IODINE deficiency can cause many problems for cattle and sheep but too much iodine can also lead to trouble - so investigation before supplementation is the watchword, writes Alwyn Jones, of SAC Consulting Veterinary Services, St Boswells.

A shortage of iodine can have a significant effect on livestock because iodine is a component of thyroid hormone which plays an important role in maintaining normal bodily functions. These include the production of proteins, immunity, appetite control, muscle function and adapting to hot or cold weather.

Thyroid hormone is also important for the development of calves and lambs and their transition from the womb to free living. This is why the most commonly reported signs associated with iodine deficiency in cattle and sheep are stillbirths, or weak newborn lambs and calves.

Newborn animals may also show signs such as goitre, a swelling of the thyroid gland in the upper neck, which strongly suggests that their dam's diet is iodine deficient.

Other, less obvious, signs of iodine deficiency include fertility problems and poor scanning percentages or poor survival rates of new born lambs and changes to the skin and hair/wool. In sheep, wool changes may show as scanty wool and hairiness of the fleece.

Even if you suspect iodine deficiency, it is still wise to investigate before supplementing as over supplementation can have a detrimental effect on animal health and wastes money. Dietary supplementation of iodine should not exceed the EU maximum permitted level, which is set to prevent excess iodine entering the food chain, particularly through milk.

First, have a look at the dam's diet. Symptoms of iodine deficiency can develop in animals on brassica crops like rape and kale. Some brassica varieties contain substances, generally known as goitrogens, which can interfere with thyroid hormone production.

Even though sufficient iodine is present in the diet, these animals can develop symptoms of iodine deficiency. The presence of brassicas should be taken into account when investigating a problem and an additional test can be used in these cases.

In live animals, a blood test can measure current iodine intakes, but the best way to diagnose iodine deficiency is by postmortem examination of a stillborn or weak newborn calf or lamb. This way the thyroid gland can be examined and the iodine content measured.

Farmers should also be aware that selenium has a role in thyroid hormone production and a deficiency of selenium can lead to a reduction in thyroid hormones. When investigating an iodine problem, selenium levels should be measured at the same time.

There are various methods of supplementation and the type most suitable to your farm will vary depending on your system and practicality of the supplement for you. This should be discussed with your vet, but available supplements include boluses, iodized salts, feeding seaweed (kelp) and mineral mixes.