The whole flaw in this is that cow manure isn't valuable to begin with. It's used a lot in fertilizers, and is very mild and will not burn plants. Organic farmers prefer this to other means.
So, it's not an "excessive waste" as you indicate. The logistics might work against it too. Cow manure is right there at the farms, where it can be used without transportation. When you start burning up a lot of gas to transport stuff, or need energy to get the gas, you cut into the efficiency of it.