One reason why some suckler cows cannot get pregnant may be because they have male (Y) chromosome fragments in their DNA, according to a U.S. Department of Agriculture (USDA) study.

Reproductive efficiency is the most economically important trait in suckler beef production. As every Irish suckler producer knows only too well, when a cow does not produce a calf, the farmer does not make a profit, but still has to pay for feed, labour and other expenses. There is also a significant loss incurred if a cow or heifer has to be culled early, because of fertility-related problems.

With the help of commercial beef producers, Agricultural Research Service (ARS) geneticist Tara McDaneld examined reproduction data on about 6,400 females from cattle herds in Colorado, Florida and the U.S. Meat Animal Research Center (USMARC) in Nebraska. Her team then genotyped the animals, using a cost-saving genetic screening method called DNA pooling. This technique combines DNA from individual animals into a single pool.

Females usually inherit an X chromosome from each parent (XX), while males inherit an X and a Y (XY). In the study, only females were tested. The researchers found fragments of the male Y chromosome only in the pool of DNA from non-pregnant animals. All the results should have been XX among the females, according to McDaneld.

To verify their findings, scientists used an additional test called polymerase chain reaction (PCR), which is an inexpensive and effective means to identify fragments of the Y chromosome. Among animals with low reproductive efficiency, the PCR studyshowed that 25 percent of those females in the Florida population and 20 percent in the USMARC group had at least one significant chromosome-Y genetic marker. None of the highly reproductive animals had these markers, indicating that females were not getting pregnant because they carried Y chromosome segments.

ARS scientists are the first to identify the occurrence of chromosome-Y genetic markers in beef cows with reduced reproductive capacity.