These animals are known as freemartins.

This happens when the twin’s two sets of blood supply fuse together during early pregnancy and is shared by the calves.

The common blood supply allows for an exchange of cells and hormones between the two foetuses.

Sexual development in the heifer calf is inhibited by the presence of male cells and hormones from the developing bull.

The ovaries fail to develop in a fully grown freemartin cow and are only about the size of a grain of wheat, and she fails to cycle and never becomes pregnant.

The uterus is also underdeveloped and the udder and teats are small (similar in size to the udder and teats on a steer). The vulva and vagina appear much the same as normal, except there is often a tuft of hair on the vulva.

There are no such concerns with the twin bull, which will develop normally.

A freemartin appears like a normal heifer, therefore you need to get a veterinarian to diagnose her as a freemartin.

There is no treatment for freemartin cows, the best strategy is to permanently identify all heifer calves born twins with a bull and assume they will be infertile.

For further advice contact your local veterinarian or DEDJTR veterinary or animal health officer, or in NSW your Local Land Services.