I've always found that it's the urine from bitches which kills grass, but the urine from entire dogs actually acts as a fertiliser. All my dogs are entire, and I never have them castrated, so I wonder if it's the dogs which are castrated which are killing off grass. Castration seems to be a fairly popular practice on here, and quite genuinely, I'd be interested to hear of the experiences of others.
On our paddocks, and when the grass is short, we have grass in vibrant patches, hundreds of them, and when I discussed it with a knowledgable visitor, she said that the patches were where a mare had urinated.
Alec.