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The grease gland is gland that produces skin protecting grease. It can occasionally become overproductive in either gender; more often in boars than in sows. The grease gland is located on the bum, a little above the genitalia. See a vet if a gentle clean cannot get rid of any crusts or growths in the grease gland.
Please only intervene if really necessary and do not overclean the grease gland as that can stimulate continuous overproduction. Too much bathing can also lead to longer term dry skin problems as the skin microbiome is constantly interfered with.