In the beef industry, if a cow does not get pregnant after breeding, she becomes an economic liability in the herd. Lack of calf production can significantly reduce annual revenue for producers.
At the Agricultural Research Service’s Roman L. Hruska U.S. Meat Animal Research Center (USMARC) in Clay Center, Nebraska, scientists are developing genetic markers for economically important traits, such as reproductive ability, which can be used by producers to select efficient animals. They have also found extraordinary answers as to why some cows are not reproducing.
The USMARC research team led by geneticist Tara McDaneld has discovered one reason for reproductive failure is that some females have introduced segments of the male (Y) chromosome in their genome.
McDaneld, molecular biologist John Keele, and geneticist Larry Kuehn collaborated with producers in gathering reproduction data on several female beef cattle populations. They examined records, which indicated whether cows became pregnant in their first spring breeding, on about 6,400 animals from herds in Florida, Nebraska, Colorado, and at USMARC.
The team then used a cost-saving genetic screening method called “DNA pooling” to genotype animals. The technique combines DNA from many individual animals into a single pool for further evaluation.
“We decided to pool the DNA because individually genotyping the 6,400 animals would be very expensive,” McDaneld says. “We had two extreme phenotypes—animals that are pregnant and animals that are not pregnant.” Multiple DNA pools were constructed for each phenotype—pregnant and nonpregnant. Each pool contained contributions from about 100 animals, and all animals within a pool had the same phenotype.
Segments of the Y chromosome were found only in the pool of DNA from non-pregnant animals. Normally, females inherit an X chromosome from each parent (XX) and males inherit an X and a Y (XY).
“Considering all the animals were females, they should all have been XX,” McDaneld says. “There shouldn’t be any Y chromosome at all in the DNA.”
The first thought was that females with the Y chromosome could be “freemartins.” This condition, resulting from twinning in cattle, causes infertility in the female calf born at the same time as her male twin. Although the male twin is rarely affected by reduced fertility, the female twin is completely infertile in a high percentage of the cases. The reproductive tracts of freemartins do not develop normally. In addition, because of the blood exchange between male and female fetuses, the Y chromosome can often be detected in the female’s blood, Keele says.