Just a note about your soils as you posted the reported amounts for your area. Just because something is in the soil does not mean it gets to your horse. Different plants have differing levels of uptake for various minerals. Somethings some plants won't pick up. That is why you test the forage. There are plants that can take up toxic amounts of some things detrimental to your animals but not the plants. Having an idea of what is there helps as it could determine what you plant.