The majority of horses in New Zealand are usually kept on some sort of restricted pasture - meaning that they live in one or perhaps two paddocks on the same property and are not able to graze extensively like wild ponies. They rely on us for their nutrient intake and to keep them healthy.
Horse pasture is quite often unfertilised or is found on the poorer land not used for commercial animal production and we know that with New Zealand being a geologically young country, large areas of land have mineral deficiencies, hence the multi million dollar fertiliser industry!
We mostly then feed our horse some sort of grass and balance out its diet with supplementary hay and/or silage and some variety of hard feed. Depending on the season and amount of grass, we will feed more or less of the hay or hard feed. However, without testing the bought in feed, you will not know if it is nutrient deficient and unless feeding high levels (5+kg) of hard feed, you cannot rely on meal to provide all the essential vitamins and minerals needed through a busy competition season.
As many of the water soluble vitamins and minerals are not stored in the body, a daily dose is required to maintain adequate levels. My horses get a daily dose of Equi Mineral Plus supplement, sometimes more sometimes less, depending on their level of work and how much feed is being given, but always enough to maintain a base level that meets their minimum requirement to keep them in peak condition and able to step up and perform when required.