Current dairy management strategies aimed at increasing milk production per cow may be leading to an increase in the rate of twinning in dairy cattle, according to a recent University of Wisconsin-Madison study.
“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,” says Milo Wiltbank, a cattle reproductive physiologist in the College of Agricultural and Life Sciences. “This is the first study we know of that shows a direct relationship between milk production and double ovulation in dairy cattle.”
“The relationship between milk production and twinning remains unclear,” cautions Paul Fricke, a UW-Madison Extension dairy scientist and co-author of the study. “Further research will determine if increases in the incidence of double ovulation actually increase twinning.”
Although some twins arise from the same egg, most twin calves are the result of cows producing two eggs during the same ovulation. Double ovulation must precede twins from two different eggs, but double ovulation does not always result in twinning because cows carrying twins experience high rates of embryonic loss and abortion.
Most dairy farmers don”t welcome twin calves because twinning reduces a herd”s overall profitability and reproductive efficiency.
“Some frequency of twinning is unavoidable with dairy cattle,” Fricke says. Cows that deliver twins are at a greater risk for problems during their pregnancy and take longer to rebreed after the calves are born. Compared with single calves, twin calves are more likely to be aborted, stillborn and have low birth weight. In addition, most female calves born as twins with male calves are reproductively sterile. The loss of female calves before or at birth creates problems for farmers, who need new heifers to enter the milking herd.
Studies show that the frequency of twinning today is greater than in the past. If twinning were related to milk production, that increase in twinning would be expected because milk production per cow has been increasing over the same period. However, researchers also have found that the percentage of cows producing twins increases from less than 1 percent for virgin heifers to nearly 10 percent for older cows.
So what”s more responsible for double ovulation, milk production per cow or age of the cows?
To answer that question, Fricke and Wiltbank studied 237 cows from a Wisconsin farm with a rolling herd average of 22,000 pounds of milk per year. The researchers used Ovsynch, a method for causing synchronous ovulation in cows. The dairy scientists used ultrasound to determine if each cow ovulated one or two follicles.
The researchers divided cows from the herd into two groups: high-producing cows averaging 112 pounds of milk per day, and lower-producing cows averaging 69 pounds per day.
The incidence of double ovulation tended to increase with the number of lactations, also called parity. However, milk production had the greatest effect on double ovulation rate in the study. The overall incidence of double ovulation was nearly three times greater for high-producing cows (20 percent) than low-producing cows (7 percent). That three-fold difference held regardless of age or lactation number.
Fricke says there were too few calvings in the study to assess embryonic loss and abortion in cows showing double ovulation. The researchers followed some of the cows through to calving. Of 58 births, there were three twins (5 percent) — all from double-ovulation cows.
“Our results show a linear increase in the incidence of double ovulation with increasing parity,” Fricke says. “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. We concluded that milk production is the primary factor affecting the incidence of double ovulation in dairy cows,” he says.
The dairy scientists can not yet explain the mechanism that increases double ovulation in high-producing cows. They believe it may be related to feeding practices and will conduct additional studies to understand and perhaps eventually control double ovulation rate.
The study was supported by: state funding to the UW-Madison College of Agricultural and Life Sciences and the UW-Extension Cooperative Extension Service, and a grant from the National Association of Animal Breeders.