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.

PD, Anderson, B Dalir-Naghadeh, and TJ Parkinson

Proceedings of the New Zealand Society of Animal Production, Volume 67, Wanaka, 248-254, 2007
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