Twins in cattle are not common- about 1 set in every 4000 births. Some dairy farmers find they have a run of twins in some seasons and these are difficult to explain. Hormone treatment to bring cows into oestrus is sometimes blamed but this has not been proven.
There are also natural twinning where some individual cows have twins regularly over their lives, showing that genetics are involved. Twinning in cows is not very strongly inherited, as their daughters don’t regularly produce twins. So it’s been very hard to breed for a twinning herd although there is one at Ruakura which is making slow progress. Twinning cows regardless of breed have been collected from all over New Zealand and positive but slow progress has been made. The problem has been to find bulls that carry twinning genes to concentrate these genes in the herd.
Twinning cows often have retained foetal membranes (RFM) and can be slow to return to heat after calving. They may even be more prone to infections of the uterus.
Twin calves are small and grow slowly and also have higher mortality. Then there is the problem of “freemartins” in cattle which is not seen in sheep which is interesting.
A freemartin is a female twin to a male, and a very high percentage of them are infertile. In the uterus the hormones from the male calf get into the blood stream of the female calf and affect its sexual development. It’s not worth keeping female freemartins - although you can have a veterinary examination to check their status before disposal.
Twins can result from two separate ovulations (eggs) of the cow. These are non-identical (heterozygous) twins. Identical (homozygous) twins come from one egg shed by the cow which then splits in two. Identical-twin heifers (which have normal fertility) are always wanted by AgResearch Ruakura in Hamilton for research purposes. There are a few tricks in confirming that they are identical such as checking head hair whorls, eyelashes and pigment pattern inside the mouth, so let them make the decision. They may not have completely identical colour markings. Even up to old age these twins show similar behaviour.