Molybdenum News Center

Etiology of Molybdenum Poisoning

The metabolism of copper, molybdenum, and inorganic sulfate is a complex and misunderstood interrelationship. It appears that the ruminal interaction of molybdates and sulfides gives rise to thiomolybdates (mono-, di-, tri-, and tetrathiomolybdates). Copper reacts with thiomolybdates (primarily tri-and tetrathiomolybdates) in the rumen to form an insoluble complex that is poorly absorbed. In addition, systemic thiomolybdates chelate copper, making it not bioavailable and causing a copper deficiency. On this basis, tetrathiomolybdate is used in treating and preventing copper toxicity (see Copper Poisoning) in sheep. Molybdenum also appears to be directly toxic to tissue reductive enzymes, which may explain the poor response to copper therapy in severely affected animals. The lack of bacterial formation of thiomolybdates in monogastric animals partially explains the relative tolerance of these animals to molybdenum toxicity.

The susceptibility of ruminants to molybdenum toxicity depends on a number of factors: 1) copper content of the diet and intake of the animal—tolerance to molybdenum toxicity decreases as the content and intake of copper decrease; 2) the inorganic sulfate content of the diet—high dietary sulfate with low copper exacerbates the condition, while low dietary sulfate causes high blood molybdenum levels due to decreased excretion; 3) chemical form of the molybdenum—water-soluble molybdenum in growing herbage is most toxic, while curing herbage decreases toxicity; 4) presence of certain sulfur-containing amino acids; 5) species of animal—cattle are less tolerant than sheep; 6) age—young animals are more susceptible; 7) season of year—plants concentrate molybdenum beginning in spring (maximum level reached in fall); and 8) botanic composition of the pasture—legumes take up more of the element than other plant species.

Molybdenum toxicity associated with copper deficiency has been seen in areas with peat or muck soils, where plants grow in alkaline sloughs (eg, western USA), as a result of industrial contamination (mining and metal alloy production), where excess molybdenum-containing fertilizer has been applied, and where applications of lime appeared to increase plant molybdenum uptake.

In the diet of cattle, copper:molybdenum ratios of 6:1 are considered ideal; 2:1–3:1, borderline; and <2:1, toxic. Dietary molybdenum of >10 mg/kg can cause toxicity regardless of copper intake; as little as 1 mg/kg may be hazardous if copper content is <5 mg/kg (dry-weight basis). Mixing errors may occur; concentrations above 1,000 mg/kg (as sodium molybdate) cause growth retardation while concentrations of 2,000–4,000 mg/kg cause death within 40 days.

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