by: Heather Smith Thomas
One of the newer developments in the beef industry is use of sexed semen. AI has been used for many years to increase the number of progeny from superior sires, and embryo transfer enables producers to increase the number of offspring from a specific mating. Now we have technology to breed cows to a desired bull and choose the sex of the offspring. This is beneficial if a producer wants to have replacement heifers from his best cows.
Sexed semen has been used successfully for several years in the dairy industry and now there are several bull studs (including Select Sires, Genex, Accelerated Genetics and ABS Global) that offer sexed semen from beef bulls. The sexing service is provided by independent companies such as Sexing Technologies.
The process is improving but there are still some drawbacks. “Companies and researchers involved in the sorting are trying to identify factors related to sorting that impair sperm quality. They are making changes, to try to have less sperm damaged in the sorting process. But we still have a situation where in any ejaculate, 25 percent ends up sorted X, 25 percent ends up sorted Y, and the other 50 percent is damaged or can't be sorted. So we're still losing half the ejaculate to begin with,” says John Hall, PhD (Extension Beef Specialist, University of Idaho, Superintendent of the Nancy M. Cummings Research, Extension and Education Center).
The industry standard now for sexed semen is 2.1 million cells per straw. The industry standard for non-sexed semen is about 10 to 20 million cells per straw. Research looking at the effects of dose has shown that once you get above two million cells, however, you generally don't get much increase in pregnancy rate.
“There are some bulls in which an increase in the sperm numbers really helps their pregnancy rates, and in other bulls it doesn't. To increase the dosage--from the small amount of sperm you get from a normal ejaculate—doesn't always make much difference,” says Hall.
“Let's say you double the dose (putting twice as many sperm in each straw). This halves the number of straws that are available for people to use. But you may only increase pregnancy rates by five percent, at most. The cost benefit is not there.”
As a result of fewer sperm in the straw, and other things that happen to sperm during the sorting process, sexed semen pregnancy rates are generally lower than with non-sexed semen. During the sorting there is damage to some of the sperm. “Also, the acrosome reaction—the change that has to take place in the sperm so it can fertilize the egg—actually gets started during the sorting process, which means the life span of the sperm is shortened,” explains Hall.
There are also some factors on the cow side of the equation that we don't fully understand. “In general, what we see is a decrease in pregnancy rates of 10 to 20 percent with use of sexed semen compared with conventional AI. So for sexed semen to be feasible, you need to be a producer (or herd) that's doing well with conventional AI before you try sexed semen,” he says. You'll have better luck. Otherwise you may not want to risk the extra expense. Currently, sexed semen sells for $25 to $75 per straw.
“We've been fortunate in our research at this ranch, in that over a fairly large number of inseminations we've been getting 48 to 50 percent pregnancy rates with fixed time AI using sexed semen in post-partum cows. Our pregnancy rates to sexed semen averaged 52 percent during the past three breeding seasons (48 percent to 58 percent), while our pregnancy rates with conventional semen averaged 58 percent (52 percent to 68 percent),” says Hall.
“One of my theories is that the mature beef cow (who is already proven, in that she's had calves) in good body condition is about as fertile as any female on the ranch. By contrast, heifers are still an unknown quantity. They might be cycling, but might not be able to conceive yet. The dairy industry, however, puts most of their semen into heifers because it's so hard to get the lactating dairy cow to breed,” says Hall.
“Sexed semen is working satisfactorily in embryo transfer programs. But we see a decrease in the number of transferable embryos when using sexed semen versus conventional semen.” Researchers noted a 28 percent to 35 percent reduction in the number of transferable embryos when using sexed semen.
The producer may still find use of sexed semen beneficial, as when trying to get more heifers from a particular mating. The male counterparts may not be worth as much in this situation as the heifers. So even with decreased number of transferable embryos by using sexed semen, if 90 percent of those embryos are the sex you want, this would be satisfactory.
“One of my colleagues back East did a large study with sexed semen and ended up with 30 percent pregnancy rate, as opposed to 55 percent for the conventional semen. We're still trying to figure out why the rate was so low. One thing that might be a factor is that they used a different synchronization system than some of the other groups have used. It's one the Beef Reproductive Task Force recommends, but maybe our research here at the University of Idaho ranch picked the one that works the best,” says Hall.
“Another thing to consider with sexed semen is that because you have fewer sperm cells (a quarter cc straw instead of a half cc straw), all the things with semen handling that a person does at the chute must be that much better,” he explains. There can be no sloppiness.
“If producers are going into this with their eyes open, I think it's a great technology that will continue to improve. Right now it's still in the early stage, but as long as producers understand the risks, some will be wanting to try,” he says. One of the biggest limitations for beef producers at this point is that a relatively small percentage of AI bulls are available for sorting, which limits the options for genetic selection.
