A fixed dog still has the desire to breed, it just cant.
Occasionally yes, but not often.
Most neutered males completely forget about breeding once their testosterone levels die down.
This is from my own personal experience with neutered males, both those neutered early, and as veterans.
These are from the latest issue of Veterinary Practice News (Aug 2005).
We are mostly aware that spaying a bitch before her first season halves her risk of mammary cancer, and obviously castration frees you from concerns about testicular cancer, which is particularly worrisome in boys with retained testicles, but what about other cancers? Here are some figures that may surprise you.
Spayed bitches had four times the incidence of cardiac hemangiosarcom as compared to intact bitches. Neutered males have a significantly greater risk for these tumors compared to their intact brethren.
Prostate cancer is four times more common in castrated dogs compared to intact ones.
Neutered and spayed dogs have up to 3 times the likelihood of developing bladder cancer compared to intact ones, and are twice as likely to develop osteosarcoma (bone cancer). Males are four times more likely to die within 2 years of diagnosis when compared with females. Dogs neutered or spayed before they were a year old had a one in four lifetime risk of getting osteosarcoma.
And my personal bitch, who was spayed after 3 years of age, died of osteosarcoma. Hopefully as time goes by we are able to collect even more data on why this terrible disease happens.
Ultimately, with the unfolding of the canine genome, we may know which cancers our dogs are most likely to get, and be able to say whether an individual is better intact or neutered, in the meantime, as with most questions, the answer to whether spaying or neutering is better from a health point of view is "it depends."
One other thing that you must touch on when discussing this subject is the possibility of the death of the bitch from whelping complications.
This can and does happen. I don't have stats, but any time you breed a bitch, you stand the chance of losing her.
For me, spay/neuter of animals that are not to be used for breeding or shown in the ring is the order of the day.
I require all my pet puppies to be S/N, and fairly early.
Control of indiscriminate breeding of dogs I breed is paramount to me.
Health is of great concern as well, but I will not tolerate the breeding of any dogs that come from me unless they meet certain criteria.