The only way to know for sure if your horse needs supplementation is to have your hay analyzed. Most US and Canadian grass hays are low in magnesium, copper, and zinc. Alfalfa is very very high in calcium so you need to watch the Ca:Mg balance if you feed a lot of alfalfa. Whether you need things like selenium depends on where the hay was grown. If your horse has no issues and it isn't feasible for you to analyze your hay and supplement accordingly, then either doing nothing, or adding a VMS or ration balancer designed for the type of hay you feed are both reasonable approaches.
The amount of ration balancer for my 300lb guy was about 3/4 cup per day (like with any feed, it has to be fed by weight, this was about 1/3lb). Last spring I dropped the ration balancer and changed to a VMS since it's a much smaller amount (about a rounded tablespoon - just a few grams), to cut more calories, and I did notice he lost weight after I did that.
I think mineral issues only show up in bloodwork if they are severe. The only mineral that horses will naturally eat as needed is salt.
The amount of ration balancer for my 300lb guy was about 3/4 cup per day (like with any feed, it has to be fed by weight, this was about 1/3lb). Last spring I dropped the ration balancer and changed to a VMS since it's a much smaller amount (about a rounded tablespoon - just a few grams), to cut more calories, and I did notice he lost weight after I did that.
I think mineral issues only show up in bloodwork if they are severe. The only mineral that horses will naturally eat as needed is salt.