For those who are wondering--no, it's almost certainly not true. I just looked up a few studies of wild yaks; nothing about dick-biting or missing dicks.
It's true that wild yak herds are mostly females, but that's because the adult males wander around in separate bachelor groups outside the breeding season, kind of like elephants. The only males who hang out in the main yak herds are young ones.
The manga author's cited source is "the Dalai Lama's home page, Japanese-language version", which is, uh, probably not run by people with a Nobel Prize in Yak. If anyone can track down the relevant quote on that site, though, I'd love to see it.
Like @
nobaka says, the animal facts in this manga--and Killing Bites, and Arachnid, and most comedy/action mangas with an animal theme--are usually somewhere between "not
quite correct" and "very very wrong." Artistic license.