no problems at all with beef. I'm a pretty simple person and I don't fuss too much about stuff. I had a springer who couldn't tolerate ham or beef (diarrea) but odin eats our leftover steak n bones w/out issue. ;-) he looks fabulous and is a great size and healthy as a 'horse'...we kept his diet pure during his first 2 yrs but have slowly tested his tolerance of human stuff (meat, vegetables, etc). he has no skin issues, clean ears, clean poo.....I wouldn't think the problem is a mastiff issue but I would check w/ my vet if I were u, as there me be an allergy (like in my springer to ham and beef).