Reproductive efficiency is the most economically important trait in cow-calf production. When a cow does not produce a calf, the producer does not make a profit, but still has to pay for feed, labor and other expenses.
According to a USDA study, one reason why some cows cannot get pregnant may be because they have male chromosome fragments in their DNA.
With the help of beef producers, Agricultural Research Service geneticist Tara McDaneld and her colleagues at the agency's Roman L. Hruska U.S. Meat Animal Research Center in Clay Center, Neb., examined reproduction data on about 6,400 females from cattle herds in Colorado, Florida, Nebraska and at USMARC.
The team then genotyped the animals using a cost-saving genetic screening method called DNA pooling, which combines DNA from individual animals into a single pool.
"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."
As a result, the researchers constructed two pools – pregnant and nonpregnant. Each pool contained contributions from about 100 animals, and all animals within a pool had the same phenotype.
Females usually inherit an X chromosome from each parent (XX), while males inherit an X and a Y (XY). 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, which is an inexpensive and effective means to identify fragments of the Y chromosome. Among animals with low reproductive efficiency, the PCR study showed that 25% of those females in the Florida population and 20% 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.
"A lot of money goes into breeding an animal and keeping her long enough for her to get pregnant," McDaneld says. "Beef producers can use these PCR screening tools for chromosome Y before breeding, to test heifers and identify those that are less likely to consistently get pregnant."
"If a female calf is tested at birth and found to carry Y chromosome segments or markers, she can be used for meat production instead of for breeding," molecular biologist John Keele adds.
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The test could also be used to identify heifers that are unable to reproduce. In rare cases, calves are born with female physical characteristics but are in fact genetically male (XY). These calves have defects in the male development pathway, Keele explains, and they are infertile. This condition can now be tested for, and the affected animals can be culled from breeding herds.
A further benefit of this test is to identify potential breeding bulls that have Y chromosome segments in their X chromosome. These bulls will produce normal male calves, but all of their daughters will have a copy of the contaminated X chromosome. Consequently, the reproductive capacity of the bull's daughters will be potentially much poorer.
USMARC scientists are the first to identify the occurrence of chromosome-Y genetic markers in beef cows with reduced reproductive capacity.
Read more about this research in the April 2014 issue of Agricultural Research magazine.
Source: Sandra Avant, Agricultural Research Service