# Freezing Stallion Semen—What Do We Need to Focus on for the Future?

## TL;DR

This paper reviews challenges in freezing stallion semen and suggests areas for improvement to enable more effective use of frozen semen in horse breeding.

## Contribution

The paper identifies key factors affecting stallion semen cryopreservation and proposes future research directions for improving freezing protocols.

## Key findings

- Frozen semen offers advantages like greater stallion choice and reduced antibiotic use, but not all ejaculates freeze well.
- Stallion sperm membranes are highly susceptible to damage, and no universal antioxidant has been found.
- Variability in sperm cryosurvival leads to classifying stallions as good or bad freezers.

## Abstract

Most artificial inseminations in horses currently use cooled semen. It has not been possible to develop a freezing protocol that is suitable for all stallions and all ejaculates. Furthermore, the thawed spermatozoa have a short life, which necessitates depositing the semen in the uterus close to the time of ovulation. Using frozen semen has many potential advantages: there would be greater choice of stallions, the supply of semen is guaranteed, and the amount of antibiotics used would be reduced compared to fresh semen. However, not all ejaculates can be frozen successfully. This review looks at some of the factors that might affect the successful cryopreservation of semen, including stallion nutrition, the frequency of semen collection, the medium that is added to protect the sperm during freezing, the speed of cooling, etc. It would be helpful to identify ejaculates that will freeze well, but as yet, there are no means of identifying them other than carrying out a test freeze.

Artificial insemination (AI) is used frequently in the breeding of sport horses, apart from Thoroughbreds. Most AIs are carried out with cooled semen rather than frozen semen because of the difficulties in identifying a protocol that is suitable for freezing most ejaculates and the necessity to inseminate close to ovulation because of the short life of the thawed spermatozoa. More widespread use of frozen semen would improve biosecurity, allow greater choice of stallions, and offer more flexibility when managing deliveries of semen to the stud. It would even decrease the amount of antibiotics used in semen extenders, since the volume of frozen semen is smaller than when cooled semen is inseminated. However, there is considerable variability in the cryosurvival of spermatozoa from different stallions, leading to the classification of stallions as good or bad freezers. Improvements could be made at the level of stallion nutrition, the semen collection regimen, the extender, the removal of seminal plasma, and the cooling protocol, among others. Stallion sperm membranes are highly susceptible to lipid peroxidation, but research on antioxidants has failed to identify an additive that would benefit all stallions. In the future, biomarkers for sperm freezability could be used as an aid in identifying suitable ejaculates for cryopreservation.