wouldn't say any are calving ease, lower, yes.
maine breed is bottle necked. genetic analysis performed by two groups i helped get genetic analysis done confirms this. interestingly, the shorthorn genetics is getting diluted, angus is up a little and "mancelle" is static.
the original cross was mancelle, and probably two other localized populations, maybe some friesian, who knows, were crossed with durham and then "closed". there really is no such thing as a breed, holstein regions are in a lot of cattle etc. there was selection for milk, and then somewhere around the mid to late 60's, maybe later, this was no longer tracked. at some point, there was a bull named poplar haven red alert 54T, and boom, there was no milk, went from epd of 25 in pedigree to 0.1, sorta like cunia (who is a carrier for monkey mouth and spastic pareisis) and marbling. people found out quickly in red alert daughters had no milk, and he fell off the map. but, supposedly, he was the first bull that was lower in his flank, so people kept those daughters, "bred around it" and people wanted the lower flank and were sorta stuck with the females (a lot of daughters were pawned off on unsuspecting buyers). this bull ended up in most pedigrees that have no milk, SLC sooner, limited edition.
translation, they are black and have bone
the reason to go back to the fullbloods is to get the milk back, stay away from the super tall animals like Twin Creek etulason (streamliner) and screen which animals might have had some other positive traits, carcass, growth, etc., while screening against undesirable traits and open up the gene pool.
semen is available, but not much, a lot got dumped during two occurrences, the cattle crash of 1974 and the when tall cattle went out, semen tanks going dry, people just dumping it etc.
there isn't a single generation magic cross, and any pursuit is going to probably take a little more determination/plan than most people are willing to put up with.