Feeding cows chicken litter isn't a big deal, it's not just chicken poo, but the uneaten or spilled food, the bedding material, and pieces of feathers. Runoff from chicken litter causes a lot of issues for rivers and streams, and it can't always be used for fertilizer. Feeding it to cows to improve their mineral uptake, it has about the same caloric value as hay, and it's higher in protein.