Cathy always has good advice. My vet said, the new protocol is to use one dewormer until it stops working (be very careful not to under dose). Then change dewormers. If you are out of Ivermectin and have the Valbazen on hand then I would do the switch, but, just continue to use the Valbazen till it stops working. Of course, you wouldn't use it on the pregnant girls. I don't deworm on a schedule here (except for the preggy does get dewormed after kidding), I check eyes (or any mucus membrane on a light goat).