Alien cows are a limited time item. The event has already passed and Zynga is not releasing alien cows any more or maybe yet. Try to acquire all the possible items when there is an event for it is time limited and might won't come back again.
no there is not they were a limited addiction that lasted until the end of September
It gives you milk just like other cows.
Cows don't any crops on farmville.
Miltonium...
you can have as many cows you can fit on your farm
Adopt them from the News Feed
They do although it takes time
You buy them from the market and fill them with cows.
Green cows were a temporary item that came along with other alien-associated items available for purchase in Farmville. Temporary items change with the seasons. For example, jack-o-lanterns and tombstones were available during Halloween, and turkeys and feast tables were were available for Thanksgiving.
The cows will be an alien cow.
To harvest cow on farmville you need to click it. Your character will then go over to the cow and harvest it.
You can get cows on Farmville when your Farmville friends gift them to you or by adopting them when they wander across a neighbors farm and that neighbor posts a notification about it allowing you to adopt the lost cow.