Hello, welcome to JustAnswer! I am a licensed veterinarian, and I'll be happy to help you in any way I can.
Food allergy typically won't improve with the use of prednisone, so a food allergy would be considered an unlikely explanation, given your description.
The diarrhea could be food-related, but I would more likely suspect that eating grass is not helping matters much.
I would strongly suggest not allowing the grass eating until this issue is more clearly defined and diagnosed.
It would be a wise idea to ask your vet for a broad-spectrum dewormer, such as Panacur, to help rule out internal parasites that might have been missed on stool tests.
The prednisone is NOT an acceptable long term management strategy.
and it is imperative to seek other treatments for allergic skin issues, such as antihistamines, fatty acids, or allergy testing followed by desensitizing injections.
You can try regular pepto bismol, 1 mL per 10 pounds of body weight, every 8-12 hours, for symptomatic relief of the bowel issues.
Benadryl, 25-50 mg, every 8-12 hours, plus fish oil, 1000mg per 10 pounds body weight daily, can help to some degree with environmental allergies.
Thank you. She is doing well! We are slowly increasing her salmon oil, and started allergy meds instead of prednizone. She is scratching a lot, but managable, still an improvement if we give her nothing. She scratches and nibbles but not to the point of hair loss. We are hoping the extra salmon oil will improve it as well. The local vet told me to only give her 1500mg or 2000 and said 6000mg was way too much. Though we are going to proceed with the oil, I think the steroids are worse than too much salmon oil. Thank you so much for your advice.