I have killed 20 cows this map, do not have enough data yet. Here's some helpful stuff.

http://minecraft.gamepedia.com/Cow Under the Usage sub-heading: Cows drop 0-2 leather, and Calves drop nothing.

I remember that killing a calf has yielded leather for me this map, will test to confirm later.

So. Cows are dropping 0-2 leather per kill for vanilla, with a Farmer chance for double drops, and a Looting chance for extra drops.

Looting III increases the range for leather from normal to 0-5. This means that even with Looting III you can be unlucky and get zero drops.

We should answer these questions:

How much leather per 100 cows should we expect as

Vanilla drop chance players?

- JavaRandom's a bitch, but as far as I can tell, we should expect ~ 33 to be 0, 33 to be 1, and 33 to be 2 = 100

How much leather per 100 cows should we expect as

Farmer players?

How much leather per 100 cows should we expect

using Looting Enchants as Vanilla drop chance players?

- Again, JavaRandom's a bitch but I expect ~ 20 to be 0, 20 to be 1, 20 to be 2, 20 to be 3, 20 to be 4, and 20 to be 5 = 300

How much leather per 100 cows should we expect as

Farmers using Looting Enchants?