Most all pasture and hay are deficient in some mineral or other; which ones in particular depend on the soil in that particular region and how the field is maintained. Hay also loses its nutritional value to older it is, so even if you had perfectly balanced hay when it was cut, it would be much less nutritious 6 months down the road.
Feeding a ration balancer
is an easy, inexpensive way to make sure a horse is getting all the required nutrients. I really like Triple Crown 30% Supplement, but most feed companies make at least one ration balancer formula.