You've got a very ignorant view of dairy farming. Now I'm sure there's some operations in those western desert states that behave that way but it is absolutely not the norm east of the Mississippi.
I've got a friend who owns a dairy farm. He doesn't keep dead cows around because the risk of introducing disease isn't worth it.
Manure gets treated in a "lagoon" (which looks like an open pit to you) wherein the water evaporated and bacteria breaks it down to become fertilizer for the next guy. The system is specifically designed to keep it from running off. Good dirt work is cheaper than having you people up his ass over pollution.
If he dumps milk it's like 20gal from purging equipment or something, not enough to matter. Milk is money so he tries to avoid dumping any. If he did want to dump a bunch of it, he could dump it straight into the lagoons because a liquid designed to be nutritious for growing mammals will be a walk in the park for bacteria that feed on what the mammals leave behind.
And 40yr ago when he dad owned the farm all of this was the same. The technology has changed a little but they weren't piling bodies then either.