“Right now we are still trying to figure out all the factors—because sometimes we are really pleased with the results and other times not so pleased. We still have a lot to learn about it,” says Hall.
The number of bulls available for sorting will eventually increase. “I talked with people from the bull studs and asked how many beef bulls they offered with sexed semen in 2008 versus 2011. We've gone from virtually none in 2007-2008 to a decent number today. The company that does the sexing is looking at someday doing more custom sexing; they may eventually be able to sort any particular bull that the customer wants to use. Currently, one bull stud is offering 14 bulls that are sexed, compared to maybe100 beef bulls they normally offer.” The opportunity for genetic selection, using sexed semen, is still limited, but slowly expanding.
THE TECHNOLOGY - The process is based on the fact that the X chromosome has 3.8 percent more DNA than the Y. The machines can read that difference in DNA content. Semen is stained with a fluorescent dye and then passed through a machine (flow cytometer) that can sort the sperm as it goes by in a stream of single droplets. The female producing sperm with the X chromosome shines brighter than the Y because the X chromosomes are 3.8 percent larger and have absorbed more dye. A laser in the sorting machine determines the gender of the sperm based on the amount of light it emits.
The flow cytometer uses focused laser light to illuminate cells as they pass by the laser beam one at a time in a fluid stream, traveling 60 miles per hour or faster; up to 8000 sperm per second are sorted and processed. This seems fast, but it takes about 3 to 4 times longer to process sexed semen than to process conventional semen for shipping or freezing.
The X-bearing sperm are sorted off in one direction, the Y-bearing sperm in another, and anything of undetermined sex passes straight through as waste. The sorted sperm are then frozen in .25 cc straws to be used for AI. An ejaculate yields fewer straws of sexed semen than conventional semen.
One advantage to the sorting process (which also helps make the sexed semen more successful in low dosage than regular sperm) is that damaged or dead sperm are sorted off. A normal ejaculate always has a certain amount of dead, dying or damaged sperm cells, so when it's sorted by the sexing process, these are eliminated -- making the remaining sperm more viable.
The highly purified groups of sorted semen are then frozen for future use in AI, to enhance AI programs or embryo transfer. For many years the drawback in using sexed semen was the difficulty in getting enough sorted sperm from an ejaculate to make it practical. Studies at Colorado State University helped make it feasible. Their theory was that since it only takes one sperm to fertilize an egg, why use 20 million (the recommended amount for optimum reproduction)? They began experimenting with low dose insemination for AI, and it worked--especially if the cow was at the optimum point in her heat period. Today, advanced reproductive techniques like in vitro fertilization (where the egg is fertilized in the lab and then used in embryo transfer) make sexed semen even more practical, since only a tiny amount of sperm is needed for fertilizing a large number of eggs. Sexed semen works well (with highest success rates) in a well-managed AI or ET program in which timing of insemination is optimum for proper fertilization of the egg, but probably works best in an IVF program. Some variability has also been noted with different bulls; semen from various bulls goes through the sorting machines differently, so some bulls are better candidates for sexed semen than others.
BENEFITS OF USING SEXED SEMEN - When using sexed semen, the results are predictable about 93 percent of the time. It can be a handy tool to increase your heifer numbers or steer numbers. This option is very useful for the dairy industry, since there is always a market for heifer calves but not for the bull calves. In the beef industry, it is handy to be able to select the gender of calves if the producer wants to keep replacement females from certain cows/bulls, or wants just steers from a terminal cross.
In any given herd, the option of sex determination could be useful for reducing calving difficulty in first calvers (since heifer calves are typically smaller at birth than bull calves), or for producing a higher number of good females to choose from as replacements if you want to expand your herd size without buying outside cattle. If you can keep a closed herd, not having to depend on bringing in any other females, you can cut down on some of the health problems that may be inadvertently introduced, and in many cases you also have better knowledge of the genetics in your own herd. If you want your best cows to have heifers, this can give you faster genetic progress in improving your cow herd, enabling you to keep more good females and cull more deeply.
A producer may want heifers from the top 10 to 15 percent of their cows, and steers from the rest of the herd. When the market is good for replacement females, breeders may opt for more heifers, and when the market is better for steers they may choose to produce mostly male calves. Seedstock producers may select bloodlines for maternal qualities for brood cows and others they are hoping to use for marketing bulls. This technology gives producers the choice, and also enables them to develop an early strategy for a potential future market.
In female sales, for instance, some breeders are finding their bred cows and heifers worth more when bred with sexed semen. Bred heifers guaranteed to have heifer calves (and less calving problems) may bring a premium. Other buyers may want females that will produce only male calves. Seedstock producers marketing pen lots of bred females or pairs may find an advantage in being able to offer cattle guaranteed to have one sex or the other, or may find a premium in offering exceptional female bloodlines in a 3-way package--a cow with heifer calf at side and bred back to have another good heifer calf.
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