SELECTIONS - Winter 2005
Sexed Semen: Is It Finally a Reality?
By Mel DeJarnette, reproduction specialistMost of us have heard the rumor that sexed semen
is "just around the corner" for as long as we have
been aware of A.I. Through the years, countless numbers
of techniques have been investigated with little to no
indication of repeatable success.
However, in the 1980s a breakthrough in semen sexing
technology was made by USDA researchers in the Lawerence
Livermore Laboratory in California. The patents for this
technology were licensed to a company named XY Inc. of
Fort Collins, Col., which has performed a considerable
amount of research during the 1990s to optimize efficiency
of these sorting procedures.
Commercialization of sexed semen in the United States has
been initiated with a 2003 license granted to Sexing Technologies
(ST) in Navasota, Texas. Select Sires and ST are partnering to
bring highly proven, Gender SELECTed™ Sires to U.S. dairy producers.
Flow-Sorting Technology
The only repeatable technique to sex sort sperm uses a device
called a flow-cytometer to detect a 3 to 4 percent difference
in DNA content between male and female sperm. The first step
in this procedure is to dilute sperm to a very low concentration
and stain them with a fluorescent dye. The sample is then sent
through the flow-cytometer at 60 mph under 40 to 60 psi of
pressure. As sperm pass through the internal laser beam,
the fluorescent dye is excited.
Because of the larger X chromosome, female sperm emit slightly
more light than male sperm, which possess the smaller Y
chromosome. Detectors measure the amount of fluorescence
and assign positive or negative charges to each droplet
containing a single sperm. Charged deflector plates then
split the single stream into three streams: positively
charged particles containing one sex go one way, negatively
charged particles containing the other sex are deflected
in the opposite direction, while uncharged droplets containing
multiple sperm or sperm with unidentified sex pass straight
through. Confirmed with tens of thousands of offspring born
in world-wide research trials, the procedure separates sperm
of the two sexes with approximately 90 percent purity.
Technology Limitations
There are several major limitations that have stifled
implementation of sex-sorted semen. Without question,
reduced conception rate has been a primary hurdle. As
you can image from the description above, sex sorting
of sperm is a highly invasive procedure that negatively
impacts sperm viability and longevity compared to normally
cryopreserved semen.
In addition, the procedure is extremely slow and inefficient.
To properly sort, sperm must be precisely oriented as they
pass through the laser and fluorescence detectors in the
flow cytometer. Due to the flat shape of bovine sperm heads,
only about 30 percent are correctly oriented and half of
these are female. Thus, only 15 percent of the sperm going
into the machine are recovered as a marketable, sexed product.
Although the 3,000 to 5,000 sperm of each sex sorted per
second sounds like a lot, this translates into approximately
1.3 hours of sorting to process enough semen for a standard
20 million sperm/straw dosage. Thus, due to the slow sorting
speed, commercialization is only possible with very low sperm
numbers per straw (about 2 million). If these limitations were
not enough, the high cost of flow cytometry equipment (approximately
$250,000 per machine) and intensive amounts of highly skilled
labor required to sort sperm dictates that sexed semen will not
be inexpensive. Because of the low sperm numbers per dose and
compromised sperm viability, sex-sorted semen is only recommended
for use in well-managed, highly fertile virgin heifers. While
many research herds have realized very acceptable conception
rates, averages indicate well-managed herds that achieve 60 to
65 percent conception rates in virgin heifers with normal semen
can expect 45 to 55 percent conception rates with sexed semen.
Sex-sorted semen is not recommended for lactating cows. Sex-sorted
semen should also not be used when flushing for embryo transfer.
Is sexed-semen ready for the market and is
the market ready for sexed semen?
Select Sires would like to state openly and honestly from the
onset with no sugar-coating of the subject: We don’t know
the answers to these questions.
After describing the facts and background of sexed-semen technology,
many producers suggest they would still like to give it a try.
Select Sires is introducing Gender SELECTed Sires to the U.S.
market in a cautious effort to try and answer these questions
while providing a valued product and service to our member
owners. Although the high rate of sperm loss precludes use of
our "most elite" sires for production of sexed semen, we are
making available semen from several genetically outstanding,
proven Holstein and Jersey sires for research herds willing
to help us answer these questions.
The return on investment for the dairy producer depends on a
complex interaction between the initial conception rate with
non-sexed semen, the percent reduction in conception (if any)
due to use of sexed semen, the price differential between
sexed and conventional semen, and the value differential between
bull and heifer calves. Most of these factors will change
considerably from herd to herd, which differentially affects
the breakeven value of sexed semen to each respective producer.
