Disregard the above post please.
Most dogs are food motivated, some just prefer different types of food that us humans think they should have. My older shiba works for prime rib, but very little else. I originally though she wasn't food motivated either.
When you have a stubborn breed, which you do, you may want to try "upping the ante" and use cut up hot dogs, small pieces of boiled chicken, or even bacon bits.
As for how to get your dog to release a ball to you, PHArrow's suggestions are great. A "drop it" and "leave it" command are very helpful.
Start teaching "leave it' with an item your dog isn't nearly as motivated by getting, in your case, you could possibly use food to teach the behavior and reward with the ball. It's a bit opposite of what many do, but no one is saying it won't work.
Also, one your dog does back away from the object, it's only fair that they can have the object back sometimes. Don't use it only to end the game. Use it to continue to play. If your dog doesn't drop the ball, no more game. All is done. He'll quickly learn there is no advantage to keeping the ball.
One other thing, you might want to put "tug" on command. That will tell your dog when it's appropriate to play that game and when you are playing fetch instead.
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Chelle and the rest of the crew including, but not limited to Kita and Taiko (the shiba inu wrestle maniacs), Adi (reserved and dignified tabby cat), and all 28 reptiles
