Hi Sue--  I'm sure knowledgeable people will be answering your questions soon, but I just wanted to say that since I see you are in North Carolina, I am wondering if your barn is feeding coastal hay?  If so you may want to look at the hay analyses in my folder.  You will see that the coastal has an inverted calcium/ phosporus ratio and extremely high zinc that needs to be balanced.  This could be going on with your hay also.

Deb and Cory in NC
July 2012
http://pets.groups.yahoo.com/group/ECHistory5/files/Deb%20Funderburk%20in%20NC/
https://groups.yahoo.com/neo/groups/ECHistory5/photos/albums/1275105710