Our milk production started to drop off during a dry spell. My neighbor Steve was down for a visit to discuss a sick cow. He is the farmer we bought our first three cows from, and the cow in question was one we got from him. We do rotational grazing, and I was complaining that it is hard to keep up rotation when you know you are going to have to put them on bad paddocks. Bad paddocks mean lower milk production. However, I need their manure on the bad paddocks or they will never be GOOD paddocks!
He gave me some good advice. He pointed out that cows pee and poop a lot during the night, but they don't eat much. Now that was news to me! I never went out and watched. Following Steve's reasoning, since they are not filling up the alimentary tract during the night, they pee and poop less during the day. To sum up - nighttime, not much grazing, lots of manure; daytime, lots of grazing (given enough fodder, a cow eats about six to nine hours a day), not much manure.
Here is how you put these fascinating facts to your advantage. Put them on bad paddocks at night, good paddocks during the day. That daytime munching on a good paddock means that they will ingest good grass with plenty of minerals and vitamins. Then when they are moved to the poor paddock at night, they will carry that nutrition to the nighttime paddock, where they will very efficiently deposit some it on the ground. Now isn't that clever? I love grass feeding. Manure is being moved around the farm without using one drop of fossil fuel!