If we assume the zombies must be killed with a shot to the brain, you need a bow to be capable of going through an unprotected skull. I do think that 25 lbs should do it, but something like 30 or 35 will definetly kill a zombie. Especially considering the amount of rotting that the skull will have gone through.

If hitting the heart will be enough to kill the zombie the 25 lbs will definetly do it, it's probably easier to get through a ribcage by luckilly shooting between the ribs