Did you notice anything curious when you last looked at your dairy’s maternity pen statistics? More twin births than usual? If so, you’re not alone.

“Some dairies we work with have twinning percentages as high as 10 percent,” says Milo Wiltbank, extension dairy reproductive specialist at the University of Wisconsin.

Researchers aren’t certain why this trend is occurring. But they do have some ideas, and one of the leading theories centers on increased multiple ovulations in high-producing cows.

Here are some reasons why your cows may experience more multiple ovulations and, therefore, more twin births.


There are several reasons why you may experience more twins on your dairy. However, high milk production at ovulation seems to play the largest role.

How much have twin pregnancies increased?
Twinning rates have increased in the last 30 years — from about 3.4 percent in 1975 to 5.02 percent by 2000. (However, understand that the 5-percent figure may be understated because it includes all cow parities. First-calf heifers don’t give birth to twins very often.)

Researchers at the University of Wisconsin and IowaStateUniversity, writing in the September 2001 Journal of Dairy Science, noted that in some Holstein bulls the projected transmitting ability for twinning may be as high as 8 percent. Keep in mind that USDA does not currently track these data, so it is not possible to attain this information on-farm.

In addition, the researchers found that sires born after 1990 had a higher incidence of twinning than those born before.

Why is this twinning trend rising?
Several factors appear to be at play: 

  • First, there is a significant correlation between high milk production (90 pounds of milk per day or higher) at ovulation, multiple ovulations and more twins.
  • Second, seasonality may have some impact.
  • Third, dam age has an impact on twinning rate.

According to recent research conducted by Wiltbank and his colleagues, cows that produce 111 pounds of milk per day or more at ovulation are more likely to have multiple ovulations. Meanwhile, cows that produce less than 88 pounds of milk at ovulation are significantly less likely to have multiple ovulations. (Only a small portion of those lower-producing cows had multiple ovulations, as shown by the chart below.) 

“Multiple ovulations really seem to take off in cows milking more than 90 pounds per day at ovulation,” says Wiltbank. “And it seems to hold true for all cows milking at that level, regardless of lactation number. It didn’t change when we looked at Ovsynch programs versus naturally ovulating cows either.”

Multiple ovulations are the most common reason for twin pregnancies. (Ninety-three percent of twins are not identical twins, which means they come from separate eggs, not a single egg that split.)

Research by the University of Wisconsin and IowaStateUniversity found that the incidence of twins was highest — 5.88 percent — in April to June. Twinning incidence was lowest — 4.23 percent — in the period from October through December.

Older cows are more likely to have twins. The University of Wisconsin/Iowa State University study also found that the incidence of twinning increases to 7.19 percent by a cow’s fifth lactation. 

Why more multiple ovulations?
As previously noted, cows that produced less than 88 pounds of milk had significantly lower ovulation rates than those producing higher levels of milk. According to the same University of Wisconsin study, 51.6 percent of the cows that produced 111 pounds of milk per day or more ovulated on a multiple basis. (It is the amount of milk produced at ovulation that is important, not the milk produced during an entire lactation.)

The level of feed intake at the time of ovulation may hold the key to understanding why this happens.  

The current working hypothesis goes something like this: High-producing cows increase feed intake, which causes more blood to flow from the intestinal tract to the liver. This, in turn, increases steroid metabolism, which seems to encourage more multiple ovulations.

“We don’t know the overall mechanism, and this anticipated cause is not definitive,” Wiltbank acknowledges. Still, he and other researchers believe they are on the right track.

Higher milk production, combined with genetic influences, appears to create a more fertile environment for twins.

Can you avoid twin pregnancies?
Unfortunately, there doesn’t seem to be a good strategy at this point. 

Scientists are working on DNA sequencing to locate the genes responsible for twinning. Brian Kirkpatrick, University of Wisconsin dairy genetics specialist, suggests that a simple test will be available in the future to allow producers to test whether cows have the “twin” genes or not. (Kirkpatrick’s lab has discovered that at least one gene each on chromosomes five and 19 are responsible for twinning, and he suspects that at least two other twin-related genes await discovery.)

Wiltbank says we may someday have hormonal therapies to add to the arsenal. 

Meanwhile, the strategy remains one of good management, including:

  • Early identification of those cows pregnant with twins.
  • Proper nutrition for those cows.
  • Extra attention paid to those cows during close-up and calving. These cows often calve at least seven days earlier than they would otherwise. So, be prepared for early deliveries and the potential for calving difficulties.