Pete Ramey has posted a new article on Equine Nutrition and how it affects the hoof. http://www.hoofrehab.com/diet.htm
After reading it, if you are wondering where to test your hay, go here to EquiAnalytical. Also, Dr. Kellon's Web Site is: www.drkellon.com . I am going to sign up for this online equine nutrition course, it sounds amazing.
Sally Hugg, a trimmer in the Orroville, CA area (Northern CA) has done a lot of testing of hay in Northern CA. They all come back with similar profiles - we are deficient in Selenium, Copper, and Zinc. She has created a custom mineral blend for horses in Northern CA called CA Trace Plus. I always carry bags of these minerals in the car with me. I also suggest adding a teaspoon of salt and CocaSoya Oil from Uckele (this provides Omega 3, 6, 9 and Vitamin E). This works well mixed into Beet Pulp (no molasses), Timothy Pellets, or the Parelli Grain Mix (Barley 30%, Oats 30%, Wheat Bran 30%, Sunflower Seeds 10%)
Definitely take the time to read Pete Ramey's article - it will really help you sort through labels and what your horse needs. Even though you may be feeding a multivitamin, your horse may still be deficient in some minerals and getting too much of other minerals.