Calves are inclined by nature to suckle a nipple, and suckling is mentally satisfying to calves of all ages. Many calves will actively seek something to suckle after consuming their milk or milk replacer meal from a bucket or open pail in order to satisfy this desire to suckle. Unfortunately, for calves housed together, this suckling desire will be exercised on a penmate's udder, ear or navel, with clinically recognized negative consequences.
Calves can continue to suckle a nipple on an empty nipple bucket or nipple bottle, which seems to satisfy their suckling desire. This cross-suckling issue is why many veterinarians advise their clients to house milk or milk replacer-fed calves separately.