Farmers with beef cows suckling their own or maybe fostered calves, which have grown well because of a good milk supply from mum should watch out for these calve starting to cycle. This can happen as early as four months of age and long before conventional weaning. If a bull is used to mate the herd, he may mate the calves as well as their mums, and a potential animal welfare problem is guaranteed as these pregnant yearlings will have calving problems.
This seems to be an increasing phenomenon 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 large mature size. Consequently they grow well and if reared as single-suckled calves, either as purebreds or in crosses with Hereford or Angus, they can reach puberty and start cycling by four months old. 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.
There is also the other side of the coin, where uncastrated male calves can also mate their herd mate heifer calves, 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 can really frustrate the herd sire by getting in the way just at the moment of action, so he gets over worked.
What to do with a pregnant yearling can be a concern. The answer is always to consult a veterinarian and have them aborted. Some farmers report that some of these pregnant yearlings can calve safely, but the risk is not worth it as inevitably the calf is too large for the immature pelvis of the heifer. She can be damaged and end up as a cull, or the effect of having a lactation is too much for her and she fails to perform the following season – again ending up being culled.
The earlier they are aborted the better but often their pregnancy is not discovered till very late term. It's often assumed their early udder development is assumed to be fat but to make sure a veterinarian should carry out a pregnancy test. If a yearling does calve successfully then she needs very special care to make sure she does not get mastitis and her lactation is short, only suckling a single calf for a only a few months. The chances are that she will be late in coming into heat again, and her lifetime production may be restricted unless she is given high levels of feeding and care.