I mix BOSS right in with my feed, I have found that my horses all have beautiful shiny coats and their manes and tails glisten and sparkle. Here is some info on BOSS that I had saved from LB some time ago-
As already noted, BOSS is an acronym for Black Oil Sunflower Seed. These can be found in feed stores, pet stores, any place that carries bird seed, etc. The black oil seed is said to contain more oil in the shell than the gray-striped version, but nutritionally they are otherwise fairly equivalent. The gray-striped have a larger shell surface, and some reports indicate that they may be better for horses who tend to not chew their food well (seniors, horses with teeth problems). On the other hand, some sources say that the gray-striped shells splinter more easily. There are no real rock hard facts about feeding sunflower seeds, but they have been a staple in the equine diet in other parts of the world, especially Australia for years, and have been used around the race tracks for a long time as well.
Sunflower seeds are high in fat, anywhere from 30-50%, but are also mineral rich, are a good source of Vitamins A and E (both good for the hair coat), and are high in omega-6 essential fatty acids which are essential for skin/coat/hoof health and a strong immune system. The seeds also contain a high level of protein and good levels of amino acids (but not lysine and threonine, two of the more important ones). They are also very calorie dense, so a little goes a long way.
If you'd like to add BOSS to your mini's diet, you should introduce the seeds slowly, like you would any new food item. The average amount given to an adult mini should be less than 1/2 cup per day. Because they are rich in minerals, anything more than that should be mineral balanced with the rest of the diet in mind. If your mini is in extra good flesh, you could use the seeds to SUBSTITUTE for part of the grain ration, instead of just adding them to what he is already getting. As far as what age to introduce them to your mini -- there's no real hard facts on this. Personally, I start my horses on them once they are about 8-9 months old and are starting to get more grinding surface to their teeth. In any case, whether you feed them to young or old horses, there are bound to be some seeds that don't get ground up and digested well and are passed in the manure. The upside to that is your pasture or manure pile gets a little more attractive as the plants grow and bloom!