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You can’t completely prevent it. Piggies need access to a lot of hay at all times, so the fact hay needs to be there means there is always going to be a risk.
you can minimise if by having Timothy hay (as it’s stalkier) in a hay rack, but again, it won’t stop the risk entirely.
mine are bedded on meadow hay so I just throw piles of hay in the cage for them to snuggle into - Piggies love nothing more than diving into a pile of hay and playing and I find mine eat so much more hay now Ive stopped using hay racks. I put their Timothy hay just into a pile in the corner. They sit next to the pile and eat it not dive into the Timothy.
after a sudden stint of near constant hay pokes I moved from using hay racks to one big hay tray and we've now gone a month hay poke free. my boys had been fine using hay racks for nearly 2 years but once they started Gundham in particular had a lot over a short space of time. I think it's because all the small pokey bits just sink to the bottom of the tray now rather than falling in their eyes.