Please do not pigeon hole all DVM's. Our practice has a doctor that specializes in pretty much everything. If they don't know, the university is just a phone call or e-mail away.
A vet can do a blood panel to see what the mare is short on. There may be nutrients her body is not utilizing correctly.
vets do ONE semester of general nutrition. so unless your vet has taken extra EQUINE nutrition it is a wasted vet trip