In this case it's chickens which get carted around a few days behind the cows. Cows eat grass. Cows poop on field. Flies lay eggs in poop. Eggs hatch into maggots, which eat poop. Chickens dig through poop looking for maggots. This spreads the poop out and gives the chickens an excellent source of protein.

Cow poop is actually a good fertilizer for grass, but it's too concentrated normally. The chickens spread it out to a more reasonable concentration and produce eggs in the process.