Are you using barley hay? This may be a long shot, but it is being used in ponds to control algae. They way it works is that is does release chlorine, which kills the algae. This process usually takes about 3 weeks to start. I wonder if perhaps wet barley hay has been used, stayed damp, and started to release the chlorine which then may have burned the feet. As I've said this may be a long shot.
Katharina