Sex-sorted semen: What researchers have learned

By Dr. Joseph C. Dalton
University of Idaho

Sex selection is arguably one of the most sought after and misunderstood reproductive technologies in history.

More than 2000 years ago, Greek philosophers suggested that the right testis produced males, and the left testis produced females. Of course, this is not the truth, as there are two populations of sperm – called X-chromosome-bearing and Y-chromosome-bearing sperm.
In 1983, a collaborative group of researchers from Oklahoma State University, USDA, and Lawrence Livermore National Laboratory reported on the use of a flow cytometer to determine the difference in DNA (genetic material) content between X- and Y-chromosome-bearing sperm from cattle, sheep, pigs and rabbits.

Unfortunately, the initial procedure killed the sperm, and it wasn’t until the late 1980’s that USDA researchers refined the procedures to sex-separate live sperm via flow cytometry/cell sorting. The USDA patented the sperm-sorting technology in 1991.
During the late 1990’s, XY Inc., a Fort Collins, Colo.-based company working with researchers at Colorado State University, further developed technology and procedures that improved upon USDA’s work.

In early 2005, sex-sorted semen became commercially available in the United States. As data is accumulated by researchers and AI studs, the answers to the following questions are becoming clearer:
• What are the results following use in heifers?
• What results can you expect?
• What are the results following use in lactating cows?
• Why is there a lowered conception rate following the use of sex-sorted semen?

Results: Heifers

Sex-sorted semen research and commercial use has traditionally focused on heifers because well-managed heifers tend to have first-service conception rates (resulting from AI with frozen-thawed, unsexed semen) of up to 70%, compared to less than 40% in lactating cows. Nevertheless, nearly all of the early research trials in dairy heifers using a similar dosage (1.5 to 2.0 million sperm cells) to what is available today (2.1 million sperm cells) provide evidence of decreased conception rates following AI with sex-sorted semen as compared to unsexed, control semen.

As shown in Tables 1 and 2, the range in conception rate following AI with sex-sorted semen was 31% to 65%. Furthermore, the conception rate achieved following the use of sex-sorted semen, expressed as a percentage of the conception rate achieved using unsexed control semen, was 44% to 90%.

Lastly, there is no evidence that semen deposition into the uterine horns enhances conception rate as compared to deposition into the uterine body (Tables 1 and 2).
What about more recent data? In a retrospective study presented by Select Sires at the 2007 American Dairy Science Association Annual Meeting, 16,587 services to sex-sorted semen (2.1 million sperm per dose) were evaluated and the average conception rate was reported to be 44%.
Furthermore, the conception rate achieved following AI with sex-sorted semen averaged 85% of that achieved with unsexed, control semen at first service. In fact, 74% of herds achieved a conception rate ≥ 70% of that obtained with unsexed, control semen at first service. In the 25 herds that used ≥ 100 doses of sex-sorted semen, the conception rate to sex-sorted semen averaged 48% (range 38 to 72%), compared to a first-service conception rate to unsexed control semen of 54% (range 38% to 70%).

What can you expect?

There is ample data in dairy heifers to support the expectation of an average conception rate to sex-sorted semen of approximately 70% to 85% of the conception rate to unsexed control semen used at first service. Consequently, if you currently achieve a 65% conception rate at first service in your heifers with frozen-thawed, unsexed semen, you can expect to achieve (with good management) a conception rate between 46% to 55% with frozen-thawed, sex-sorted semen at first service. As can be seen from Tables 1 and 2 and the data in the preceding paragraph, there is large variation in conception rates following AI with sex-sorted semen. This is not surprising as the level of management plays a role in the success or failure of any new technology, and therefore must be considered.

Most reports describe an accuracy of approximately 90% in the predetermination of the sex of calves born. Furthermore, calves born as a result of the use of sex-sorted semen are normal, as no differences in neonatal death rates, birth weights, weaning weights, gestation length, or incidence of abnormalities have been reported by researchers.

Results: Lactating cows

Although the current recommended use for sex-sorted semen is in virgin heifers only, recent research has focused on use in lactating dairy cows. Researchers from Finland reported an average conception rate of 21% with sex-sorted semen (2.0 million sperm per dose) and 46% with conventional, unsexed semen (15 million sperm per dose) following first service in Holstein cows. No synchronization programs were used in this study. The conception rate achieved with sex-sorted semen was 45.6% of conventional, unsexed semen. No synchronization programs were used in this study.

