A practical approach to managing the risks of iodine deficiency in flocks using thyroid - weight: birthweight ratios of lambs.
OBJECTIVE
To establish a protocol for determining the iodine status of a flock and managing the risks of iodine deficiency, using thyroid-weight:birthweight ratios of newborn lambs and serum iodine concentrations of ewes.


METHODS
Data were collected from iodine supplementation studies in Southland and Rangitikei, of sheep fed exclusively pasture or pasture plus brassica during the latter half of gestation (n=350, or 89 per treatment group). The ewes were supplemented pre-mating or at the time of pregnancy scanning with an injection of long-acting iodised oil. Serum iodine concentrations were measured in ewes prepartum in Rangitikei and postpartum at both locations. The thyroid-weight:birthweight ratios (as g/ kg) in 229 newborn lambs were determined at post-mortem examination and compared between iodine supplemented vs unsupplemented flocks using probit analysis. Samples of pasture and kale were analysed monthly for determination of iodine and selenium.


RESULTS
Initial mean serum iodine concentration of all ewes was 41 mug iodine/L. Supplementation increased serum iodine concentrations regardless of forage fed, and concentrations remained high for between 127 and 206 days. The range of thyroid-weight:birthweight ratios in lambs from supplemented ewes was 0.09-0.70 (mean 0.35, standard deviation (SD) 0.147) g/kg. Among lambs from unsupplemented ewes, the range was much wider (0.21-8.5; mean 1.61, SD 1.95 g/kg). About half of those ratios were >0.8 g/kg and clearly indicative of goitre, including 62% from the groups on brassica and 18% from the Rangitikei group fed exclusively pasture. Probit calculations showed that a ratio of 0.40 (95% confidence interval (CI)=0.29-0.47) g/kg predicted with 35% probability, and a ratio of 0.80 (95% CI=0.70-0.99) g/kg predicted with 90% probability that a lamb came from an unsupplemented flock.


CONCLUSIONS
Compared to iodine concentrations in forages, thyroid-weight:birthweight ratios more accurately reflected the iodine status of the flock and could be used to identify which flocks to supplement the following year. Serum iodine concentrations of ewes measured before or after lambing did not reflect forage fed, but values near 40 mug/L were associated with goitrous lambs.


CONCLUSIONS
Thyroid-weight:birthweight ratios >0.8 g/kg were indicative of iodine deficiency, and ewes should be supplemented pre-mating or during pregnancy to prevent goitre the following year. Ratios <0.4 g/kg rarely occurred among deficient flocks, so the probability of benefit from supplementation was low. Intermediate ratios were ambiguous, and the iodine status of the flock could not be determined from biomarkers. In such cases individual-farm supplementation trials might be required to detect and manage the risks of marginal deficiency.
Food, Metabolism and Microbiology Section, Food and Health Group, AgResearch Limited, Private Bag 11008, Palmerston North, New Zealand. scott.knowles@agresearch.co.nz
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D000821:Animal Feed; D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D000831:Animals, Newborn / Q000097:blood; D001724:Birth Weight / Q000187:drug effects* / Q000502:physiology; D001937:Brassica; D019587:Dietary Supplements; D005260:Female; D007455:Iodine / Q000008:administration & dosage / Q000097:blood / Q000172:deficiency; D009751:Nutritional Requirements; D009752:Nutritional Status; D009929:Organ Size / Q000187:drug effects* / Q000502:physiology; D006109:Poaceae; D011247:Pregnancy; D011897:Random Allocation; D012307:Risk Factors; D012643:Selenium / Q000097:blood; D012756:Sheep; D012757:Sheep Diseases / Q000175:diagnosis* / Q000453:epidemiology
D007455:Iodine; D012643:Selenium