“Sex is determined genetically,” Dr. George Seidel, Colorado State University, said. “In my opinion, sex is the most important genetic trait.”
The Dairy Cattle Reproduction Council recently hosted a webinar presentation, “Ten Years of Sexed Semen in North America,” given by Dr. Seidel. Dr. Seidel became involved in the industry when it was publically funded by universities, during the 1990s. By 2002, the first commercial license for sexed semen was obtained by a private company.
“It has undergone more commercialization since. Funding has changed from public to private,” he said. “Private funding has made sexed semen practical.”
Sexed semen was made commercially available to the dairy industry in 2006, and its use increased dramatically until 2009. Since then, its popularity has remained relatively stable, with about 18 percent of dairy heifers in the United States being bred with sexed semen.
Process of sexing
Initially, semen was studied by the United States government, testing the effects of radiation exposure, during the 1980s. From there, it was discovered that separation of sperm, depending on whether they had an X or a Y chromosome was possible. This separation can’t be done simply by physical characteristics, however, as phenotypically the sperm are identical, “so you get a 50/50 sex ration” naturally, Dr. Seidel said.
Researchers found that the X chromosome has more DNA, and began to dye the sperm. This caused those with more DNA — the X sperm — to fluoresce a bit brighter, but only by an average of four percent, a difference which cannot be discerned with the human eye. The process requires computer analysis.
This analysis occurs in a flow cytometry system, where the dyed sperm are exposed to a given wave length of light — causing them increased activity — via a laser. The sperm are pumped at high rates of speed, in a stream of fluid, and sorted into three categories: X, Y, or unsure. Sperm carrying the X gene are given a positive charge, those with the Y a negative charge, and those not distinguishable are not given a charge. The sperm, suspended in their charged droplets, are then sorted out as they pass through an electrical field.
Once sorted, the droplets containing the sorted sperm are discharged, at 50 mph, into a test tube containing fluid, to which food coloring is added. If the sperm has damaged cell membranes, it will take up the dye, and is discarded. Intact sperm are then separated out for use.
“Only live sperm are collected. We throw out a lot of sperm in the course of sexing,” Dr. Seidel said.
The sperm are further sorted by the actually likelihood of carrying either an X or Y chromosome, discarding those who are least likely to be correctly marked. In this manner, 40 percent of the sperm — including those in droplets that contained more than one sperm — are discarded. This leaves 30 percent with the most likelihood of having an X chromosome, and 30 percent most likely to have a Y.
The system is “very expensive. These systems are remarkable,” Dr. Seidel said. “The more pure, the more expensive.” The industry standard is currently at 90 percent purity.
The sperm are “insulted” multiple times throughout the process. The multiple steps, including the use of lasers and dye, speed, dilution, centrifugal forces and freezing, can all damage sperm. But the system works, and is the only process available that does work to sex semen, he said.
“There’s just no difference in the calves,” bred via sexed semen compared to those bred with unsexed semen, Seidel said.
Calves from sexed semen can be bred to sexed semen, with no impact on the next generation.
Costs and methods
A typical dose of sexed semen contains two million sperm. Raising this dose to 10 million slightly improves conception rates.
“If you increase the number of sperm five times, you increase pregnancy rates a little bit,” Dr. Seidel said, but also increase the cost.
The cost of sexed semen today is about $15/dose above the cost of unsexed semen. But the pregnancy rate with sexed semen is lower than unsexed, which is the biggest expense, he said. Semen needs to be collected from the bulls more frequently if it is being sexed. When using A.I. on lactating cows, the sexed semen pregnancy rate is lower than unsexed, making sexed semen unprofitable in this group.
“You’ve got to use like four doses of sexed semen to get one calf,” he said, with pregnancy rates at about 25 percent in A.I. lactating cows.
Sexed semen is not as viable as unsexed semen, so it can’t be waiting around for an egg to fertilize. To increase the pregnancy rate with sexed semen, it is best to breed late so the egg is waiting, Dr. Seidel said. Breeding 12-24 hours after estrous is detected is prudent.
Producers using sexed semen need to “breed on heat. Pregnancy rates are similar no matter how you synchronize. It doesn’t really matter which way you synchronize, as long as you breed on heat,” he said.
If Fixed Time A.I. is being used with sexed semen, the untripped pregnancy rate drops to a very low two percent. If, however, producers will wait until the next day to breed, that rate increases to 34 percent.
“If you are going to use Fixed Time A.I., then you have got to change the way things are done,” he said. “Those that have untripped patches you need to breed the next day.”
The industry is beginning to increase the number of sperm per dose to four million, and the fertility rates are increasing to about 95 percent. But the cost of sexed semen will double.
“You only get half as many doses for the same amount of work,” Dr. Seidel said.
The use of sexed semen for in vitro fertilization (IVF) is developing. In the future, it may be possible to select for traits other than sex when selecting semen, he said.
The cost of using sexed semen, calculated via a cost analysis which takes into consideration the accuracy of the sexing, the pregnancy rate per dose, and the expected loss of some calves, will be about $64 per heifer, plus the cost of semen, he said. It is a bit higher right now, but should remain below $100/heifer moving forward.
“One sex, or the other, is always more valuable. If all these things work ideally, eventually unsexed sperm could become obsolete.”