Molybdenum is an ultra-trace mineral that serves as a cofactor for several enzymes involved in sulfite detoxification and purine metabolism. For horse owners, molybdenum is more relevant as a copper antagonist than as a nutrient to supplement. High soil molybdenum levels (common in certain regions, particularly parts of the UK and low-lying wet pastures) interfere with copper absorption, potentially causing secondary copper deficiency. This interaction is well-documented in cattle and applies to horses as well, though horses are somewhat less sensitive.
There is no established NRC requirement for molybdenum in horses — the amounts needed are minuscule and universally present in feed. The practical concern is excess rather than deficiency. If your horse grazes on wet, alkaline, or peat soils, consider having your pasture or hay tested for molybdenum. If levels are high, increase copper supplementation to maintain the copper-to-molybdenum ratio above 4:1.
True molybdenum deficiency has not been documented in horses under normal feeding conditions. The amounts needed are so small that virtually any diet provides enough.
Excess molybdenum from grazing on high-molybdenum pastures can cause secondary copper deficiency, leading to coat color fading (especially in black and dark bay horses), rough coat, poor hoof quality, anemia, and poor growth in young horses. Very high molybdenum can also cause diarrhea.
High dietary molybdenum interferes with copper absorption by forming insoluble copper-molybdenum-sulfur complexes (thiomolybdates) in the gut. This can cause secondary copper deficiency even when dietary copper intake appears adequate. Horses on pastures with high-molybdenum soils are at particular risk.
What this means: If your pastures are on wet, alkaline, or peat soils known for elevated molybdenum, increase copper supplementation to maintain a copper-to-molybdenum ratio above 4:1. Watch for coat color fading in dark horses as an early sign of copper insufficiency.
