It prevents testicular and prostate cancer. Apparently if the dog is entire, prostate problems are more difficult to treat. Testicular cancer is easy to cure if it remains in the testicules as they just castrate them, but if the dog is old when it gets testicular cancer, then the risk of anasesthetic is high and could be a problem for an older dog.

Some people say they would never keep an entire dog because if they smell an in season bitch (i met one the other day off lead in a busy park, so if you live in a fairly busy area, the chances of meeting one are high), then they will chase after the scent and will ignore recall commands, so could be dangerous. Castration will also help the problems with your dog, although don't expect marking to go away completely (my dog was castrated last July and he still marks (although he never marked excessively)) so your problem may decrease and he may stop marking in the house, but will still mark outside.

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I OWN A CHOCOLATE ROAN COCKER SPANIEL
RIP David xx Run free at the bridge. Love you always xx 19th February 2011
RIP Jack and Mickey xx Run free at the bridge. Love you forever xx 20th June 2011