Figure 1 illustrates how the conception rate of both non-sexed
and sexed semen interacts to influence the breakeven value of
sexed semen to the producer. By breakeven value I mean: How
much extra (not counting genetic value of semen) can the producer
afford to pay for sexed semen and still show a return on investment.
Even when sexed semen conception rates are 100 percent of non-sexed
(i.e., no reduction), it’s clear from the back row that herds with
higher conception rates have a higher breakeven value and thus a
greater opportunity to harvest a return on investment. However,
irrespective of the initial conception rate, reduction in
conception with sex sorted sperm can be tolerated and still be
profitable. Gender SELECTed Sires are being offered at a price
of $45, which includes the genetic value of approximately $15 per
sire. Thus, only $30 “extra” is being paid for the sex sorting
procedure. The green bars in the figure above correspond to
conception scenarios where a profitable return on investment
is highly likely. The red bars illustrate non-profitable scenarios
and yellow bars are basically borderline or breakeven. Previous
research has demonstrated that many herds can easily harvest a
return on investment from the use of sexed semen at these prices,
however, we are equally confident that some herds will not.
It is for these reasons that Select is cautiously introducing
Gender SELECTed Sires to the U.S. market in the form of a
controlled research trial. To optimize opportunity for success
and considering the fact that lower conception rate herds have
marginal economic capacity to tolerate conception rate reductions,
we must insist that research herds have a track record of achieving
conception rates of 60 percent or better in virgin heifers with
non-sexed semen. Research herds must also agree to abide by the
following "Keys to Success" to ensure optimum probability for
conception.
Keys to Success
Use of sexed-semen will require a breeding gun designed
to accommodate the smaller diameter ¼-cc straws. Straws
are to be thawed and handled identical to their ½-cc
counterparts. However, the smaller diameter and compromised
semen quality will make them much more sensitive to cold
shock and errors in semen handling. To maximize potential
for success:
- Thaw straws in 95°F water bath for 45 seconds.
- Semen thawing and handling environments should be
warm and draft free.
- Warm all semen handling equipment including guns, sheaths,
and paper towels prior to contacting straws.
- Only highly experienced technicians should use this product.
- Use only in well-managed, virgin heifers that have achieved
greater than 60 percent of their mature weight by 14 months
and are in moderate or better body condition.
- Inseminate heifers eight to 12 hours after observed estrus (AM/PM Rule).
- Use of estrus synchronization and breeding to observed estrus
is encouraged, but use of timed-A.I. in the absence of observed
estrus is discouraged.
Can Anyone Purchase Gender SELECTed Semen?
Once the initial research semen has been allocated, there will
be limited opportunities for other herds to purchase semen from
Gender SELECTed Sires. However, Select Sires only condones use of
this product in virgin heifers and only if 60 percent or better
conception rates can be achieved with non-sexed semen. Again,
Gender SELECTed Sires should not be used in lactating cows or
in animals super-ovulated for embryo transfer (cows or heifers).
Semen Identity and Packaging
Sex-sorted (female) semen will be packaged in pink ¼-cc straws.
As shown above, each straw will be labeled with the sire's complete
identification as would be on a normal straw. Additionally, the
phrase "Sexed Female" will be printed at the beginning of the
identification line and the number "203" will be inserted just
prior to the freeze code to represent the semen processing
laboratory. Straws will be packaged with 10 straws in a single
goblet placed on the bottom of each cane, rather than the typical
two goblets with five straws each. Each cane of sexed semen will
be identified by a pink cane top with black lettering identifying
the sire’s NAAB code number, short name, and the phrase "Sexed-F."
Female Probabilities
Research has consistently demonstrated that this technology
produces sexed-sorted sperm with about 90 percent purity.
However, that still leaves 10 percent male sperm available
to compete for fertilization. Odds and ratios tell that in
about 26 percent of the herds, 80 percent or less of the
offspring will be female. Seven percent of the time, 70 percent
or less of the offspring will be female. These are simple
mathematical probabilities of which the herd owner should
be aware before purchasing Gender SELECTed sires.
Summary
There is no question sex-sorted sperm for gender selection
is a reality. Whether or not the technology is sufficiently
developed to offer a return on investment to enough producers
that it will be a commercially sustainable product is yet to
be determined. In an effort to answer these questions while
offering our customers the opportunity to use the latest in
animal breeding technologies, Select Sires is cautiously
introducing Gender SELECTed Sires to the US market. Pending
results of this research phase in mid-to-late 2005, the program
will either be maintained, expanded or discontinued completely.
We’ll keep you posted as we proceed.
Select Sires makes no guarantees, expressed or implied, as to the actual conception rates and (or) gender bias that may be
achieved in any given herd.
Return to SELECTIONS Table of Contents |
