Have had experience with No Worries and I-80 both are highly recommended for ease of calving and also quality calves.The heifer is as important as the bull used and heifers that have good condition have had exercise and are built right to be a females.Small framed, shorter bodied,overly fat heifers that are heavily muscled with a small pelvic area can be trouble Heifers younger than 24 months in many cases are not as well developed.Many ranchers rely on pelvic measurements when keeping replacement heifers.Actually calves under 80#s are plenty for most heifers. A live calf is the main goal .