The field sprayer is rinsed out in the same yard as the chickens. Glyphosate is used in the yard and around the coop throughout the year to suppress undesirable plant growth. It's sprayed in the evening once the chickens are roosting and the chickens are released to free-range the next day. The birds have been fed grain from the fields that are sprayed, for 6+ years. There have been no issues as stated.
Glyphosate inhibits the enzyme 5-enolpyruvylshikimate-3-phosphate synthase. It's a key component of the shikimate pathway. Inhibition of the shikimate pathway blocks aromatic amino acid biosynthesis in plants, resulting in the death of those plants. The shikimate pathway is found only in microorganisms and plants, not in animals. Mammals don't have this pathway. Neither do birds. The spraying you see, the majority is water. The covering of the seed might be exposed to the spray, but not the seed itself. A quick search shows mixing directions as 5 tablespoons in 1 gallon of water. I believe a pop amount sprays a football field. Pesticides are expensive, no one wants to use more than they have to. To give context, alcoholic beverages are classified as a Group 1 carcinogen.
Glyphosphate is patented as an antimicrobial agent, for several families of susceptible organisms that cause grief such as toxoplasmosis.