Your local cooperative extension service will test hay for free. I don't know how extensive your testing would be, but it's worth looking in to. Texas AgriLife Extension Service
Generally though, if you're getting the same kind of hay from the same supplier, then your hay is going to be relatively stable in nutrient content. As long as you get it tested once or twice a year, you should be fine. Then use those tests to figure your diet.
Your local extension service will know all about your soil and nutritional needs as well. They can do soil/pasture testing to get the exact make up of your pasture. All of this testing is generally free!
I really like Uckele's Vitamin & Mineral formulas. They have a wide variety available. Their prices aren't cheap, but their products seem to be good quality. I got my first batch of the Equi-Base Grass and so far so good! Both horses are picky eaters and they are eating it fine. I'll know more in another month on how it's doing overall though. Uckele Equine Nutrition
I mix it with a 2qt scoop of chopped alfalfa hay (they get pasture and bermuda as their main source of hay) and just a dash of oats. They like their oats and I barely give 1/2 a 2qt scoop a day, so it's not enough sugars to mess up their diet. The chopped alfalfa hay does great at mixing everything up and is forage, so safe to give them. I like adding it for something different since they get straight bermuda hay.
Both horses are healthy and barefoot!