Zinc (Zn)
New Zealand's zinc question is almost never a deficiency — it is the facial-eczema dose, and what a long dosing programme does to copper.
Often searched as facial eczema, zinc dosing, zinc oxide.
What the pasture test should show
Shown for reference — NZ pasture zinc is generally adequate and nutritional zinc deficiency is not documented in NZ (Grace & Knowles 2012). International guidance flags <20 mg/kg DM as marginal.
Source: The <20 mg/kg DM marginal line is White (1993), an INTERNATIONAL band offered as context only — NZ pasture sits above it and nutritional zinc deficiency is not documented here (Grace & Knowles 2012).
What it changes in the tools
- Facial-eczema prevention rate
- 20 mg of elemental zinc per kg liveweight per day, to hold protective blood serum levels. Zinc oxide is 80% zinc by formula, so the product rate is higher than the elemental one — the health tool does that arithmetic. It reconciles with DairyNZ's own label rates: 3 g zinc oxide per 100 kg liveweight works out at about 24 mg Zn/kg LW.
- Source: The 20 mg/kg LW/day rate is the widely-cited NZ figure (PGG Wrightson knowledge hub, after DairyNZ and Dairy Australia); DairyNZ's own Facial Eczema management guide publishes product label rates rather than this figure. The 80% is stoichiometry: Zn 65.38 ÷ ZnO 81.38 = 80.3%.
- Zinc is toxic in excess
- Start 2–3 weeks before spore counts rise, and do not exceed the preventive rate without veterinary advice. Sheep are especially sensitive to zinc.
- Source: DairyNZ / NZ veterinary guidance.
- Zinc ↔ copper
- Less clear-cut than it is often stated. In Romney sheep on ryegrass, drenching 12 g zinc oxide a week for a full 52 weeks did NOT significantly change liver copper — the drop showed up instead after a short intraruminal bolus (40 days), and the authors concluded the facial-eczema benefit outweighs a short-term reduction. Worth a copper check on high-molybdenum country where copper is already tight, but not a reason to skip eczema prevention.
- Source: Rounce, Knowles, Grace & Lee (1998), "Effect of zinc oxide treatment for facial eczema on the copper status of Romney sheep grazing ryegrass pastures", Proc. NZSAP 58:199–201.
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.