Choosing the Sex of Your Foal: Breeding with Sex-Sorted Semen

Imagine being able to choose whether your next foal is a colt or a filly before conception. Thanks to advancements in reproductive technology, this is no longer a distant dream. Sex-sorted semen—a tool widely used in cattle breeding—is gaining traction in the equine world. But is it ready for prime time in horses?

What is Sex-Sorted Semen?

Sex-sorted semen is sperm that has been processed to separate X-bearing sperm (producing females) from Y-bearing sperm (producing males). This is typically done using flow cytometry, which identifies and separates sperm cells based on DNA content. The X chromosome is slightly larger and contains more DNA than the Y chromosome, allowing for physical separation. In cattle, this technology has been in widespread commercial use for over two decades, but several unique challenges have limited its application in horses.

Why Use Sex-Sorted Semen in Horses?

Choosing the sex of your foal can offer several benefits:

• Economic Strategy. Owners may prefer fillies for breeding or colts for performance.

• Competition Goals. In disciplines like polo or racing, one sex may be favored.

• Genetic Planning. Sex selection can help maintain desirable traits in a breeding program.

Challenges in Horses Compared to Cattle

While sex-sorting is well established in cattle, horses present some specific hurdles:

• Sperm Fragility. Equine sperm are more sensitive to physical and chemical stress than bovine sperm.

  
  

• Lower Sperm Concentration. Stallion ejaculates typically have fewer viable sperm, making losses during processing more critical.

  
  

• Shipping and Timing. Unlike cattle, horse breeders often rely on shipped semen, which complicates the logistics of sorting and insemination.

  
  

• Variable Semen Quality. There is more individual variation in semen quality among stallions than among bulls.

Recent Breakthroughs in Equine Sex-Sorting

Thanks to new technologies and protocols, significant strides have been made:

• Improved Equipment and Extenders. STgenetics® developed proprietary technologies like the SuperGen™ sorter and Ultraplus™ semen processing system. These advances minimize sperm damage during sorting and improve post-sorting quality.

  
  

• Shipping and Processing Innovations. Researchers demonstrated that stallion semen can be extended in specialized media, cooled to 8°C, and shipped overnight for sorting without significant loss in sperm quality. A critical component is the use of centrifugation with Equipure™ and cooling extenders like BotuGold™, which maintain sperm motility and viability.

  
  

• Lower DNA Damage. Early concerns about increased DNA fragmentation in sorted sperm have been largely addressed. In fact, the sorting process, especially when paired with careful handling, can actually reduce DNA damage compared to raw semen.

Fertility Results: How Well Does It Work?

• Artificial Insemination (AI) with Fresh Sex-Sorted Semen

  In a 2024 study conducted at Texas A&M University, researchers demonstrated that when mares were inseminated with fresh, sex-sorted semen, embryo recovery rates were comparable to those achieved with conventional semen—about 40%. Importantly, all embryos produced using female-sorted sperm were confirmed to be female via PCR, proving the effectiveness of the sorting.

  
  

• Frozen-Thawed Sex-Sorted Semen for ICSI

  Intracytoplasmic sperm injection (ICSI)—injecting a single sperm into an egg—is becoming more popular in equine reproduction. While earlier attempts using sex-sorted sperm for ICSI yielded poor results, newer protocols show promise. In recent trials across multiple labs:

  
  

  • Cleavage Rates: 53.2% for sex-sorted sperm vs. 68.3% for conventional.

  • Blastocyst Rates: 23.7% (sorted) vs. 30.5% (conventional).

  • Embryo Sex Accuracy: 95%+ accuracy in desired sex using sex-sorted semen.

    
    

These outcomes indicate that, although slightly lower than conventional sperm, the results are consistent and reliable for producing foals of a desired sex.

Practical Considerations for Horse Owners

If you're considering using sex-sorted semen, here are a few things to keep in mind:

• Cost. Sex-sorted semen is more expensive due to the extra processing and lower sperm yield per straw. You may also need to utilize specialized breeding techniques, such as deep uterine insemination or ICSI.

  
  

• Stallion Suitability. Not all stallions produce semen that responds well to the sorting process. However, studies show that with proper processing, over 90% of stallions tested could be successfully sorted.

  
  

• Time Sensitivity. Due to reduced sperm numbers, the timing of insemination is critical. AI should be timed precisely around ovulation, often requiring hormonal synchronization and frequent ultrasound monitoring.

  
  

• Breed and Discipline Preferences. Some disciplines or registries may have preferences for colts or fillies, making this technology more attractive for certain breeders.

  
  

The Bottom Line: Is It Worth It?

Sex-sorted semen is no longer a novelty in the horse breeding world. It's a scientifically supported, increasingly practical option for breeders with specific goals. Thanks to recent advancements, especially in semen extenders and cryopreservation, sex-sorting has moved from an experimental procedure to a commercially viable service.

For horse owners aiming to produce foals of a specific sex, whether for economic, competitive, or genetic reasons, sex-sorted semen is a powerful tool now worth considering. While sex-sorting isn't for every breeding program, it opens new possibilities for those with clear objectives. Talk to your veterinarian or reproductive specialist to determine whether your mare, stallion, and breeding setup are good candidates for this technology.

Reference

Samper, J C et al. “The use of sex-sorted semen in horses.” Journal of equine veterinary science vol. 145 (2025): 105251. doi:10.1016/j.jevs.2024.105251