*on subway rats, at least
Basically the idea is to stop rats from making more rats, rather than just killing/trapping them directly. I suppose it could be considered more humane, in a sense. I seem to remember reading about a similar plan to be used against mosquitoes in an effort to fight malaria.
"A sweet-salty, sterilizing chemical called Contrapest will be placed in subway bait boxes. When a female rat ingests some, it'll target the ovarian follicles, permanently sterilizing it. If it's already pregnant, some of its female children will even become sterile. Along with those more traditional techniques, Metro Transit Authority officials are hoping this puts a dent in the rodent population."excerpt from the article
P.S. I hope this doesn't count as link spam or anything.