I recently got 2 pullets at age 6 weeks. They came from a place with 280 chickens and were raised on medicated feed. I switched them over to unmedicated feed as I only have a few chickens. My older chickens were never on medicated feed. Healthy backyard chickens should be fine on un-medicated feed. Some people feed medicated for up to 16 weeks or while they are on starter feed from what I understand. It is supposed to prevent coccidiosis in chickens.
Grown chickens are supposd to be immune to coccidiosis
I got four pullets in July. They were started at the breeder's on medicated feed, and I continued with it until they were eight weeks old. They were in a chicken "nursery" in the basement until that time, as they were too little to be with the other hens, and it would have been impossible to keep the other hens from eating the medicated feed if they were all together. I was told that you can't eat eggs from chickens eating medicated feed. They stopped medicated feed after eight weeks and are fine.
Feed medicated with Amprolium is a great thing for chickens. It is not an antibiotic, it's similar in structure to vitamin B1 and keeps the cocci parasite from overwhelming a chicks sytem and killing or seriously injuring them. Even healthy chicks are suceptible to coccidiosis infection. It's found in the soil and all chicks will be exposed to it.
Can't I just treat coccidiosis after they get it?
Not a great idea, once the chicks get coccidiosis the damage is irreversible. It can cause death in only about 96 hours, and it may have no symptoms.
It is only when a bird is infected with a relatively small quantity of oocysts, that the bird then has an opportunity to fight off the infection and build immunity to future infection.
That's what Amprolium allows the chicks to do. It mimics thiamine which is the food source for the Cocci parasite, but unlike thiamine can't be used by the cocci so it lets the chick become exposed and develop immunity without being over exposed.
They can be kept on medicated feed until you switch to grower at around 8 weeks, or left on it until you switch to layer. As mentioned it's not an antibiotic and there is no withdraw time. Layers can have feed medicated with amprolium with no adverse effects.