Wouldn’t excrement be too valuable as a fertilizer and for producing methane power to just let it all run off into streams etc.?

A lot of it is sold off but the cows aren't always spending time where it can be collected. When they are out in the pasture the crap just stays where it falls and it will inevitably get picked up by rainwater.

I also wouldn't be surprised if this is including the manure that is being used as fertilizer as well. Because when it rains that manure isn't just going to stay on the field. There's one road in my area where, whenever it rains, you can watch a crap river flow down the road because of the sheer amount of manure running off the nearby field.