Iodine-dependent hormones produced by the thyroid gland are essential for growth and development of the foetus, and crucial for the survival of the newborn lamb.
The foetus must produce his/her own thyroid hormones, and this is dependent on sufficient amounts of iodine derived from their mother’s daily intake being transported across the placenta.
Goitre (enlarged thyroid glands) is the most obvious clinical sign and is commonly seen in newborn lambs when ewes have been grazing on iodine deficient or goitrogenic fodder in the last half of pregnancy. These lambs may also be born with pink bare skin with little or no wool.
More importantly are the subclinical effects that iodine deficiency has on the newborn lamb. They may be born small and/or weak rendering them highly susceptible to starvation and exposure resulting in reduced lamb survival.
Brassica species like kale (which is also low in iodine) may contain high levels of goitrogens which block the uptake of iodine by the thyroid gland. Some clovers can also contain high levels of goitrogens. In comparison, swedes and turnips tend to bear little or no risk.
Poor lamb survival associated with iodine deficiency should also be considered in ewes that have been overwintered on pasture. Whilst the ingestion of soil in years of drought or when there are high stocking rates over the winter is said to reduce the incidence of goitre, there is considerable variation in incidence from one year to the next and therefore tends to not to be that predictable.
The most effective way of preventing iodine deficiency in ewes grazing goitrogenic crops or iodine deficient pasture is to supplement the ewes with iodine. The preferred options are to:
- Inject the ewes with an iodised oil
e.g. Depodine one month before mating or not less than two months before lambing. This product will provide a long-term depot in the muscle which is slowly released. It is recommended that ewes are treated at least two months prior to feeding goitrogenic winter crops.
- Drench the ewes with potassium iodide This is best done at eight weeks and then four weeks prior to the start of lambing to prevent goitre in lambs. However, treating ewes earlier in gestation (e.g. at scanning) may be more effective in terms of improved lamb survival than treating ewes nearer to lambing. This is because severe iodine deficiency in ewes can cause a reduction in foetal brain development and body weight from as early as 70 days into pregnancy.
Please feel free to contact your veterinarian for further information.