I don't know if it is statistically significant or not but my small base herd of "purebred" commercial Hereford cows and heifers bred by my T21 registered Hereford bull last June all calved within 12 days of each other (1st heat). The only ones left to calve are the registered Hereford heifers I bought as bred and that had a later due date based on later exposure prior to me buying them.

Frankly I think there are a lot more factors involved in creating a tight calving season and quick breed back after calving than whether the cows/heifers are purebred or crossbred.

Jim at Dawn