Clinical signs of iodine deficiency (e.g. non-visible oestrus, low first service conception rate, stillbirths), plus low (<45 nmol/L) serum thyroxine (T4) concentrations were observed in dairy cows from 12 farms in the Manawatu. Goitre, as determined by diffuse thyroid hyperplasia,plus significantly (p<0.05) increased thyroid size, weight and thyroid:bodyweight ratio, was found in 16% (6 of 38) of stillborn calves from these farms. Goitre was present in calves from 42% (5 of 12) of the herds. One of these farms was chosen to investigate temporal changes in concentrations of the thyroid hormones, thyroxine (T4) and tri-iodothyronine (T3), as an indicator of iodine status, and to attempt to determine the optimum time for sampling. Blood samples were collected from a cohort of animals from the herd (20 representative animals from 400 cows) 8 times over a 12 month period, for measurement of total serum iodine (TSI), T3 and T4 concentrations in both individual cows and pooled (10 cows) samples. T4 was maximal in June (58.6 nmol/L) and lowest (39.2 nmol/L) in October (P<0.05). Between November and Augusts meanTSI varied (NS) from 31 ± 9 to 47 ± 9 µg/L, but was significantly (P<0.01) higher in October (91 ± 16 µg/L). T4 concentrations were(P<0.05) related (r = 0.35) to TSI between November and August (7/8 blood samplings), but were widely divergent (r = 0.08) in October. Concentrations of T3 were unrelated to TSI (r= 0.07). In pooled samples, concentrations of TSI, but not T3 or T4, were closely related to the mean concentrations in individual animals. An iodine-response study was also carried out on the same herd. Half the herd (200 of 400) were given (6 mL i/m) iodised oil (Flexidine, Bomac) before mating and before calving. The pregnancy rate in anoestrous cows treated with iodine was 13.9% higher than in untreated cows. It was concluded that T4 (individual animals) or TSI (pools of 10 cows) give equivalent information about the iodine status of a herd, except in the post calving period. However, the reference range needs to be validated by further response trials.
Proceedings of the New Zealand Society of Animal Production, Volume 67, Wanaka, 248-254, 2007
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