Zinc-enriched and zinc-biofortified feed as a possible animal remedy in pastoral agriculture: Animal health and environmental benefits

Zinc is used in New Zealand agriculture to protect livestock against fungal infection (facial eczema), and supplementation can improve growth performance and feed efficiency. Zinc is commonly administered via a drench or bolus, but most is excreted in faeces. Mass balance calculations indicate that excreta is increasing the average Zn concentration in some NZ pastoral soils at a rate of 0.78 mg/kg/ha/yr. This represents an unsustainable level of Zn increase that threatens environmental quality guidelines. To determine whether current supplements could be replaced by naturally Zn-enriched or biofortified fodder, 20 sheep were administered one of four Zn treatments over seven days (drench, control hay, biofortified (Zn) hay and willow with a naturally elevated concentration of Zn; Salix purpurea, clone Pohangina). Blood and faeces were sampled regularly and analysed for Zn. Drench rapidly increased blood serum and whole blood Zn concentrations to above the established threshold for protection. The plant treatments also increased blood Zn relative to the control, but there was insufficient Zn in the fodder to reach the threshold level. The concentration of Zn in faeces was significantly increased by drench and willow. For animals fed willow, the faeces Zn concentration remained elevated for longer than for those that were drenched. This was despite the animals on the willow treatment receiving only 11% of the Zn given to the drenched animals. We propose that willow, grown on soil elevated in Zn to yield a higher concentration of Zn in fodder, may represent a biogeochemical system for Zn supplementation where the trace element is cycled from soil, to plant, to animal, and back to soil. Such a system could reduce the flux of trace elements entering the pastoral environment from animal remedies, mitigate future environmental risk that may become apparent due to current farming practices, and potentially lead to increased livestock production.

► We compare the efficacy of drench and high-Zn fodder to administer Zn to sheep.
► Zinc via drench causes a rapid but short-lived increase in serum concentration.
► This Zn is excreted and builds up in soil, threatening environmental guideline values.
► Zinc via willow increases in serum and faeces more slowly, at lower concentration.
► Biofortification may promote trace element cycling in the soil–plant–animal system.