As previously said, they should breed on their own if the pond is established and if the LMBs are of adult size. The extra forage will greatly help to feed the parents and the babies, but it's not absolutely necessary; as long as the parents have enough food then there shouldn't be any problems. Mud or sand definitely does help since they like to nest in the mud/sand.
I had my SMBs breeding in my pond the first year I added them; the pond was a bit over a year old at that point in time, so it doesn't take very long for a pond to get established.