Molybdenum

Functions
The regular occurence of molybdenum in very low concentration, in a wide
range of animal tissues has raised its interest in the past four decades.
This interest in the biological significance of molybdenum received greater
attention from the discovery of its role as a cofactor in many enzymes
(oxydase, reductase) and from the demonstration that their activity is highly
dependant upon its presence.
Since then, it has been well established that this trace element is essential for
mammals, in particular ruminants, because it favours the rumenal fixation of sulfur,
its reduction in sulfide followed by its subsequent oxydation into sulfate.
Also well known is the antagonism between copper and molybdenum.
It finds its origin in the inhibition, by the thiomolybdanes, of the ‘copperdependant’
enzymes which catalyse the intestinal and hepatic absorption of copper.
The presence of dietary inorganic sulfate together with molybdenum may also
exert an adverse effect on copper retention.
Consequently, to prevent any direct or indirect molybdenum toxicity,
attention has to be paid to the recommended Mo supplementation depending
on the animal species. Only very small concentrations are required.

Molybdenum sources
Two sources of excellent bio-availability are used in animal nutrition :
- Sodium molybdate dihydrate - Na2MoO4.2H2O -, by far the form under
which molybdenum is supplemented.
- Ammonium molybdate tetrahydrate - (NH4)6Mo7O24.4H2O - is much less used,
for suppliability and cost reasons.
Contrarily to other trace elements, the authorized level of Mo supplementation
is very low and fixed at maximum 2.5 mg/kg of the complete feedingstuff by E.U. directive.

Quality parameters
Biological availability
The water soluble properties of the sodic and ammonic salts ensure an
excellent bio-availability of the supplied molybdenum source.

Purity (undesirable elements)
As for the other trace elements, there is no directive regarding the
undesirable substances edicted for their use in animal premixes or
feed minerals, but one can recommend the following :

	both Mo salts
	(mg/kg)
Pb	< 50 ppm
As	< 30 ppm
Cd	< 10 ppm
Hg	< 5 ppm

General criteria for selection
- total guaranteed Mo assay
- few SiO2 / few heavy metals
- perfect solubility