I prepare several "horse businesses" tax returns.  Number 1 Item to be aware of - EVERY ONE of my returns that mention "horses" has been audited~ ~ that is just a given!  So alert your clients that their bookkeeping needs to be accurate!

All of my horse clients have farming involved with their equine businesses.  The very first auditor, and every auditor since then, has told me that i can either do a Sch. C for the horses and Sch F for the rest of the farming income....or put all of it on Schedule F.  I make notes and separate income and expenses (noted on the return) that show horse or farming.  They all told me that it would be so much simpler to put it on an F, otherwise it is difficult to deduct feed, vet, etc on the Sch C.  

If it's horses and then land rent, I would do Schedule F for horses and then the Sch E for rent.  I would figure out the acreage and income for both and then divide expenses accordingly on the two forms.

Good luck!  And, like I said, be prepared for an audit!