1. What do 'dung's do?

2. What does the recycler do?

and how can i kill cows they are so annoying (if i can)

1. If you make a fertilizer storage, you can convert the dung into fertilizer.  You then use it on ground you have just cleared of a weed, and then plant cereal on it.

2.  If you put in a crafted item, it spits out it's constituent parts.

3.  You can't; you can just fence them in somewhere near grass, and use them for their dung.

oh ok thanks :D