Icelandics are generally very easy to feed. The main problem is not allowing them to become too fat. If you have good grass throughout the year they will probably not need any extra food unless you are working them very hard.
In the spring and summer, you may need to restrict your horsesīgrazing time to stop him getting too fat. Electric fences are great! "Starvation paddocks" are not so good for Icelandics - better a concrete or gravel yard, as even if thereīs hardly any grass at all your Icelandic will still pick at it, and he may eat pebbles or sand resulting in colic. Strip grazing can be very effective espacially if you 'back fence' the area they have already grazed down
Provide a salt and mineral block with free access, and of course plenty of fresh water that wonīt freeze in the winter. Depending on your grass and hay you may need to feed a good quality vitamin and mineral supplement.
If you have a "real" winter or limited grass, then your Icelandic will need good hay or haylage. Silage is too rich, but good quality straw can be used as a "filler" for greedy horses.
If you are working your horse hard, or using it for breeding, or it is very old, then it may need extra food. Remember that even though Icelandics are tough, any food which they have must be of good quality.