In this instructable I will show you how to make an automatic cow farm.
Step 1: The Cow Pen
To begin you make a 12x10 1 block high square made of any type of block. On the 12 block side, break the second and fifth block.
On top of the blocks, place fence peices so the cows can't get out. On top of those fence pieces place another row of blocks, this will prevent certain mobs from entering the cage.
Step 2: The Rails
When the cage is done place rails in the pattern I have shown. Add mob spawners inside the pen and spawn cows in it.
Step 3: Working the Rails
Add a switch station if you want to have multiple farms.
Now make a rail line to where your house will be. You can use detector rails and powered rails for more dependable service.
Step 4: Farming Station
Now for the farming station. Make a 5x5 3 block deep hole and begin filling it with hoppers. To place hoppers you will have to crouch down using the crouch button in the middle of the direction controls.
Walk away from where you started following the pattern in the fourth photo, then place rails and netherrack in the pattern in the fifth photo.
Step 5: Farm House
Build your house on top of the farming part.
Where the minecart enters the farm, have a 1x1 hole so other 1x2 mobs can't enter.
Finish the rails going from the house to the spawner, shown in the third and fourth photos.
Light the netherrack with flint and steel and send minecarts from your house down the rails. 1)The cart goes down the rail. 2)The cart enters the spawner and gets a cow. 3)The cart exits the spawner with a cow and goes down the rail to the farm. 4)The cow and cart enter the farm and the fire breaks the cart and the cow gets cooked. 5)The cows drops, (cooked beef and leather),and fall into the hopper and gets fed into the first hopper.