Your kidding right? a good Dresage horse wouldn't find it a challenge at all, and short of using a hot wire on the fence wouldn't discourage many a horse from eating their way through them.
Lee
how about start with half-posts - use a dado or table saw or other fast cutting saw to make the slots, then glue the posts together with a waterproof glue. That will be much faster than any other means and will give you nice square edged slots
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