The new discovery of a lethal gene may explain why pregnancy rates in dairy cattle have dropped sharply in recent years, a University of Wisconsin – Madison researcher says.

Hasan Khatib, an assistant professor in the dairy science department in the College of Agricultural and Life Sciences, has found a gene that causes embryonic death in cattle around the fifth day of pregnancy.

Dairy farmers have found it increasingly difficult to get their cows bred, a problem that is especially acute among higher producing cows. Getting cows pregnant once a year is an essential part of the milk production cycle on commercial farms.

“In the last 20 years, there has been a 20 percent drop in pregnancy rates in Holstein dairy cattle,” explained Khatib. “Based on our preliminary data for in-vitro fertilization experiments, we believe these problems can be attributed to this gene.”

The gene is only lethal when it’s homozygous; that is, when both parents contributed the lethal variant of the gene. In fact, Khatib’s data show the frequency of the gene in its heterozygous form, where a cow carries both the lethal variant and the nonlethal variant, is as high as 50 percent among Holsteins, the most common dairy cow breed.

Khatib figured that if this gene is so frequent, there must be a genetic advantage. Otherwise, genetic selection would have eliminated it. This proved to be the case. Through further analysis, he found the gene also increases milk production and fat and protein content in milk.

“Based on a study with 2,000 individuals, we believe that 65 to 70 percent of the Holstein population has the heterozygous genotype,” said Khatib.

“We don’t want to eliminate the lethal variant because it is linked with milk production traits, but we want to eliminate the homozygous effect that causes early embryonic death,” said Khatib

Since the homozygous condition only occurs if both parents carry the gene (in that case, the odds of a homozygous embryo are 25 percent), the solution is to avoid breeding heterozygous cows to heterozygous bulls.

“It is easy to predict, with simple DNA testing, if the cow will have dead embryos or not,” Khatib said.

The high frequency of the gene may be attributed to the narrow genetic base of cow families, says Kent Weigel, an associate professor and extension dairy genetics specialist who works closely with cattle breeding firms. “According to the data at hand, if a commonly used dairy bull had this gene, then it would have likely carried on through his daughters and sons.”

Weigel says that there should be a positive correlation between the frequency of this gene and pregnancy rate of a cow’s daughters.

“Since the embryonic loss occurs so early during pregnancy, you would never really detect a pregnancy, so it looks like that cow would have a longer number of days open,” says Weigel. “It would also decrease the estimated relative conception rate for bulls that were mated to that cow.”

Khatib says that there is interest from a variety of companies, breeding firms in particular, in potential applications for this gene. He intends to continue research to better understand the gene and provide information that will be useful to both breeding firms and dairy producers in the near future.

That research extends beyond cattle. He has confirmed the presence of the gene in sheep and is investigating whether it is present in pigs.

Khatib research focuses on two major areas of study. “My first objective is to identify genes of economic importance in dairy cattle, including ones that affect fertility, milk components, longevity, somatic cell count and other health traits,” Khatib explained.

“I (also) spend a lot of time looking for imprinted genes, those expressed according to the parent of origin, meaning (the gene from one parent) is expressed while the (gene from the other) is silent. PD

References omitted due to space but are available upon request.

—From News and Features, College of Agricultural and Life Sciences, University of Wisconsin-Madison

Molly Florent, University of Wisconsin – Madison