A heiferette is any young cow that will still grade on the rail. Usually these are 2 yr olds that fail to re-breed and calve as 3's, but the odd 3 or even 4 year old will still pass.
A first calf heifer is a heifer that just had her first calf. A second calf heifer is usually a 3 yr old that has had one calf but not the second. A few still call them second calf heifers while they are nursing their second calf.
Jason Trowbridge Southern Angus Farms Alberta Canada