This is directly from Xylax:

- Lion Meat/GMO cows will work the same no matter what your lion's Fertility is.
- Lion Meat/GMO cows work separately from Fertility, meaning, they can affect one cub on its own, and your lioness' fertility can affect another cub in the litter anyway
- Cotton Root Bark will override any fertility boost/debuff
- GMO cow WILL override CRB's effect BUT it most likely WILL make GMO's mutation rarer if CRB's % triggers. So they benefit each other you could say, IF they both work at the same time. Each item has its own % chance to actually trigger, they may fail however.
- GMO Cow will override Bark's % and use cow's % mutation chance BUT the bark might make the mutation super rare.

Meaning, CRB's effect will indeed override item's effect, but can combine with GMO Cow/Lion Meat.

Lion Scrotum works differentely - it will make mom's mutation pass more likely - in this case does not combine with CRB.

Information from other mods:
The CRB will roll first, then fertility if CRB won't give you muties.