Historical clue formulations

• Calfless cow — 2021-03-10
• Cow sans calf — 2019-04-17
• Young cow — 2018-02-08
• Mate for a bull — 2017-12-20
• What Zeus transformed Io into — 2006-07-08
• Cow that hasn't had a cow — 2005-11-02
• Member of a herd — 2002-02-12