Here is the reader's question to Dear Abby:
"Why don't they put something in pet food to keep dogs and cats from getting pregnant?"
To which Abby (a.k.a. Jeanne Phillips) replies, and I'm paraphrasing: so far, we just haven't figured out a cheap and effective way to produce contraceptive pet food. But there are trials under way to develop oral contraceptives to control wild animal populations like coyotes. So (presumably) something like this is certainly conceivable for the population of pets somewhere down the line.
How exactly would this work? Would all dog and cat food be laced with contraceptives? Or just dog and cat food fed to shelter animals? Or would contraceptive food be just another option among others at grocery and pet stores, shelved alongside senior, anti-allergy, and reduced fat diets? What would it be called? Reproductive Control Plus?