Often it is caused by a heifer sucking other calves. There seems to a be one every so often that can wreck havoc with her herdmates. We usually put a nose ring in the offender and remove it after they calve( of course our latest one is nursing her milking herdmates now??) Otherwise cleaniness as the heifer grows up is important an even more so as they start to spring up, which often is when they get the least care or attention. Sometimes we use a dry cow tube and milk out a quarter if it looks uneven or too much swelling at the wrong time.