That's true Harmony. However, they definitely last longer if you keep them dry. When we just left them in the field, we ended up losing some every time we tipped the water off.
We found that this wasn't so much of an issue with the red molassessed buckets because the sheep liked them more and slurped the liquid off the top. However, if it was just a mineral bucket, they ate less and then ignored it once it got wet and went sludgy.