Straw is also incredibly dusty and prone to molding. Won't even use it with my horse. It's also not absorbant and will start to stink very quickly. Non-absorbant bedding that isn't soaking up the urine leads to the rats standing in their own pee. That can create a bacterial situation very quickly and bacteria leads to bumblefoot(which HURTS from what I've seen).

It's very inexpensive to use a good quality aspen bedding. Pine smells disgusting when wet(think guinea pigs peeing a gallon a day...), but aspen just smells like wet tree when wet. No biggie.