I am intrigued as to where this novel idea of poisonous urine originated from! I have seen it mentioned before on here, and though I understand that people have their own beliefs and superstitions about their animal care, this one is unusual!
If urine was poisonous we would all be in trouble, if you think about what comes out of our taps! While I agree about the risk of infection between dogs and other species via sniffing urine etc, especially nowadays with the huge influx of tens of thousands of foreign dogs coming into the country via import dealers, many of whom manage to avoid even basic health checks, resulting in the awful problem of canine brucellosis and such like that we have now got, the comments I have seen suggest that there is an idea that there is something different about canine urine itself that makes it poisonous!
While I would agree wholeheartedly with being careful where grass is picked from, mostly because of the risk from weed killers etc, I do find this obsession with dogs' urine very strange.
My guinea pigs have shared their grass with multiple dogs, horses, sheep, you name it, but consistently always with the dogs, for decades. They eat more grass than anything else. I can assure you this does not seem to prevent them living long healthy lives so why this almost cult like obsession?
Where did it come from?