Pregnant ewes wintering on brassica crops such as rape or turnips are likely to have much higher iodine requirements than those grazing pasture due to the presence of goitrogens in the crop.
Veterinarian Dr Ginny Dodunski says ewes grazing brassicas, and that have not been treated with iodine, are at increased risk of giving birth to lambs with hypothyroidism and an enlarged thyroid gland (goitre).
Iodine is a constituent of thyroid hormones which control basal metabolism, heat production and the growth of tissues such brain, central nervous system and lungs.
This means that lambs born with iodine deficiency tend to have low birthweights and have less control over their body temperature. They are therefore at much greater risk of dying, particularly in cold weather.
To ensure at-risk ewes have adequate iodine going into lambing, Dr Dodunski says they should be drenched at eight weeks and four weeks before lambing with 200mg of potassium iodide or iodate.
If farmers suspect hypothyroidism could be causing losses at lambing, they are encouraged to get help to weigh and post-mortem 10-20 newly born dead lambs and remove and weigh the thyroid gland.
‘’It’s tiny, so you need to know what you’re looking for, and remove it carefully – a skinning knife is not the right tool!’’
The weight of the thyroid should then be calculated in relation to the lamb’s bodyweight (g/kgLW).
Lambs with a ratio of >0.8g/kg confirms iodine deficiency. Lambs with a ratio of <0.4g/kg indicates that iodine deficiency is not a problem.
For example, a 6kg lamb with a 5gm thyroid is a ratio of 0.83. This suggests an iodine deficiency.
Find out more
See our factsheet on trace element nutrition in sheep (PDF, 433KB) or contact a vet or animal health professional.