Guide: Animal Feed Through

With the introduction of Animal Feed Troughs you can now purchase a new building for your Farm that, once full of animal feed, can attract one of a number of atypically available animals which you can adopt for your farm. Once you adopt one, your Feed Trough will need to get filled up again in order to attract another farm animal.

Feeds and Troughs
The next time you log into FarmVille after the update you’ll receive an Animal Trough to place on your farm. If you do not choose to place it now it will go into your Gift Box for placement later. Once you click on the Animal Trough and choose “Look Inside” you’ll see this window:

Next you’ll need to get the Animal Trough filled—it’ll take 20 units of Animal Feed to top off the Trough. You can acquire Animal Feed one of two ways—you can buy it by clicking on the “Buy Feed” button and purchasing Animal Feed one unit at a time for 1 Farm Cash each.

When your neighbors visit your farm, they can click on your Animal Trough to give you one unit of Animal Feed (or, if you visit them, you can click on their Animal Trough and gift them a unit of Animal Feed). If you click on the “Ask For Feed” you’ll be prompted to put a notice up on your Facebook feed asking for more Animal Feed. You can also acquire more Animal Feed through feed posts.
The Animal Feed Trough window shows visually how full your Trough is, and has six mug shots of the animals known to be nearby that may wander onto your farm. Animals will drop out of the rotation eventually and new animals inserted, so the six won’t stay the exact same for long.
The visual state of the Animal Trough changes as it fills up with Animal Feed:

Adopting a new farm animal
Once the Animal Trough is filled with Animal Feed you’ll get this notice:

Sometime in the next day or so an animal will stop by and enter the Animal Trough. You’ll be able to tell by the fact that the light above the building entrance is green, the doors are closed, and the feed has been drained (see comparison image above). If you visit the Animal Trough in time, you will be successful in adopting the animal for your farm (and you’ll have the option to share one with your neighbors):

All of your Animal Feed will be used up and you’ll need to fill your Animal Trough back up again to get another animal.
However, if you wait too long (more than a couple of days), the animal will go away. You’ll see this window instead:

By losing the animal you’ll only lose 2 units of Animal Feed, so you’ll have 18 units instead. The mug shot board in the Animal Trough will reflect which animals you’ve adopted and which still remain to be adopted.
Source: Official Farmville Forum