chaumsong, the latter isn't true at all.
The information put about mammary tumours is a myth and the research which started it all off was very flawed, as Avidog point out:
"One important note: there are now several recent (2020) publications that show that mammary cancer rates, though higher in intact bitches, fall far, far short of the 25% threatened by many vets. Ben and Lynnette Hart’s 2020 paper show it to be highly variable but usually less than 6%. For example, intact Australian Cattle Dogs had no cases of mammary cancer while 8% of bitches spayed between 2 and 8 years had mammary cancer. In beagles and Bernese Mountain Dogs, no spayed or intact bitches had mammary cancer.
In comparison, rates of other cancers, such as hemangiosarcoma and mast cell tumors, were increased in some breeds like golden retrievers up to 17%, in Irish Wolfhounds to 25%! Owners must consider that spaying (and neutering) may increase the risk of untreatable cancers"
This is the paper which debunks the myth that spaying prevents mammary tumours:
https://www.researchgate.net/publication/225080364_The_effect_of_neutering_on_the_risk_of_mammary_tumours_in_dogs_-_a_systematic_review Quote: "This review concludes that, "due to the limited evidence available and the risk of bias in the 1969 study, any evidence that neutering reduces the risk of mammary cancer and that age at neutering has an effect on it, is judged to be weak and not a sound basis for firm recommendations on neutering."
This is the paper, from 1969, which is criticised above and debunked:
https://academic.oup.com/jnci/article-abstract/43/6/1249/910225?fbclid=IwAR0bHB0ENPq4JITkzIe81evUKAQde-HgDVNgu_i_8FRDe7cSgzZR7h1Pn-o Unfortunately this is one which people read or are told and then pass on to others, and no one really looks directly at the research and evidence behind it.