Horses, like most monogastric animals, are very resistant to the effects of Mo as compared to ruminants. Cattle are commonly cited as slightly more susceptible to molybdenosis than sheep, and limited quantitative data suggest mule deer are relatively resistant compared to cattle. Therefore, drinking water Mo concentrations that are safe for cattle are probably also safe for horses, other classes of livestock, and wild cervids. Although there is quite a bit of variability in the reports summarized above, and some (large) amount of dietary Mo may cause poisoning regardless of Cu status, the bottom line seems to be that total dietary Cu:Mo ratios of less than 2-4 can result in chronic toxicity and decreased production in cattle, especially if dietary S is higher than absolutely necessary.
As with many substances, the effects of forage and water Mo concentrations are additive, and, in some areas of the western United States, forage Mo concentrations are already toxic or very nearly toxic. Under these conditions, any additional Mo intake contributed by drinking water is potentially dangerous. In these areas, however, producers are likely already aware of the problem and feeding supplemental Cu. A more normal situation would be cattle grazing "typical" Wyoming forage containing 7 ppm Cu (or supplemented to that level) and negligible Mo (~1 ppm). Under these conditions the critical safe 4:1 ratio would be exceeded whenever drinking water contains 375 μg Mo/L.
We recommend that, in the absence of other data, drinking water for livestock and wildlife contain less than 0.3 mg/L.
If dietary Mo is higher, which is not unusual in this region, water Mo concentrations should be adjusted downward accordingly.