That's not correct Verona. Some nutrients, such as water soluble ones, that could be said about, but copper & zinc for eg are both mins that are cumulative & per day means per day, not per feed/amount.
Puck, yes, frustrating isn't it?? For one, I think that's why it's so vital to do a diet analysis & then find the *appropriate* supplement for your horse's particular diet, and that what's said/advised on the lables & product advertising... ain't necessarily so. That's why I rave about feedXL.com - takes the confusion out of the equation!
The other thing to consider is that many types of minerals are more or less 'bio-available'(absorbable, useful to the body) than others, and the 'elemental' level of a certain mineral may be far less than the amount of product. For eg. Organic sources of minerals are generally far more bio-available than man made stuff like copper sulphate, iron oxide, etc. So the elemental levels of the min must be far higher in the product than with organic stuff, for the body to absorb the same level. More or less confused now?? FYI I don't know whether there's an article on FeedXL on copper, but I have a good article from Dr Richards(nutritionist) on weighing up pros & cons of different forms of Mg that I can forward if you're interested, which explains it better than I do!