Fence boards

there is what is called continuous fencing, it is like a gage or a tube cattle panels but with out the ends, and one fastens them to posts,

https://www.google.com/search?q=cont...w&ved=0CDMQsAQ

many are using them around here, no more expensive than wood and last much longer, if your in a area where you crowd them then put in more posts,

Reason for square posts: If you have to join two boards at a post, it is easier to securely and neatly join them on a square post.

I think you said you were going to use treated posts, which IME means pine wood. Pine is not strong enough for 3 inch corral posts, recommend 6 inch, possibly 8 inch at the point where the head gate is fastened to them. I have a head gate fastened to a homemade cow length working chute made from 2 inch iron pipe. It was used when I got it and the bottom was open so that the cow stood on the ground and could push against the chute-head gate assembly, making it necessary for the assembly to be fastened to large posts deep in the ground. I modified it by welding fastening points to it so that a new commercial head gate could be bolted to the chute. I also welded cross pieces to the bottom and fastened a wooden floor to them, so that the cow has to fight the whole assembly which is strong in all directions. No need to fasten it to any posts.

Just something to think about, i'm sure you have your plans worked out.

COWS