Im having problems finding out how much ash my horse needs in its diet. I can find individual measures for all the macrominerals but not a whole number. Is it as easy as adding all the macromineral requirements together and hoping they come somewhere near the hays known ash content?
Is there a good book/web anywhere explaining this?
Answer on How much ash would a 15.2hh medium build horse need?
I wouldn't worry about it in quite that much detail. Remember that overfeeding vitamins and minerals is worse than any slight deficiency in hay.
If you are feeding good quality hay, you can buy a pelleted feed balancer like bailey's lo cal to get anything they are missing.
Sarah Pilliner's Horse Nutrition and Feeding is a good book and demonstrates how to properly calculate a horse's feed ration.
Just remember that horses have survived for millions of years on just grass and leaves. They don't need extra this and that added.
If your horse is not in work, or only does light hacking out/ riding, he would (in most cases) be perfectly healthy and well fed on a diet of ad-lib hay or haylage and grass when turned out. If he needs a bitmore condition or energy, most big feed companies provide a balanced ration in the form of general horse and pony nuts.