The maize inbreeds are maintained by

The maize inbreeds are maintained by Selfing.
The nucleus seed of inbred lines can be maintained by self-pollination, sib-pollination, or a combination of the two procedures (hand pollination). Some breeders prefer 'sibbing" because it maintains vigour. "Selfing" is used to stabilize inbred lines if a change in breeding behavior is noticed.
Selfing or self-pollination, where ovules are pollinated by the plant's pollen, is the ultimate form of inbreeding or mating between relatives.