We get asked a lot about if our slow feed hay nets will freeze if they are used out in the weather, the answer to this is Yes, then can freeze. Most hay nets freeze when it gets super cold and they are damp or wet from rain/saliva/snow and are out in the weather. Any fiber that is not a solid strand of material can entrap water and the water freezes when it gets cold enough outside. The good news is the hay nets unthaw pretty quick as the polypropylene doesn’t absorb water, so as soon as the net warms up some they soften back up . Even the sunlight can be enough to unthaw a hay net.
When we lived in western NY, we dealt with plenty of the wet /cold & snow. For myself I kept a small rotation of hanging nets. Any froze ones I would bring back into the barn and replace it with a freshly filled hay net, My horses hay nets hung outside in the weather, they were not undercover at all so they took the brunt of whatever mother nature threw at us. Normally just the temperature in the barn was enough to unthaw them so I could re-stuff for the next feeding. Nets used in the stalls were never a issue during the freezing weather, just the body heat alone seemed to be enough to keep the nets from freezing.
If you experience an abundance of frozen net weather and your horses struggle to get the hay out of the nets, one option is to go up a size in the mesh for the freezing conditions. Horses require more fuel during extreme cold so feeding additional hay is a good thing, another option is using them where they can be protected from the elements so the nets don’t freeze as hard.