
I have a 12 year old bitch that developed a small malignant mammary tumour at age 5 (spaying early was not in the cards as I wanted to breed from her, and she had two litters). We had a regrowth in the same spot at age 8 when she had a patial mastectomy.
Other than now loosing some of her muscle on her hindquarters she is fit and well.
I have had no problems with her daughter age 9, who wasn't spayed until nearly 7 years of age, or her Grandaughter who is 6 and a half and expecting a litter any day, or her Great Grandaughter who is 4 and a half and has had one litter now aged 8 months.
None of my girls were bred from early, so had already had at least four seasons before being bred from.
Fellow breeders in my breed inform me that they ahve very rarely had maammry tumours, and one of them has bred countless generations over 50 years.
A friedn in another breed (Welsh Springers) had mammary tumours in all her mature bitches later in life.
The study that maintains the protective effect of sopaying was discredited a few years ago as being of too narrow a smaple to be conclusive.
Experiences of breeders in various breeds would seem to suggest that this Cancer appears at very variuable incidence according to breed, but is not as high as often is quoted by the Veterinary proffession.
The decison of course had to be yours.
Some say the operation has no effect on mental or physical development, but all the prepubescently spayed bitches of my aquaintance have remained, evben into old age very puppyish and immature, often lacking the confidence of a finished adult.