"The proper method of stacking square hay bales is a skill more likely to be passed down from one farmer to another than read about in a book or an Extension guide sheet," said Bob Schultheis, the agricultural engineering specialist with MU Extension in Webster County. "Still, it is a farm skill that has some definite do's and don'ts."
Schultheis says the stacking pattern he used was similar to Pattern #2 found at http://www.wikihow.com/Stack-Hay, with the following changes.
One, the stack would hold together better if the bales were stacked flat (twines up) rather than on edge. "The bottom layer could be stacked on edge, if desired, to reduce twine rot from moisture wicking," said Schultheis.
Two, build the stack to its full footprint and then build from the corners toward the middle, making sure to keep the corners square and plumb. "This gets really important when the stack gets 20 layers high," said Schultheis.
Three, for stability, make sure the stack height does not exceed 1.5 times the shortest base dimension. For example, if the stack is 20 feet wide and 40 feet long, the height should not exceed 30 feet.