Zinc status of grazing sheep: seasonal changes in zinc concentrations in plasma, wool and pasture

Abstract

Zinc status of grazing sheep was surveyed for three years on six properties in the south-west of Western Australia. The concentration of zinc in the plasma of rams and ewes varied both among sheep and among properties, and was up to 21% lower in autumn than in spring. The zinc status of ewes, but not rams, remained low in winter, as reflected by both plasma and wool levels. Pasture zinc levels were lowest in autumn and highest in winter, and regular recordings below 20 �g g-1 were made. The low zinc status of sheep in autumn was accompanied by low pasture zinc content and a low pasture availability. However, the data suggest that the decrease in maternal zinc levels during winter are due to reproductive stress. Plasma zinc levels regularly fell below the presently suggested optimum, and seasonal variation in both plasma and wool zinc levels indicates that in autumn and possibly winter there is an inadequate uptake of zinc. This is indirect evidence for a widespread marginal deficiency of zinc in grazing sheep.