In my (Sue's) experience, the biggest single factor in health of sheep is the correct balance of minerals, which permits their immune systems to function optimally. Thus for example, most sheep should not have much copper, though all need some, with downland and Texel sheep being particularly susceptible to copper poisoning, fine-woolled sheep intermediate, while Gotlands and Finnsheep are more tolerant than other breeds and indeed need a greater amount.
This is why it is important never to feed sheep with pig or poultry feed, which contains too much copper. My sheep broke into the feed store and gorged themselves on pig food once and the greediest had the worst hangovers – luckily a vitamin injection was sufficient to get them back into a state of normality (as far as Gotlands are ever normal …).
Both sulphur and molybdenum reduce the absorption of copper so these are often added to sheep feed to reduce the risk of copper from grass, hay and straw. However, too much molybdenum in the forage may risk a copper deficiency and failure to thrive.
Sulphur is also contained in garlic, which is good for reducing the risk of fly strike by altering the pH of the skin slightly and also helps the immune system and can be obtained in malt tubs in the spring.
Calcium is particularly important in late pregnancy when the lamb is growing rapidly and a deficiency will cause twin-lamb disease or even death if not diagnosed and treated promptly. So in general salt and lime are important …
The balance of minerals in the soil, and therefore the grass, is both variable according to the weather and according to the type of grasses and other plants growing. Mixed and older pastures are likely to have a better balance than newly seeded and heavily worked pastures. Adding back what the sheep have taken by spreading back the muck from the winter housing will help, while adding too much artificial fertiliser without careful soil assessment may cause more problems than it solves by unbalancing the potassium, nitrogen and phosphorus balances. This is why many farmers are turning to boluses, where the minerals are directly absorbed by the sheep, not randomly from where and how they graze and are absorbed at the rate at which the individual sheep metabolises.
Generally Gotlands thrive on extra zinc, which helps with wool and horn growth and thus with foot health and this can be obtained in many mineral licks and blocks. Zinc sulphate is also used effectively externally for foot baths. However, zinc also inhibits copper absorption so can cause copper deficiency … which is not good in late pregnancy when sheep need a little more copper.
So testing, experimenting and balancing are important and here talking to the neighbours is often particularly helpful, as they are most likely to have similar soils and their experience could be invaluable. Similarly talking to other breeders within the breed society will establish what might work best for your particular sheep.