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Twin’s gender impacts ewes’ production

by Wyoming Livestock Roundup

Laramie – Females born twin to males are affected by uterine exposure to testosterone in varying degrees. In cattle, over 92 percent of heifers born twin to a male are freemartins and unable to reproduce. In littering species, the presence of a high number of males results in the females reaching puberty later and having a shorter reproductive life than those born to litters with higher numbers of females .
University of Wyoming professors Brenda Alexander and Gary Moss say these notable differences led them to ask about the impacts of a female lamb born twin to a ram in sheep.
“I’m very interested in reproductive development and find it very interesting that while females born co-twin to a male in cattle are almost always freemartins and infertile, sheep have multiples that are often of the opposite sex time and we think nothing of it. Even if it’s subtle, I questioned if there was impact on the female’s reproductively if she was born twin to a male,” says Alexander.
“There’s data out of the U.S. Sheep Experiment Station in Dubois that shows two male twins have more sex drive than a male born twin to a female, so that raised the question of what was going on from a production standpoint in the female twin,” adds Moss.
Their questions re-sulted in a study of lambing records from UW’s purebred sheep flock to determine if the sex of a co-twin affects number of offspring, flock longevity and age at first lambing. A Western Section of the American Society of Animal Science (WSASAS) paper says that nine years of lambing records from 1995 through 2003 for Columbia, Hampshire, Rambouillet and Suffolk ewes were analyzed.
“Some of our early challenges were deciphering, organizing and entering all the data into an Excel spreadsheet. Some of the older records were hand written and it took a lot of time,” notes grad student Valerie Uthlaut.
The WSASAS paper states the sex of a ewe’s co-twin influenced her subsequent reproductive lifetime prolificacy.  Ewes born co-twin to a female produced approximately 10 percent more lambs in their lifetime than ewes with a male twin .
“Another interesting thing we found is that ewes born co-twin to a ram have the same level of productivity as ewes born as singles. A lot of people don’t keep ewes born as singles as replacements because they think they will be less productive, but if they’re born co-twin to a ram their production is similar to single born ewes, according to our results,” notes Uthlaut.
“Next we need to determine where the production loss occurs and how meaningful it is. For example, if the impact is on longevity and the producer is culling everything after six years of age and the loss occurs after that, there is no effect. But, if the impact is on the age of puberty it becomes much more meaningful to the producer.
“Our job now is to figure out where that production loss occurs and to assure it’s not an affect specific to a certain breed, which I would be surprised if that were the case,” says Alexander. She adds that UW is currently working with Montana State University and the USDA Meat Animal Research Station in Nebraska to increase the current dataset on this project.
“I think it’s really exciting from a developmental standpoint. It’s also fun to be involved in a project that is both scientifically important and producer relevant,” says Moss. “There are implications from this that extend beyond sheep as far as understanding heredity of mammals and neurological development.”
Moss adds that while determining co-twin sex may be difficult in range flocks, those who shed lamb, or dock and brand at lambing, could easily introduce an ear-notch or other form of identification to show which ewes are born co-twin to another ewe. That form of identification could later be used when selecting herd replacements.
“Even though the propensity of twinning is a lowly heritable trait, it is economically important and easily selected for,” notes the ASAS paper. The potential to increase herd production by 10 percent simply by selecting replacement ewes born co-twin to another ewe is worth considering for producers.
For more information contact Brenda Alexander at 307-766-6278 or Gary Moss at 307-766-5374. Heather Hamilton is editor of the Wyoming Livestock Roundup and can be reached at

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