Use of Gender-Sorted Semen
by Troy Smith for Angus Productions Inc.
NASHVILLE, TENN. (Aug. 5, 2010) — It can be argued that the most important genetic trait is gender. Depending on the specific mating and the target market, the sex of a calf can make a significant difference in its value. So it’s not surprising that producers would be interested in a reproductive technology allowing them to choose whether the result of a mating will be male or female.
CSU's George Seidel said one of the most practical applications for commercial producers using sexed semen might be to decrease dystocia by breeding first-calf heifers to have heifer calves.
During the Applied Reproductive Strategies in Beef Cattle (ARSBC) Conference in Nashville, Tenn., Colorado State University animal scientist George Seidel discussed the development and application of sexed semen for artificial insemination. He called it appropriate for some situations — but not without its limitations.
Seidel said the current method of sorting sperm according to gender is its greatest limitation. He explained how sperm are sorted one at a time, in series, and there are limits as to how fast this is accomplished. Sorted by flow cytometry, X-chromosome-bearing sperm (which produce females) can be distinguished from Y-chromosome-bearing sperm (which produce males) due to differences in DNA content. Actually, said Seidel, it can be accomplished at the rate of more than 20,000 sperm per second. The procedure is about 90% accurate.
However, more than half of those evaluated are undetermined. That results in fewer than 5,000 sperm of each sex sorted per second. That’s just one or two standard AI doses. And since a semen sorter costs around half a million dollars, sexed semen is more expensive, costing roughly $20 per dose more than conventional frozen semen.
According to Seidel, another limitation is that semen from some bulls does not sort well, so sexed semen is not available for all Ai sires. Additionally, sperm viability is compromised if semen is frozen, thawed for sexing and then refrozen. Consequently, bulls must be kept near the sorting laboratory so fresh semen can be sorted prior to freezing.
The fertility of sexed semen is lower than that of conventional semen, added Seidel, partly due to fewer sperm per dose but also because the sperm undergoes greater stress. Still, under ideal circumstances, pregnancy rates are only slightly depressed.
“With excellent management, pregnancy rates are 70 to 90 percent of that achieved with conventional semen,” said Seidel. “It works best with heat detection. When used with estrous synchronization, it works best when inseminating 12 to 24 hours after the onset of heat. It does not work well with timed AI.”
When used with superovulation, the number of good embryos produced is roughly half the number achieved with conventional semen. However, Seidel says when sexed semen is used with in vitro fertilization; it offers the advantage of fertilizing many oocytes with one dose of semen.
Due to the higher cost of purchasing sexed semen and even higher cost of lower fertility, most sexed semen is currently used by seedstock producers wanting to produce bulls or bull mothers from specific matings, or females to sell.
Seidel said one of the most practical applications for commercial producers might be to decrease dystocia by breeding first-calf heifers to have heifer calves. Another would be to increase herd biosecurity during herd expansion by targeting replacement-quality females and avoiding purchased females.
“Commercial producers may find sexed semen to be profitable under some circumstances. In my own herd, over the years, steer calves have been worth an average of about $50 more than heifers at weaning. This probably isn’t enough to justify sexed semen at current costs and success rates,” said Seidel. “The difference in value would probably have to be greater than $200 to make it work economically.”
In time, added Seidel, costs of sexed semen will decline and fertility may improve. New products, like the now available 75% accuracy sexed semen, will become available at lower cost. It’s also likely that sexed semen from more bulls will become available.
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