Pasture youngest, oldest females separately
Drovers.com
First-calf heifers typically are the toughest females on the ranch to get rebred, says Oklahoma State University Extension cattle reproduction specialist Glenn Selk. These young females, he says, are being asked to continue to grow, produce milk, repair the reproductive tract and have enough stored body energy to return to heat cycles in a short time frame. In addition, their mouths are going through the transition from baby teeth to adult teeth.
The Beef Blog 