First, rats will eat eggs, chicks, and if you have any hens that don't roost up high, they'll attack them from underneath when they're in that night-time stupor and they'll definately kill them if they can.
cgmccary is right about not all cats being good ratters... heck, my housecat watches mice for entertainment without making a move to catch em! We have a feral cat that won't come near us but is good for rats...
If the rat problem is that bad, the poison blocks are mostly safe if the block is where the hens or pets can't reach it since rats crawl back into their burrows to die, as long as you keep an eye out for sickly rats, it's honestly worth the risk. Even if a pet does get one, it'll usually only throw it up, that's the thing, rodents can't up-chuck, which is why those poisons work well on them. (not to say it's SAFE for mammals, just not as dangerous in the secondary form - as in a dead rat)