The effect of a temporal change in ingestion rates of fluorine (F) in soil on the concentration of F in serum and bone of young sheep

AIM: To determine the changes in concentrations of fluorine (F) in serum and bone of young sheep given a high intake of F, as F in soil, followed by a low intake of F, to simulate the varying intakes experienced by grazing sheep. METHODS: Sixty autumn-born, weaned, mixed-sex lambs were randomly divided into two groups (n=30). One group (Control) was fed a low-F (12 mg/kg dry matter; DM) lucerne/ grain diet for 345 days, while the other group (High soil-F) was fed a high-F (224 mg/kg DM) lucerne/grain diet containing 10% soil for 94 days and then a low-F diet for a further 251 days. Blood samples were collected on Days 1, 32, 94, 108, 153, 214, 280 and 345, to determine concentrations of F in serum. Groups of five or six animals from each group were killed on Days 1, 94, 153, 214, 280 and 345, to collect radii and humeri for determination of F content. The sheep were weighed at about 6-8-weekly intervals. RESULTS: Initial liveweight of the sheep was 22.1 (standard error (SE) 1.12) kg and the mean liveweight gains were 270 (SE 22.1) and 170 (SE 12.4) g/day from Days 1 to 94 and 95 to 330, respectively. The mean initial (Day 1) concentration of F in serum was 0.03 (SE 0.005) mg/L, and this changed very little for animals on the low-F diet. In sheep on the high soil-F diet, the concentration of F in serum reached 0.38 (SE 0.021) mg/L at Day 94 but after being placed on the low-F diet concentrations decreased to 0.04 (SE 0.006) mg/L after 14 days (Day 108) and then remained at about this low level for the remainder of the duration of the study. The mean concentrations of F in the humerus and radius of the lambs at Day 1 were 161 (SE 19.4) and 159 (SE 20.4) mg/kg DM, respectively, which increased to 2,784 (SE 80.6) and 1,805 (SE 51.5) mg/kg DM, respectively, at Day 94 in lambs fed the high soil-F diet, then decreased to 1,075 (SE 58.1) and 1,064 (SE 61.6) mg/kg DM at Day 153, and then progressively increased to 1,669 (SE 57.7) and 1,312 (SE 30.8) mg/kg DM at Day 345. CONCLUSIONS: The ingestion of F in soil by sheep markedly increased their concentrations of F in serum and bone, but when the soil was removed and they were fed a low-F diet, concentrations of F in serum decreased to baseline levels within 14 days while concentrations in the bone only decreased to 60-70% of the peak concentration, indicating that some of the F in bone was remobilised.