by STgenetics Director Global Quality Assurance Leonardo F. C. Brito
Although use of sexed semen used for artificial insemination (AI) is rightfully considered a reproductive biotechnology, it could be argued that it should, also, be considered genetic selection since gender is a genetic trait. Most genetic traits can be manipulated through selection, but before sexed semen was available, producers had to accept the 50/50 probability for obtaining females and male calves. Due to the impact that gender has on animal production systems, it has been described as “the most important genetic trait.” As such, sexed semen will continue to be one of the main drives of cattle production efficiency and sustainability.
Sexed semen production has greatly improved since the beginning of commercial application, but still continues to evolve rapidly. Incorporation of the most recent advancements into the production of sexed semen resulted in a differentiated product, SexedULTRA 4MTM, that now allows producers to obtain fertility rates comparable to those obtained with conventional semen and more than 90 percent calves of the desired gender.
Brief history of sexed semen technology
Sexed semen technology was initially developed at United States government research centers, initially at the Lawrence Livermore National Laboratory in the 1970s, where scientists developed techniques that allowed precise measurement of sperm DNA content, and later at the USDA Beltsville Agricultural Research Center in the 1980s and 1990s, where further advancements in cell separation technology led to breakthrough research resulting in the producing live offspring obtained after insemination with sexed semen. In 2007, Sexing Technologies (ST) acquired all technology rights and refocused the commercial approach to allow bull studs access to growing amounts of sexed semen of consistent quality at reasonable costs. Today, all of the world’s largest bull studs use ST technology to offer sexed semen from a diverse group of top bulls as an essential and important portion of their product portfolios.
Overview of sexed semen production
Gender in mammals is determined by the pair of sex chromosomes individuals carry in their genome. There are two types of sex chromosomes, X and Y; females carry two copies of the X chromosome (XX) and males carry a copy of each chromosome (XY). When gametes (egg and sperm) are formed, the pair of sex chromosomes is split; eggs always carry a X chromosome, whereas sperm carry either a X or Y chromosome. Therefore, offspring gender is determined by the sex chromosome carried by the sperm that fertilized the egg. Sexed semen production is based on the difference in size and DNA content between X- and Y-sperm (approximately 4 percent in bulls).
After collection and evaluation, semen must be prepared for sorting. This involves extension with appropriate buffers, removal of seminal plasma, and adjustment of cell concentration to optimal range. The sample is then incubated with optimal concentrations of a DNA-specific fluorescent dye. Stained sperm are then processed and sorted using a flow-cytometer (Figure 1). Sorted sperm are collected into tubes containing appropriate buffers to protect cells during the sorting and cooling processes. After sorting, tubes are slowly cooled to 5oC, additional extenders containing cryoprotectants are added, and tubes are centrifuged to obtain concentrated sperm pellets. The number of recovered sperm is determined and extenders added to obtain the desired concentration. After a period of equilibration, semen is loaded into straws and frozen in a programmable freezer using the optimal freezing curve. Post-thaw quality control involves evaluation of sperm motility and acrosome integrity after three hours of incubation at 35oC, analysis of purity, concentration, and bacteriology. Altogether, sexed semen production involves over 20 sub-processes.
Continuous research and development investment in sexed semen production technology have resulted in significant improvements in semen quality and fertility, so much so that a new product label was created. The SexedULTRATM label was officially launched in 2013 as the culmination of a series of innovations that combined to create a product significantly different from that produced using XY Inc. legacy technology. Some of these innovations included developments in flow-cytometry technology and complete redesign of equipment, optimization of media and extenders, large scale media and extender production for global distribution, optimization of staining conditions, and worldwide adoption of modern, standard equipment (Figure 2 and 3).
Results from field trials conducted by ST and commercial partners demonstrated that conception rates in dairy herds were greater when SexedULTRATM product was compared with product produced using XY legacy technology (Table 1). In addition, data compiled by researchers from the USDA on sexed semen usage for Holstein females in the United States demonstrated a consistent reduction in conceptional rate differences between sexed and conventional semen coinciding with global introduction of SexedULTRATM in 2013 (Figure 4).
Prior to introduction of SexedULTRATM production technology, increasing insemination dosage from the 2.1 million sperm used as the industry standard resulted in little-to-no-significant gain in conception rates. However, a study conducted in collaboration with a commercial partner demonstrated that fertility rates obtained with SexedULTRATM 4M (4 million sperm per straw) resulted in conception rates comparable to conventional semen (Figure 5).
STgenetics officially launched SexedULTRA 4MTM in 2017. Some bull studs, also, conducted internal trails and have recently announced the release of similar products (see SELECTed™ SexedULTRA™ 4M from Select Sires and GenChoice™ 4M from Genex). Several other bull studs are currently conducting trial and the expectation is that SexedULTRA 4MTM will soon become the new industry standard for sexed semen.
SexedULTRA 4MTM use in beef cattle
Exciting results have recently been reported with the use of SexedULTRA 4MTM semen in beef cattle in Brazil, Argentina and the U.S., demonstrating that conception rates approximately 85 percent of that obtained with conventional semen can be obtained (Tables 2-5). Research aimed at adapting fixed-time AI protocols and developing specific recommendations for breeding time and strategies for sexed semen are expected to further improve results. Sexed semen from Brangus and other beef breeds is available at stgen.com.