It’s amazing how much seed is in the bank and how long it can lay dormant and viable waiting for the right signals from the soil microbiome to germinate. A neighbour of ours did some work years ago a came to the conclusion there was between 1200 to 1400kg per ha of sub clover in the top couple of inches of soil. It answered the question as to why he was having establishment issues sowing a couple of kg of other varieties anyway.

Your chickens might not eat all the seed but their presence in your system will be a change and send some different signals.