Joseph Meehan wrote:
I have seen large U shaped nails that appear to be used for that exact
purpose. I don't know where you might find them, I have seen them on post.



Andy writes:
They are called "fence staples" and are used to fasten wire, such as
barbed wire, into wooden posts..... They might work if the split is
narrow.
However, the best fix is to get some galvanized wire, such as is used
for
electric fences, and wrap about 10 turns around the top of the post, in
effect binding the split together.....

Andy in Eureka, Texas