Eleanor Kellon, VMD, indicates true food allergy has never been formally proven in horses. A major difficulty is diagnostics. If only by chance, the blood IgE tests are bound to get it right sometimes but research really does not support their use as a diagnostic tool.
Source: Horse Network, May 10, 2017.
The horse cannot be allergic to a food he has never been exposed to, so if you see positives for things you’ve never fed you can write that off. Also realize that when showing the signs you suspect are a food allergy it has to be something he is eating then.