Cattle Twins

Twinning in Cattle

Twins in cattle are rare. On average, only one cow in every 4000 will deliver a set of twins. Occasionally, dairy farmers report a run of twin births in a season, but the reasons are often unclear. Hormone treatments used to bring cows into oestrus are sometimes suspected, but no solid link has ever been proven.

Genetics and natural twinning

Some cows are naturally prone to having twins throughout their lives, which suggests a genetic component. However, this trait is not strongly passed on. The daughters of twinning cows do not reliably produce twins themselves, making it difficult to breed a reliable twinning line.

At Ruakura, there has been a long-running effort to breed a twinning herd by collecting cows from all over New Zealand that have a history of twinning. Progress has been positive but slow. The major hurdle has been identifying and using bulls that carry genes for twinning. Without this, it is difficult to concentrate the genetic traits needed.

Health and production challenges

While twins might seem like a bonus, they often bring more problems than benefits. Cows that deliver twins are more likely to retain their foetal membranes and can be slow to return to heat after calving. They may also be more prone to uterine infections.

Twin calves are generally small, grow more slowly, and have higher mortality rates. These factors make twinning in cattle a management challenge, especially for lifestyle farmers with limited stock numbers.

The freemartin issue

One of the more unusual complications from twinning in cattle is the freemartin. This happens when a female calf is born as a twin to a male. In most of these cases, the female is infertile. This is due to the male’s hormones affecting the development of the female’s reproductive system while they are still in the womb.

If you suspect you have a freemartin, a vet can examine the calf to confirm whether she is fertile. However, in most cases it is not worth keeping her for breeding.

Identical versus non-identical twins

Cows can produce two types of twins. Non-identical (heterozygous) twins result from two separate eggs being fertilised. Identical (homozygous) twins form when one fertilised egg splits in two. These identical twin heifers are fertile and of particular interest to researchers.

AgResearch at Ruakura in Hamilton is always interested in hearing from farmers with identical twin heifers, as they are valuable for research. Even if the twins do not have identical colour markings, they often show remarkably similar behaviour into old age. Identifying them involves looking at things like hair whorls, eyelash patterns, and pigmentation inside the mouth, so it is best to let the researchers make the final call.

If you are raising cattle on a lifestyle block and want to be better prepared, our What You Need to Know to Keep Cattle and Cattle Breeding courses offer practical guidance to help you manage these events confidently.