Natural peanut butter gave Chase the runs (REALLY badly) when he was three months, but the regular kind was okay so long as he only had a amall amount. I wouldn't overstuff the Kong to begin with. He eventually got used to the natural PB around 5 or 6 months old.
Good healthy treats that you can get started with are apples, banana, frozen melon balls (these are VERY popular in my house!), carrots and broccoli. Just start with only small amounts and don't give more than one new thing in a day to avoid tummy upset.
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Chase (Friendly Resident Wigglebum) - Border Collie/Lab/Shepherd X
Kailey (Misunderstood Gentle Beauty) - GSD & foster failure #1
Rupert (Gold-Medal Winner of the 3 a.m. Kitty Destruction Olympics
)
Heidi - RIP my sweet baby girl