Originally Posted by stevenson
Most horses don't need additional supplements . Hay, water and free choice salt .
I'm going to disagree with you on that one. It's possible that a horse on fresh grass with water and free choice salt might not need supplementation, but a lot of those still do. The reason being that the nutritional content of the grass itself is based on the soil it's grown in. The soil can be deficient in minerals because of regional deficiencies (e.g. Selenium) or due to the farming history of that particular plot of land (certain crops will strip out specific minerals, which must be replaced by well thought out crop rotation or other means) Excess minerals can also be a problem- iron is a common culprit and blocks absorption of copper and zinc when they get out of balance.
There are additional things to consider when you take a horse off fresh grass and put them on hay. Vitamins A & E in particular deteriorate very quickly once the grass is cut and need to be supplemented. Omega-3 fatty acids also deteriorate rapidly. Other nutrients also deteriorate, though more slowly, so even if you had hay that was nutritionally balanced (minus the vitamins & omega-3's mentioned above) when it was cut, by the time late winter rolls around, it's much less nutritious.
Most vitamins and minerals have a wide margin of safety (even selenium has a range of safety that is unlikely to be exceeded unless you live in a high-selenium area or are feeding multiple products containing selenium) Feeding a ration balancer is a good way to ensure that your horse is getting sufficient nutrition year round. I'd consider it a requirement for horses on hay, and highly recommended for horses on fresh grass unless a pasture analysis shows sufficient levels of all macrominerals.