We have always used chicken breast for treats. I bought one of those little food dicers and after I boil and cool the chicken, I put it through the dicer and it made perfect sized treats. I have also used turkey, eye round beef or even pork tenderloin as these are very lean cuts. I would say I stick mostly with chicken but I'll switch to eye round if it's on sale or whatever. Pork is least used, and usually only if I make it for us which is pretty rare. For training purposes, I use meat but for indoors reinforcement of training, I will use veggies like a sweet potato also cubed or green beans. This is the food dicer I have.
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