Where can I find a PhD in equine nutrition?

there are no such institution specialise in equine nutrition. may be try to search the internet to see if any professors or academic expert in this area, then proceed further! it depends on what umbrella you wanna focus on your research.

Try Harper Adams university college (Shropshire) or Moulton college (Northamptonshire)(affiliated to HAUC)both are on the web and have an interest in this area also being smaller I find people have more time to talk - particularly if you already have an idea for a project. Get a member of staffs name in the field and ask to meet up for a chat sometime - might give you other leads even if it comes to nothing.