So far the cost on the recycled/composite "wood like" stuff is about 2.5-3x pressure treated prices at the local Home Depot/Lowes. If we are talking a 25+ year endeavor then great, otherwise tough to justify.

Oh, on thing I forgot to mention on horse fencing....make sure you put the boards to the inside of the pasture. That way when someone/thing leans on them they don't just pop out. I'm sure most already know it but I'd rather nag now than...

Screws should work fine...just never seen it done on something the scale of pasture fencing. If you have the time/patience go for it!