photo by Patricia Stroup

Dairy management methods designed to increase milk production per cow may be contributing to an increased rate of twinning in dairy cattle. "Our results agree with the idea that high milk production near ovulation can increase the incidence of double ovulation and that this increase may result in increased twinning," said Milo Wiltbank, a cattle reproductive physiologist from the University of Wisconsin-Madison (UWM) who participated in the study.

The study does not prove a definitive connection between higher double ovulation rates and increased twinning frequencies, cautioned Paul Fricke, a UWM Extension specialist in dairy reproduction who co-authored the report. "Although we would like to assume that an increase in the double ovulation rate might increase the incidence of twinning, there is no data available to prove that relationship at this point. Further research will determine if increases in the incidence of double ovulation actually increase twinning."

Future research may help control or eliminate twinning in calves, Fricke added. "Understanding the physiologic mechanisms that regulate twinning may eventually enable us to reduce or prevent twinning in dairy cattle. I would like to further investigate the relationship between the incidence of double ovulation and twinning in dairy cattle, and assess management strategies to minimize the negative impacts of twinning."

Most twin calves develop when cows produce two eggs during the same ovulation, although twins occur from the same egg occasionally. Double ovulation does not guarantee twinning, however, since cows carrying twins frequently suffer high rates of embryonic loss and abortion.

There are currently no effective methods to prevent twins from occurring among dairy cows, Fricke noted, and twin calves are not popular among most dairy farmers. Twinning reduces a herd's level of profitability and reproductive efficiency. Cows carrying twins face increased health risks during their pregnancies, and they take longer to breed back once the calves are born. Twin calves also have a greater chance of being aborted, stillborn or have a low birth rate than other calves.

As if all this weren't enough, a majority of female calves that are born as twins with male calves are reproductively sterile. This puts another strain on dairy farmers who need new heifers to join the milking herd.

Is milk production or the age of the cow more important?
Scientific data show that an increase in milk production measured during a 10-year period was highly correlated with the increase in twinning that occurred during this period, Fricke noted. Is increased milk production a foolproof indicator of increased twinning rates?

Fricke and Wiltbank knew that the percentage of cows producing twins jumps from less than one percent for heifers to nearly 10 percent for older cows. The researchers wanted to learn whether milk production or the age of an animal contributed the most to double ovulation.

The scientists studied 237 Holstein cows from a Wisconsin farm that had a rolling herd average of 22,000 pounds of milk annually. Fricke and Wiltbank used Ovsynch to create synchronous ovulation in the cows. They also used ultrasound to detect whether each cow ovulated one or two follicles.

The cows were separated into two groups: high-producing cows that averaged 112 pounds of milk per day, and lower-producing cows that averaged 69 pounds of milk per day.

While the incidence of double ovulation tended to increase with the number of lactations, the study showed that milk production had the biggest impact on double ovulation rates. In fact, the 20 percent rate of double ovulation in the high-producing cows was nearly three times the seven percent rate observed in low-producing cows. In addition, the three-to-one ratio remained consistent, regardless of age or lactation number.

When the cows in the study calved, five percent of the 58 births produced twins. All the twins came from double-ovulation cows. There were not enough calvings in the study to analyze embryonic loss and abortion among the cows that had double ovulations, however, Fricke said.

"Our results show a linear increase in the incidence of double ovulation with increasing parity," Fricke said. "The apparent reason for this increase was that the proportion of cows with high milk production was greater for the older cows. Therefore, something associated with high milk production in and of itself appears to increase the incidence of double ovulation independent of either age or parity.