Abstract

Fluoride (F) is an essential element for animal growth, not readily taken up by plants from soils, yet cases of acute fluorosis in grazing animals caused by ingestion of phosphatic fertilisers, volcanic ash, and industrial wastes remind us of its potential hazard. Fluoride concentrations in topsoils slowly increase where annual inputs through atmospheric pollution and phosphatic fertilisers exceed losses. This paper reviews information on the fate of F in grazed pasture systems with the aim of assessing the potential toxicity of accumulating soil F. A preliminary F-cycling model for grazed pastures, based on the review of international literature and F concentrations in selected New Zealand pasture soils, indicated that grazing sheep and cattle obtain over 50% of their dietary F (and this may be >80% during winter) from soil ingestion. The model suggests that at the extremes of the ranges of the measured winter soil ingestion (143–300 g d^-1 for sheep and 900–1600 g d^-1 for cattle) and dietary F absorptivity (bioavailability) of soil F (20–38%), total topsoil F concentrations in the range of 372–1461 µg F g-¹ could cause chronic fluorosis in sheep and 326–1085 µg F g^-1 in cattle. We recommend that research is undertaken to measure F accumulation rates and soil F dietary absorptivity for a range of intensively managed New Zealand pasture soils.

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