Question:  I live in Arizona and our soil is quite alkaline. The equi based grass formula used to have iron in it, and now it does not, I feed this to both my horses in addition to timothy and Bermuda hay grown in Arizona. One horse is a 15 year old gelding and one 8 year old mare. She eats a lot of dirt, the gelding not so much. If I add iron to their feed they stop eating dirt. What is the best source of iron for them? I have added Red Cell currently with added but it also has other minerals they are already getting in the Equi base grass formula.

Dr. Kellon:  Despite your alkaline soils, which do limit iron uptake by the plants compared to acidic soils, there are no cases of diet related iron DEFICIENCY in horses on any soil or hay type.  You can check your horses' iron status by sending blood to Kansas State's Comparative Hematology Laboratory, the only lab in the world that can do this testing.

I have seen hundreds of these specialized screens from all over the US and have never found an iron deficient horse.  There is also no evidence that horses eat dirt, chew bark, etc because of mineral deficiencies or that they have any natural drive to seeK out minerals except salt.
If you just feel more comfortable feeding a supplement that also has iron added, consider Base Mix P55.
Another option is to have hay analysis done on your hays so that you know exactly what they are eating and the best supplement for them.