Molybdenum (Mo)
Not a deficiency to fix but an antagonist to watch: high pasture molybdenum ties up copper the animal has already eaten, so a copper figure that looks adequate stops being adequate.
Often searched as moly, copper lock-up, induced copper deficiency.
What the pasture test should show
Antagonises copper above
1 mg/kg DM
High
> 3 mg/kg DM
What it changes in the tools
- Herbage molybdenum
- Above 3 mg/kg DM is the published line that matters: there, molybdenum cuts the efficacy of oral copper supplements by 30–50%. Our tool opens a watch from 1 mg/kg DM, which is OUR conservative buffer and not a published cutoff — Morton (2024) found pasture molybdenum up to 7 mg/kg had no effect on lamb liveweight gain where pasture copper was 7–8 mg/kg or better. There is no lower limit; low molybdenum is not a problem.
- Source: Grace & Knowles (2012), Vet. Med. Int. 2012:639472 — ">3 mg Mo/kg DM can reduce the efficacy of oral Cu supplements by 30–50%", verbatim. The 1 mg/kg watch is ours. Morton (2024), J. NZ Grasslands 86:201–212.
- What high molybdenum does to a copper drench
- Above 3 mg/kg DM, molybdenum reduces the efficacy of oral copper supplements by 30–50% — so the supplement is itself only half working, which is why treating the copper number alone can fail.
- Source: Grace & Knowles (2012).
- Sulphur makes it worse
- Sulphur works with molybdenum to form insoluble copper thiomolybdates in the rumen, and the two together are worse than either alone — that mechanism is published. The 0.4% DM figure our tool uses is OURS, not from that paper, and it sits only just above the NZ median pasture sulphur of about 0.36% — so it will fire on a fair share of ordinary pasture.
- Source: Mechanism: Grace & Knowles (2012), Vet. Med. Int. 2012:639472 (no threshold given). NZ median pasture S 3.6 g/kg DM: Knowles & Grace (2014), J. Anim. Sci. 92:303–310. The 0.4% line is ours.
Read it for your farm
Put your herbage test in and it reads the trace elements against these bands for your stock class.
These are published reference bands, not a diagnosis. Mineral deficiencies and toxicities are diagnosed from the animal — blood, liver or tissue — not from a pasture test alone, and the treatment rates are veterinary decisions. Talk to your vet before dosing stock.