The grass found in your horse pasture is the perfect food for horses. But unless your horse is free to range over hundreds of miles, taking in a wide variety of grass types from regions with different soils, there's a good chance he won't be getting an adequate array and balance of minerals in his horse pasture.
The best place to start looking for advice regarding what minerals are likely to be in short supply in your horse pasture is your state university's agriculture department. Information is often available down to the county level.
While that can give you some solid, general advice, you should also have your pasture analyzed. To do this, snip some grass about an inch above ground level, taking many samples from the areas where you have observed that the horses prefer to graze.
Most state universities offer forage analysis services, as do private testing labs such as Litchfield, www.litchlab.com. Someone from the agriculture department, your vet or a nutritionist can then help you determine what supplementation is needed.