Cow manure is used here as well as a fertilizer on crops. It is not done by everyone, but it is used. As for using it for cannabis, I would think you would have to mix it into your soil, you could give it a try and let us know, but I have no idea how much you would put in.
Yes! It can absolutely be used for growing marijuana! I've used it before and would definitely use it again. I'd definitely use something else for flowering time though. In fact, here's one of my plants from last year grown in cow manure:
Usually you want to compost cow poop for at least a year before you use it. Allow the bacteria to do the work of breaking it down for the plant to use. Cow poop is very nitrogen rich and you can burn your plants if you use it too much or too early.