Its likely that it is haylage or silage out in the feeders - it does seem to keep better in the big bales, and even straw is left out in all weathers uncovered in the big bales. I think it takes more time for the rain to penetrate through those big bales, and it does not seem to bother it too much, yet small bales are no use if soaked through. Hay in big bales seems more water resistant too. I am not saying it does not deteriorate with being out in the rain, but its usually ok to eat.
With my ponies, its not there long enough for weather to damage it - they eat it so fast!!