In a study conducted by U.S. scientists, Holstein cows (lactation 1 to 4 and between 20 to 140 days in milk) received AI with sex-sorted semen or conventional, unsexed semen at natural heat, or after synchronization with prostaglandin or Ovsynch. The overall conception rates were 25% and 37.7% for cows in the sex-sorted semen and conventional, unsexed semen groups, respectively.

Positive conception figures

The conception rate achieved with sex-sorted semen was 66.3% of conventional, unsexed semen. Cows receiving AI with sex-sorted semen following a natural heat had a greater conception rate than cows that were synchronized with either prostaglandin or Ovsynch. Furthermore, conception rates following AI with sex-sorted semen at 100 days in milk (or more) were approximately 8 percentage points higher than earlier in lactation, and greater than 6 percentage points lower in older cows (third and fourth lactations) than younger cows.
Consequently, these researchers concluded that the highest conception rates following AI with sex-sorted semen may be achieved in first and second lactation cows exhibiting natural heat and greater than 100 days in milk.

Selecting ‘repro normal’ cows

Another study conducted by XY Inc. focused on the efficacy of selecting only “reproductively normal” lactating dairy cows for timed AI (Ovsynch) with sex-sorted semen. All cows (lactation 1 to 9) were initially enrolled in Presynch (two injections of prostaglandin 14 days apart). Ultrasonography was performed approximately 14 days after the second prostaglandin injection, and only those cows with normal ovarian and uterine status (55% of the total evaluated; average days in milk = 72) were enrolled in Ovsynch. Timed AI was performed 16-19 hours after the second GnRH injection of Ovsynch, and occurred, on average, at 82 days in milk.

The overall conception rates were 40.4% for cows inseminated with sex-sorted semen, and 55.6% for cows inseminated with conventional, unsexed semen. The conception rate achieved with sex-sorted semen was 72.6% of conventional, unsexed semen. Although the researchers concluded that conception rates of presynchronized, reproductively normal lactating cows following timed AI were acceptable, the researchers cautioned that the study was very small (115 animals) and should not be over-interpreted.

Why the lower rate?

Why is there a lowered conception rate following use of sex-sorted sperm?
The decreased conception rate following the use of sex-sorted sperm may be due to:

1. sperm injury during the process of staining prior to flow cytometry,

2. exposure of sperm to a powerful laser beam during sorting,

3. centrifugation to concentrate the sperm prior to filling straws, and

4. the current inability to determine before separation if a semen sample will be able to withstand sex separation, freezing and thawing, and retain acceptable fertility. It is widely known that sperm from different bulls differs in the ability to withstand freezing. Therefore, sex-sorted sperm from different bulls will most certainly differ in the ability to tolerate sexing and freezing.

Final thoughts

The inherent lower fertility of lactating cows makes the use of sex-sorted semen in cows more problematic than in heifers. Consequently, usage of sex-sorted semen in lactating dairy cows is still not recommended. The current recommendations for sex-sorted semen are:

• Use in well-grown, well-managed heifers.

• Administer AI approximately 12 hours after observation of heat.

• Thaw straws using warm water (95 to 98 degrees F) for a minimum of 45 seconds.

• Do not use sex-sorted semen in timed AI programs.

Accurate heat detection and well-trained inseminators will be mandatory to maximize fertility with sex-sorted semen. In general, herds with higher conception rates have a greater opportunity to reap a return on their investment as these herds may be able to tolerate a reduction in conception rate as compared to herds with marginal to low conception rates.

Overall management crucial

In addition, sex-sorted semen will most likely generate a greater return when heifer values are high and when it is used to inseminate the most genetically advanced heifers. Herds with poor management, inaccurate heat detection, and improper semen handling will likely experience plummeting conception rates while using sex-sorted semen.

FYI
■ To contact Joseph C. Dalton, University of Idaho Extension dairy specialist, Caldwell Research and Extension Center, call 208-459-6365 or e-mail jdalton@uidaho.edu

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