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March 2015
The Importance of Micro-Minerals: Molybdenum
By David Donaldson, B.Agr, N.Sch, P.A.S.
olybdenum is required in small amounts in the diets of Figure 1.
farm animals.
No discussion on the requirements of Molybdenum
Trace Mineral Interactions
(Mo) can be considered without also considering the Copper
requirements of the animal. Early interest in Molybdenum
(Mo) was less concerned with the daily requirement for Mo
than the effects of high concentrations, which is known to
reduce Copper absorption, see Figure 1.
Soils vary widely in their Mo content and the subsequent
uptake by forage crops. Maize (Corn) silage tends to have the
lowest concentrations, grass and alfalfa are intermediate and
pasture is the most variable. Concentrations of over 60ppm
(parts per million) have been measured in pasture, see Figure
2.
Soil concentrations of Mo can range from 0.1ppm to over
20ppm. Sandy soils tend to hold the least. Typically around
10% of the Mo is available to the plant, but this amount rises
as the pH of the soil increases. Poorly drained soils, peat and organic rich soils, alkaline soils and liming all
increase Mo uptake and reduce Sulfate uptake. Agronomists are often aware of soils that are low in Mo, as crop
yields will respond to additional Mo in these situations.
Plants and grains typically range from 0.5 to 2.5ppm
Figure 2.
Mo in the DM, so Mo concentrations are almost always
Mineral Concentration in Soil Based on pH
sufficient to meet the requirements of the animal. At
concentrations above 2ppm DM, Mo can form an insoluble
complex with Sulfur in the rumen. This complex binds
with Copper, thereby reducing Copper availability.
It is rarely necessary to supplement Mo in diets, but
in ruminants, on high fibre diets with high Copper:Mo
ratio’s (greater than 4:1), most of the Mo can be excreted
in the manure. The only element known to tie up Mo is
Tungsten (chemical symbol W). Tungsten concentrations
in soil are typically very low, but is known to accumulate
around surface mining sites.
Cattle are the least tolerant of farm livestock to high
concentrations of Mo, closely followed by sheep. Pigs
and poultry can tolerate levels more than 10 times higher
than cattle. When cattle
M
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are placed on High Mo pastures (over 20 ppm), they
quickly develop severe scours.
Mo is required in a few enzyme systems,
principally Xanthine oxidase (XO), Sulfide oxidase
(SO) and Aldehyde oxidase (AO). The first indication
of an essential role for Mo came from the discovery
that the enzyme XO contained Molybdenum and that
it’s activity depended on the presence of Mo (de Renzo
et al,1953; Richert and Westerfield, 1953).
These enzymes are found in Milk and many
tissues (Mills and Davis, 1987). Mo, as a co-factor
in these enzymes, aids in catalyzing the oxidation or
metabolism of sulfur containing amino acids, purines,
pyrimidines and aldehydes. In poultry, Mo is involved
in the formation of Uric acid, which is their method of
excreting excess protein.
Absorbed Mo is stored in tissues as molybdopterin,
bound to XDH and AO in the cytosol and to SO in
mitochondrial membranes.
Non-ruminant animals and milk fed calves
metabolize Mo quite differently than ruminants.
In single stomached animals, Mo is absorbed very
efficiently across the stomach (van Campen and
Mitchell 1965) and abomasum wall (Miller et al 1972).
Absorption is via an active system (enery dependant),
shared with and inhibited by Sulfate. Since very little
Sulfur leaves the rumen as Sulfate, this antagonism is
only a feature of non-ruminants.
Excess Mo is excreted in the urine. In the renal
tubule sulfate prevents resorbtion of Mo and increases
Mo excretion (Bishara and Bray, 1978). Both nonruminants animals and ruminants demonstrate this
same antagonism in the kidneys.
Deficiency
Very rare in all animal species and is likely as a result
of high Copper and Sulphur. Symptoms include:
• Reduced feed intake and growth
• Impaired reproduction.
• Poor mandible ,leg formation and vigour in Chicks.
Toxicity
NRC quotes a maximum of 10ppm in the dry matter
for cattle, but even at much lower levels, Copper
availability will be affected. Pigs and poultry can tolerate
much higher concentrations. Toxicosis symptoms are
essentially the same as Copper deficiency, namely;
• Anaemia.,
• Fragile bones,
• Poor growth and reproductive efficiency,
• Reduced immune function.
Requirements
The precise requirement for Mo is hard to define
because of the relationship to the Copper and Sulphur
content of the diet. A dietary dry matter concentration
of 0.5ppm is sufficient for all species except when
Sulphur and Copper levels are higher than normal. AK
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