Missing 13 y/o Border Collie Mix - Longbranch

She is a 13 y/o female border collie/Australian shepherd mix and is black, white and tan mix. She went missing Wednesday morning. I went out around 8:00am to feed her and she was nowhere to be found. Have been out searching every day but cannot find her. This is very uncharacteristic of her to leave and not come home. We live on 20 acres that she pretty much never leaves.