Terramycin is an antibiotic. Antibiotics are for treating bacterial infections. Terramycin has a spectrum that covers SOME bacterial infections. Does nothing for viruses, and does nothing for many common bacteria that cause disease. Many chickens carry chronic mycoplasma, which terramycin has some efficacy for- but you can't generally cure it, only suppress it.
Adding antibiotics to the feed or water because of changes in the weather has no basis in sound medicine. It would be akin to me running to the GP or pediatrician and asking for some antibiotic because a storm is coming.
Wonder why Terramycin is not working as well as it used to? Maybe because it is available over the counter at the local feed store to every cowboy, farmer or pet owner who gives it to their pet(s) at the first sign of a sniffle. Ore even worse- gives it as a preventative? Good husbandry and sometimes vaccines are/is the preventative, drugs are for treating existing known disease.
Please think about what you are doing in the grand scheme of things before you add more probably unneeded antibiotics into the food chain.
As for drug residues- if you read the bag, it probably says for use in meat birds of a certain age, maybe with a withdrawal of 10-14 days. It probably also says not for use in laying hens. Period. Published withdrawal time does not exist, meaning withdrawal time of life.