If your male dog won't leave a female in heat alone, this is perfectly normal, as he recognizes the scent of the pheromones the female gives off when she is in heat. While your dog won't necessarily know that a female dog is spayed, the spayed female doesn't generally produce the pheromones that attract male dogs.
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Spayed female dogs
Unspayed female dogs go into heat approximately twice per year. During this time, their hormones fluctuate. Not only do they produce pheromones that attract male dogs but the hormonal changes can also cause behavioral changes and problems. While some females will have a silent heat with no symptoms, many female dogs will become stressed, irritable, and even aggressive toward other dogs in the home.
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Spaying is a surgical procedure that removes the ovaries, fallopian tubes, and uterus. Not only does this leave the female unable to have puppies but it also prevents hormonal fluctuations and pheromone production. Spayed females tend to have a more even temperament and fewer behavioral problems.
Male dogs and spayed females
An unneutered male dog with a spayed female probably won't understand that she has been spayed, but he will know that she is not in heat. This is because she isn't producing the scent that alerts the male dog that she is ready to breed.
It is important to note that this is not limited to intact males. While neutered males have less testosterone and less desire to breed, they can still have sex and can be attracted to unspayed females.
Nonsexual mounting behavior
If your male dog is still mounting your spayed female, this may be perfectly normal, nonsexual behavior. Mounting can be a play behavior and can become excessive, especially in dogs who haven't been well socialized. Dogs may also display mounting behavior to establish their social standing. Excitement and stress are other reasons that your male dog may be mounting a spayed female.
It is important to note that in these circumstances, the male dog does not generally have an erection. Some medical conditions, such as urinary tract infections, may cause a male dog to mount excessively. If your male dog is mounting a spayed female excessively, consult your veterinarian to rule out medical problems and also consult a professional dog trainer to help you modify this behavior.
Male dog won't leave female alone
If you have ruled out nonsexual reasons for mounting and the male dog will not leave a spayed female alone, there could be another problem. In some cases, the male dog may be smelling the pheromones of another female dog in heat and reacting with the spayed female.
It is also possible that there was a problem with the spaying procedure. If any part of the reproductive tissue was left behind during the surgery, the female may have ovarian remnant syndrome. While the spayed female cannot get pregnant, she may still be producing hormones that are attracting male dogs. This condition is rare and can be corrected with another surgery to remove the ovarian reproductive tissue.
Finally, certain medical conditions can change the female dog's scent and cause male dogs to become attracted to her. Some examples include vaginitis, bladder infections, and cervical infections. The specific reason male dogs are attracted to female dogs with these conditions is unknown.