Anything over 3/8s bison would be considered a bison hybrid.
About 15 yrs ago I seen bison / maine hybrid cows that were rumoured to be close to 20 still producing calves ...in the eastern interlake of Manitoba
I don't have any experience raising hybrids, but here on bluestem range with no supplemental feed, bison cows are at their peak (highest breed-back percentage and largest calves) between about 7-10 years old. By the time they're 15 they calve less often, and the calves get smaller and smaller. The calves with 20-year-old mothers are runts, and often enough at that age both mother and calf fail to survive winter.
Most bison herds are not pure
This is true. Many herds can claim to be free of cattle genes in mitochondrial DNA, but every herd that has had nuclear DNA analyzed has found cattle gene introgression. Escaped cattle often ran with bison herds before the bison were wiped out, and the bison that were saved from destruction were held in corrals and shipped on railcars mixed with cattle. On a practical level though, the small amount of cattle genes in most herds has little effect. Bison are still bison.
There were a little over 60 million bison in America, compared to a little over 30 million head of cattle now, yet we are somehow supposed to believe that bovine methane expulsions are only a problem now.
Keith
Not trying to start any arguments, but current best guesses are that there were between 30 and 60 million bison in North America (60 million being a little far-fetched), and there are currently close to 100 million cattle in North America.