Horses should have access to salt at all times. I provide each of my horses with two salt blocks. One is a plain white salt block that is simply table salt; sodium chloride. The other is a calcium/phosphorus trace mineral salt block. It is sometimes called a 12:12 block because it contains 12% calcium and 12% phosphorus or an equal ratio of calcium to phosphorus, which is good for most adult horses.
Cherry Hill's Horsekeeping 