Iodine (I)
The element where the herbage number tells you least: what matters is whether a pregnant dam is eating brassica, because the goitrogens in it block iodine uptake regardless of how much is there.
Often searched as goitre, goitrogens, brassica goitre.
What the pasture test should show
Indicative only, not diagnostic — herbage iodine is a weak status indicator (sheep at risk ~<0.5, cattle ~0.3–0.5 mg/kg DM). The real risk is goitrogenic brassica in pregnant stock — see the diet’s goitre watch.
Source: Indicative herbage figures only, not diagnostic — NZ authorities decline a confident pasture-iodine requirement because the brassica goitrogen interaction dominates it (Grace & Knowles 2012; B+LNZ Fact Sheet 95).
What it changes in the tools
- Goitre watch
- A pregnant dam on goitrogenic brassica (kale, rape, swede, turnip) is the real read — the glucosinolates hydrolyse to thiocyanates that block thyroid iodine uptake. The feed tool raises it from the diet, because a herbage iodine figure will not tell you.
- Source: Judson & Barrell (2011), Proc. NZSAP 71:66–70; B+LNZ Fact Sheet 95.
Read it for your farm
Build the diet and it works the mineral balance out for you — including the indices below.
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.