Hay should be fine, but you should change it daily until they are at least a 6 weeks old. Ducks are messy and the wet hay can turn moldy really fast without even showing a sign of it. This is due to the fact that all hay has mold spored on them. Mold digested is extremely toxic to ducks. I personally prefer pine shavings, because they are high in tanic acid, which means mold it naturally not present, but it can grow on it if introduced and they shavings stay wet. I found that peewee pads and old bath towels work the best in the brooder. I'm sure there are others that say hay is not an issue, but I bet they also clean it very often to avoid any issues. So you really need to figure out what works best for you. BTW I don't think they should need the brooder any longer at all. Mine are usually out before they are 3 weeks old. With their age hay shouldn't be an issue at all. Keep it dry and clean, and make sure they cannot slip on it.