It's because of the contrast between the poop and snow. Your dog is self-conscious and is too nervous to perform. Spray paint the snow brown to calm his/her nerves.
Who could argue with that logic? Don't have any spray paint, maybe I can sprinkle some cocoa powder instead.
That seems like it might work. The color would (hopefully) be similar. And also don't be afraid to poop in the snow first, to let your dog know that it's okay.
I can tell you're an accredited dog trainer.
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