Sexed Semen Primer

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Consistently delivering heifer calves has gone from dream to reality, thanks to cell-sorting technological advancements in recent years. And, sexed semen has moved from the research lab to commercial use.

As with any technology, challenges and opportunities exist with sexed semen. It can yield substantial benefits to producers who incorporate it properly; likewise, it may vex dairy managers with its lower conception rate and higher per unit cost. Therefore, do your homework to determine how it may best fit your operation.

Here is updated view of sexed-semen technology from the following experts: Bill Thatcher, dairy reproductive specialist at the University of Florida; Mike Overton, veterinarian at the University of Georgia; John Fetrow, veterinarian at the University of Minnesota; and Kent Weigel, extension dairy cattle genetics specialist at the University of Wisconsin.

Q: What is sexed semen?

Essentially, sexed semen is semen that has been mechanically manipulated to contain significantly higher-than-normal concentrations of X- or Y-bearing sperm. Understandably, semen that contains mostly X-bearing sperm is of great interest to the dairy industry, since it offers a chance to increase heifer-calf births. Conversely, beef producers gravitate toward semen with higher concentrations of Y-bearing sperm.

The concept has been around for years. But it’s only been in the last several years that flow cytometry — the technique for separating X- and Y-bearing sperm — has been advanced for commercial use, allowing sexed semen to become more widely available.

Q: What are its benefits?

Increasing your heifer-calf population is the immediate answer, and the one that most interests producers. Sexed semen is usually about 90 percent pure for the chosen gender, so your odds of getting a heifer calf from a specific straw of semen increase dramatically compared to conventional semen.

Therefore, producers should be able to create internal herd growth faster with sexed semen. And these additional heifers should enable you to aggressively cull poor-performing heifers, as well as those with lower genetic merit.

Theoretically, once a large-enough portion of the industry adopts sexed semen, there will be an adequate supply of female dairy calves to meet the industry’s demand for replacements — currently a limiting factor.

Secondly, and maybe more importantly, the technology offers biosecurity benefits. By sourcing heifers from within your own herd, you avoid the risk of bringing in infectious diseases, or increasing their prevalence. Plus, these heifers are covered by your vaccination protocols, not someone else’s protocol — or lack thereof.

In addition, heifer calves reared on your operation are exposed to existing pathogens in the environment, and therefore, be more likely to have some degree of immunity at first calving — unlike imported animals.

Finally, much has been made about the sexed semen’s potential to decrease dystocia in first-lactation cows. Since heifer calves are usually smaller than bull calves, sexed semen use is logically assumed to reduce dystocia rates and its affiliated cost. This can be true on an individual-animal basis, but the effect on a herd-wide basis is almost negligible. According to a recently developed comprehensive computer model, the herd-level impact of sexed semen on dystocia is only about 0.6 percent at a savings per calving of about $1.

Q: Are there drawbacks to the technology?

First and foremost, conception rates for sexed semen are significantly lower than for conventional semen. The sorting process is hard on sperm cells; in fact, 70 percent or more of sperm cells fail to be sorted due to damage or because the flow cytometer could not determine the sex of the cell. Furthermore, a conventional semen straw contains about 20 million cells, while a straw of sexed semen contains about 2 million. These straws are more sensitive to cold shock and semen-handling errors.

Add these factors together, and conception rates for sexed semen consistently decrease about 15 percent or more versus conventional semen. For example, if you achieve a 55 percent conception rate for your heifers with conventional semen, expect a conception rate of about 39 percent with sexed semen.

This reduction can add months to age at first calving if you are not careful. If you struggle with conception rates with conventional semen, improve your overall reproduction program before adopting sexed-semen technology.

In addition, sexed semen is not generally available from bulls with the highest genetic merit. 

Remember that while flow cytometry has significantly improved, it is still a relatively slow method. Machines can generate about 150 to 200 straws of semen per day, but the entire industry uses about 44,000 straws of semen daily. This adds to semen unit cost and availability.

Q: What’s the best strategy to use?

For now, sexed semen is only recommended for use on well-grown virgin heifers. And you must have accurate heat detection and well-trained inseminators to institute the technology. So, the best sexed-semen strategy for your dairy depends on a number of factors, including economics and genetic merit. Be sure to bring your entire reproductive-management team on board before implementation.

One approach is to use sexed semen for first-service breeding of heifers, and then follow up with conventional semen for subsequent inseminations. This has been shown to result in female offspring of 62 percent or greater.

Another option is to use sexed semen with in-vitro fertilization and embryo transfer. If used with marker-assisted selection, the technology would enable you to generate calves with more desirable traits, or to select away from troublesome traits like complex vertebral malformation.

The key to any strategy is to improve the genetic merit of ensuing heifers. If you use sexed semen without paying attention to genetic merit, then you gain nothing genetically from the cow side of the equation. However, if you source more of your replacement heifers from the better cows in your herd by using sexed semen, then you gain genetic merit from those female offspring from both sides of the breeding equation. This means you must be able to reliably rank your breeding pool based on genetic merit, and there certainly are tools and services to help you do so.

Ultimately, the decision to use sexed semen depends on three drivers: the magnitude of conception rate drop, the additional cost of sexed semen and the differential value of heifer calves versus bull calves. Most computer models suggest judiciously using sexed semen for first and sometimes first and second service for virgin heifers, depending, of course, on these three factors.

Q: What’s the latest sexed-semen research?

Recent research has focused on the use of sexed semen in lactating dairy cows at specific points in the reproductive cycle. Although initial results are promising, don’t implement the technology in your lactating herd just yet; it is still only recommended for use on well-grown virgin heifers.

Management helps overcome conception-rate disadvantage

Semen type

 

Sperm/unit

 

Herd reproduction level

 

Conception rate

 

Sexed

 

2 million

 

Below average

 

21%

 

Sexed

 

2 million

 

Average

 

37%

 

Sexed

 

2 million

 

Above average

 

35%

 

Unsexed

 

20 million

 

All

 

58%

 

Source: Journal of dairy science 87: E120-E130

 

Results from this Wisconsin field trial with 816 Holstein heifers indicate that your herd’s reproductive performance has a significant impact on the success of sexed semen.

 

 



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