microbial
protein. If the nitrite intermediate accumulates in the
rumen, it is absorbed into the blood and there it unites with
hemoglobin and produces methemoglobin. Compared
to hemoglobin, methemoglobin has a reduced ability to
transport oxygen, and impaired oxygen delivery to tissues
will cause death by asphyxiation. Gradually increasing
the nitrate concentration of the diet will permit microbes
or animals to adapt to nitrate and increase their tolerance
to it.
Tannins
Condensed tannins (CT) are polyphenolic compounds
present in plants that when eaten may have either positive
or negative effects on animals depending on the concentra-
tion in the forage and ability of the rumen environment to
degrade tannins. Proline rich proteins present in the saliva
of certain herbivores including goats bind tannins and
reduce their adverse effect on rumen microorganisms.
However, the complexes formed are indigestible and are
excreted in the feces (Hagerman et al., 1992). Condensed
tannins at low levels, 2-5% of dry matter (DM), will
bind with ruminally degraded proteins to increase the
supply of protein reaching the SI and, when digested,
these proteins can improve the amino acid balance of
animals. However, higher levels of CT (above 5% of DM)
generally have negative effects on ruminal digestion and
decrease digestibility. Tannins have shown promise as an
SECONDARY PLANT METABOLITES
Certain compounds found in feeds, forbs, and browse are
produced as defenses against invasion by pathogens
and, in some cases, against consumption by herbivores.
Compared with cattle and sheep, goats have a very high
tolerance to such compounds.
