Grain zinc (Zn) content is important for seedling vigour. Make sure grain Zn levels are adequate if you are saving grain to seed next season’s crop. Retaining seed grown on Zn sufficient paddocks can reduce the risk of Zn deficiency in young crops.
Less than 500 micrograms Zn per seed can limit wheat seedling growth. In a 32 mg seed this is a concentration of around 16 mg/kg. Larger seed could have a lower concentration but still supply the 500 micrograms of Zn. On the other hand, smaller seed may have less Zn available to the developing seedling.
Typical grain Zn concentrations for Australian cropping regions are around 20 mg/kg for wheat. Lower grain Zn levels are most often seen in alkaline, calcareous soils. Grain Zn content can be measured through tissue testing at commercial labs.
Foliar fertilisers are useful to improve grain Zn content and address immediate Zn deficiency. Early applications, up until the start of stem elongation, have the best chance of giving a yield response. Zinc chelate and zinc sulfate are equally effective when applied from mid-tillering. Foliar zinc is best applied when there are new flushes of growth where there’s a large leaf area to take up the applied zinc. Late applications, around flowering, may benefit grain Zn content, but won’t boost grain yield.
Zinc in seed dressings
Zinc dressings applied to seed will help the young crop, but there will be little or no carryover for the next crop. The Zn supplied in the seed dressing may not be sufficient to boost Zn levels in grain.
Soil applied zinc
Applying zinc fertiliser to the soil at sowing will support seedlings and improve grain Zn levels. Fertiliser can also improve the paddock’s zinc status, and benefit future crops. Soil applied Zn is most appropriate in paddocks with a long term deficiency. Apply up to 2kg Zn/ha for a severe deficiency, and use tissue tests to gauge if Zn is still limiting.
Zinc and food quality
There is no price premium for growers producing Zn rich grain. However, dietary zinc is a problem for people on grain-based diets with little seafood or vegetables. UNICEF estimates that more than 116,000 children die each year due to zinc deficiency. The key strategy for maintaining Zn levels in the human diet is to apply adequate fertiliser zinc to grain crops. The Harvest Plus target for Zn levels in wheat grain is least 33 mg/kg.
The average Zn levels in wheat grain from all Australian regions are <33 mg/kg. In the Lower Eyre Peninsula and the Victorian and South Australia Mallee, average levels are around half the Harvest Plus target. Australian values are at the lower limits of grain from other countries, such as Canada (48 mg/kg), Russia (27 mg/kg) and India (48 mg/kg).
Photo courtesy of Ross Brennan, DAFWA.