I'm curious why you make the changes to their feed?
I always soak the feed in the warmer months and actually ferment it slightly in the cooler months because not only does it increase hydration, but also increases the nutritional content of the food. Apple Cider Vinegar (aka ACV) encourages the proliferation of healthy 'gut' bacteria and serves as a prebiotic environment in the digestive tract. Then giving them probiotics in their feed (or even an occasional yogurt treat) introduces the favorable gut bacteria. You never want to use both ACV and probiotics at the same time because the acid can actually kill off the preferred bacteria, but introducing the ACV at least one full day before probiotics primes the gut.
Supplements like garlic, oregano oil, and various herbs and greens all have medicinal properties and can help fight off parasites and infections or just boost nutrition to strengthen the immune system.
These nutritional boosts function much like multi-vitamin and other supplements work for humans; they bolster the immune system to make it easier for the animals to fight off viruses, bad bacteria and parasites. For example, all chickens are exposed to coccidiosis, but only those that are unhealthy or have compromised immune systems will typically succumb to it. Heat stress, molting, being picked on or heavily mated, etc. can all lead to a stress reaction that may weaken the immune system and allow the 'bad stuff' to take hold. Supplements help to re-balance their bodies. It's not foolproof, but it helps.