I fed my chickens almost one year with Purina Layena Crumbles. Almost all this time my chickens had dirty butt

This was driving me crazy , so I ask the owner of a local independant feed store / tack shop. She told me to put apple cider vinegar into their water. This didn't really help much.
Then one day we went at local Tractor Supply to buy Purina Layena Crumbles . All of the bags of Purina feed were not very fresh based on the dates printed on the bags.
I decided at that time to buy Nutrena Naturewise layer feed.
Ever since I quit using Purina feed I have no longer had any problems with dirty butts