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Toxicity to other above-ground organisms

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Description of key information

Four studies are considered reliable: Kincaid (1980), Kistner et al (2017) and two studies by Thorndyke et al (2021a,b).

Kincaid (1980) reported no effects at the highest tested concentration of 50 mg Mo/L. Kistner et al (2017) observed no differences for final bodyweight (P > 0.98) in steers that were exposed for 151 days to five different levels of Mo in drinking water for 151 days with exposure levels ranging between 0 (control) and 960 µg/L. ADG, DMI, feed efficiency, water intake, hot carcass weight, dressing percentage, yield grade and marbling score did not differ across treatments. These data indicate that water Mo concentration, within the range studied, had no impact on performance, mineral status, water intake, and carcass characteristics in feedlot steers.

Thorndyke et al (2021a) investigated the influence of molybdenum in drinking water or food on apparent absorption and retention of copper and molybdenum in twelve Angus steers and found that molybdenum intake was significantly greater in Mo-water and Mo-diet steers when compared to controls, but similar between Mo-water and Mo-diet steers. Apparent absorption and retention of Cu were significantly greater in controls when compared to Mo-diet supplemented steers. Apparent absorption and retention of Cu in steers in the Mo-water treatment, however, did not differ from controls or those receiving the Mo-diet. These data indicated that Mo consumed in water could impact Cu absorption and retention to a lesser extent than Mo supplemented in the diet.

The same group of researchers also reported the outcome of an extensive research program that investigated the effect of Mo-exposure via drinking water or food on molybdenum metabolism and production performance of beef cattle consuming a high forage diet (Thorndyke et al, 2021b). Cattle that received diets containing less than the NASEM (2016; 10 mg Cu/kg DM) Cu requirement for beef cattle became Cu deficient over the course of the experiment, as determined by liver and plasma Cu concentrations. However, no Mo toxicity or Cu deficiency signs (e.g., reduced growth rate, reproductive performance, or immune function) were observed throughout the course of the experiment.

All other experimental studies in which the effect to ruminants of increased levels of molybdenum in feed were studied, were of insufficient quality as copper levels in the feed of the control treatments were below the required or recommended levels against primary deficiency. Several of the field studies suffered from the same shortcoming.


Nonetheless, molybdenosis is a well-recognised phenomenon that has been observed in the field, and that requires proper assessment. It has further been observed that the onset of molybdenosis is highly dependent of the dietary copper level, its bioavailability to the ruminant and the dietary Cu:Mo ratio. Derivation of one specific PNECoral for molybdenosis can therefore not be considered relevant. Instead, it is more relevant to use a dietary Cu:Mo ratio as the threshold.


Based on an in-depth review by Smolders and Buekers (2009) making use of a weight-of-evidence approach the proposed minimum dietary Cu:Mo ratio threshold is 1.30 (For the avoidance of doubt, this means there should be 30% more copper than molybdenum in the diet). This value was derived as the 90th percentile of on an extensive dataset on cattle composed of 12 experimental and 7 field studies. At lower ratios, there is a risk of molybdenum-induced copper deficiency.


This threshold is also shown to be protective for other ruminant species such as sheep and deer.

Key value for chemical safety assessment

Long-term EC10, LC10 or NOEC for mammals:
200 mg/kg food

Additional information

Toxicity to ruminants

Molybdenosis is found only in ruminants such as cattle, deer, sheep and is caused by molybdenum induced copper deficiency, but is not found in non-ruminants. Effects in cattle correspond with symptoms observed when dietary copper is deficient e.g. diarrhoea, alopecia (loss of hair), achromatrichia (change in hair colour), reduced milk yield and loss of body weight. This effect has been studied extensively, especially in cattle, and in, for example, the UK Somerset area and New Zealand where the soils and pastures are naturally enriched with molybdenum.


A detailed description of this phenomenon is given in the Background Document “Environmental effects assessment of molybdenum”, which is attached in the technical dossier in IUCLID Section 13, and provided more information on mode of action, required daily intakes of Cu for ruminants, the influence of dietary Cu:Mo ratios on molybdenosis for ruminants, and results of experimental field studies on ruminants with molybdenum. Information on the use of sodium molybdate as a remedy against copper toxicity is also included in this Background Document, together with an outline of risk management measures to prevent molybdenosis in ruminants living in the vicinity of a local industrial site emitting molybdenum.