Merricks makes a really good lamb lung treat, single ingredient.
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My Scottie loves it. And unlike liver, lung can be given more frequently as a treat.
Liver should only be given once a week or your pup may get diarrhea because it's too heavy in nutrients - think of it like a weekly vitamin (unless you have a dog with an iron stomach
).
You can also make your own sweet potato treats, slice them up and bake at 200 degrees for an hour or so.