If moisture seeping up from below the hay stack is going to be a problem them a ground cover is essential.
Some simple examples of these are used grain bags or silage plastic
But the easiest of the lot is a single bale width black recycled plastic rolls. These are available from Hay Cap Pty Ltd. The rolls are 2m wide (single bale), 50m long (approx. 40 bales long) & 200um thick (which is the same as silage plastic). This is a cheap way of getting good moisture and dirt protection for the bottom bales.
To use – unroll and re-roll back up at full width over a pine post - makes very easy unrolling when stacking up your hay and stops the wind getting under the plastic. You don’t need to get out and unroll by hand, instead use your front end loader to roll it back one bale space as each block of hay is stacked. This keeps you away from the face of the stack in the event of a hay fall, and makes the job quick & easy.
Something to note: if you are going to use grounds sheets of any sort under hay, it must be narrower than the bale. If you can see it sticking out from under the bale – it is too wide and water will fall onto the ground cover and seep in under the hay.