When mine were younger and I took them to a dog park I always bought their ball along. They were allowed free play time with other dogs then I engaged them in a game of ball play, I found this worked very well for us as they always seemed much more at ease and happier when they had a job (the ball). However, I always made sure I was the one in control of the ball and they got the ball at the park and then I took the ball when we had finished and put in a bag which I kept in my car - they eventually learned that they got the ball at the park and even though I took it off them when we had finished, I would always give it back to them next time - they learned to trust me.
When another dog came along and took their ball, I always took control and either asked the dogs owner if they would retrieve the ball from their dog or if I knew the dog, I took it back. Again my dogs learned to trust me and there was not one instance of aggression from them when another dog took their ball because they knew I would get it back.
So I had no problems, but I know some people do.
if your GSD is eating and eating and eating and losing weight - please consider testing for EPI.
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