Well-grown young calves can start cycling as early as four months of age, and long before weaning. If a bull is running with the herd, he may mate the well-grown calves as well as their mothers, and a potential animal welfare problem is guaranteed, as these pregnant yearlings will have difficulty calving. They will be worse if the sire of the calf they are carrying was a large breed of bull.
This situation seems to be increasing as more Holstein Friesian (HF) genes have been introduced into the Kiwi Friesian. The HF was selected in North America for large milk yields and hence large mature size. Consequently they grow well and if reared as single-suckled calves, either as purebreds or as Hereford or Angus crosses, they can reach puberty and start cycling by four months. Being long legged a bull has no problem serving them.
However, even small Jersey weaners can show early heat and they are even a bigger welfare problem if they get pregnant.
Uncastrated male calves can also mate their herd-mate heifers, or they can mate the cows in the herd including their mothers! If their natural instincts don't click in, being a spectator when the herd bull is in action soon gets them going. Indeed, they often frustrate the herd sire by getting in the way just at the moment of truth, so he ends up being over worked.
The best practice is always to consult a veterinarian and have any pregnant yearlings aborted. Some farmers argue that pregnant yearlings can calve safely, but the risk is too high, as inevitably the calf is too large for the yearling's immature pelvis. She'll be damaged and end up as a cull, or the effect of lactating is too much for her and she'll fail to perform the following season – again ending up as a cull.
The earlier that a yearlings are aborted the better. If a yearling does calve successfully, then she needs very special care to make sure she doesn't get mastitis and her lactation is short. Only let her suckle a single calf for 4-6 weeks. The chances are that she'll be late in coming into heat again, and her lifetime production will be restricted unless she is given high levels of feeding and care.