For a show horse, racehorse, or performance horse, we recommend supplementing pretty much year-round. For a pleasure horse used for spring and summer riding, we suggest supplementing just during that period of activity. Additionally, he says one can more easily overdose a horse on single nutrient supplements, but this is less likely with a multi-nutrient supplement. Should you wish to supplement a horse, Herthel and others recommend first discussing this with a veterinary nutritionist or a veterinarian with nutritional training. You can also make it worse by feeding the wrong nutrients.
Minerals can affect one another and their ability to absorb a given mineral.
For example, adding iron to the diet may cause reduced absorption of other minerals. The issue of supplementation will always be there because people feel they need to do something for their horse. Horses were doing just fine for thousands of years without supplementing their diet with anything, although they didn't live and work as they do today. So I'm not dismissing all supplements. I'm saying they are marketed and presented in a light that really has not been shown to bear the brunt of research and to be shown as beneficial.