While on the subject of ancient haystacks, I located this picture of one surely the descendant of those on Trajan’s column. This photo of a modern haystack in Romania, by Paul Nasca, is available courtesy of Wikimedia Commons (the Wikimedia Commons is a database of media files to which anyone can contribute and which can be freely re-used as well):
Notice how the outermost hay has been groomed to carry water down the sides of the stack. Notice also how a small cap of hay has been added to the top of the stack (it looks a little tousled there) where the post comes out. This view doesn’t show it well, but you can see some extra sticks applied to the top of the stack. They help hold down the cap. Here is another look, showing the sticks. This photo is also by Paul Nasca, and from the Wikicommons:
Scroll down to the July 10, 2007 post to compare the pictures of the Trajan’s Column haystacks.