The effect of age on the fluoride concentration in the metacarpus of grazing sheep in New Zealand

AIM: To determine the concentration of fluoride (F) in the metacarpi from ewes of varying age, which had been grazing pastures that had been regularly topdressed with phosphatic fertilisers, which contain F as a contaminant, for at least 30 years. METHODS: Three groups of 10 ewes aged 6-8, 18-20 and 60-72 months were selected from seven research farms with a known fertiliser history, representing the major sheep farming regions of New Zealand. Lambs were born and remained on the property. The sheep were collected and slaughtered in autumn, and the right metacarpus removed for determination of F content. Representative samples of topsoil (0-30- and 30-60-mm depths) were also collected for determination of F content. RESULTS: The total concentration of F in topsoil on the farms ranged from 161 to 712 mg/kg. The overall mean concentrations of F in metacarpi from 6-8-, 18-20- and 60-72-monthold ewes were 170 (SE 9.9), 308 (SE 13.9) and 480 (SE 28.4) mg/kg dry matter (DM), respectively. On six farms, lambs which had been exposed to limited or no ingestion of soil had the lowest concentration of F in their metacarpi. Generally, there was a curvilinear relationship between age and concentration of F in the metacarpi of sheep. On the seventh farm, the concentrations of F in the metacarpi from the 6-8- and 18-20-month-old sheep were similar but lower than those of the 60-72-month-old ewes. The area of soils on the farm where the 6-8-month-old-sheep were grazing had higher total concentrations of F in topsoil (271 mg/kg) than those soils grazed by the 18-20-month-old sheep (205 mg/kg), and this may be one reason for the elevated concentration of F in the metacarpi of the younger ewes. The concentrations of F in metacarpi, at any age, were not related to the concentration of F in topsoils. CONCLUSIONS: Many factors can influence the amounts of F in soil ingested by grazing sheep and the concentration of F in their bone. The concentration of F in the metacarpi of ewes generally increased curvilinearly with age, and the rate of accumulation of F in bone was greatest in young sheep. The potential risk of chronic fluorosis occurring in sheep grazing pastures in New Zealand is low for animals managed in a manner similar to the flocks studied, given the highest mean concentration of F was 601 mg/kg DM, observed in metacarpi from 60-72-month-old ewes, and that a concentration of at least 2,400 mg/kg DM is the threshold for chronic fluorosis.