Received: 13 November 2023 | Accepted: 20 December 2023 DOI: 10.1111/eve.13943
R E V I EW A R T I C L E
Stallion semen cryopreservation: Frozen semen preparation, handling and breeding
Lauren Pasch
Rhinebeck Equine LLP , Rhinebeck , New York , USA
Correspondence: Lauren Pasch Email:
lrpasch@gmail.com
Summary The use of cryopreserved stallion semen is a significant aspect of equine reproduction, despite the tendency for lower per cycle pregnancy rates than cooled or fresh semen. This decreased fertility of thawed frozen semen is due in part to the innate subfertility of the chosen male breeding population, but also to the direct insults to the sperm cell during the freezing and subsequent thawing process. The mare aspect of frozen semen breeding is intensively managed to compensate for the aforementioned damages, and acceptable pregnancy rates can be achieved. Here we describe the stallion semen cryo- preservation process including semen collection, expected causes of damage in process- ing, freezing and thawing, mare management and future directions.
K E YWORDS horse , assisted reproduction , frozen semen , pregnancy , stallion
INTRODUCTION The artificial
PREPARATION FOR SEMEN FREEZING insemination of mares with thawed- cryopreserved
stallion semen is an essential aspect of equine breeding man- agement. The ability to cryopreserve stallion semen allows for increased genetic diversity, improved safety for horses and hu- mans, and international accessibility of renowned stallions. However, unlike other species, the male portion of the equine population has not been selected for fertility. Rather, the decision to collect and preserve stallion semen is generally based on the sire ' s phenotype, athletic performance, and pedigree. Expected per cycle pregnancy rates for frozen- thawed semen are gener- ally lower than for cooled or fresh semen (Barbacini et al., 1999 ; Loomis & Graham, 2008 ; Vidament et al., 1997 ), due largely to the deleterious effects of cryopreservation on sperm structure and function (Neild et al., 2003 ; Watson, 2000 ). The freezing process is unavoidably stressful; a significant population of sperm will not survive, and those that do will be compromised. As a result, the mare aspect of the breeding process is tightly managed to opti- mise the time and location of frozen- thawed semen deposition. This review provides a summary of stallion semen cryopreserva- tion, including a process overview, causes of sperm damage, and a discussion of the particular challenges associated with freezing equine semen.
Equine Vet Educ. 2024;36:375–380.
Prior to beginning the cryopreservation process, the stallion must be trained for semen collection in an artificial vagina. Serum titre for equine viral arteritis (EVA) should be evaluated before semen collec- tion, and previously vaccinated stallions must have a proof of nega- tive titre prior to vaccination (without this documentation, semen is submitted for virus isolation). Equine arteritis virus (EAV) testing is of particular importance ahead of freezing semen given its ability to survive cryopreservation and storage (Timoney, 2011 ). The stallion should be well trained for semen collection and collected on a regu- lar schedule to ensure that he has been ‘cleaned out’ as indicated by repeat ejaculates reaching calculated or expected daily sperm out- put (DSO), but consensus is lacking regarding the number of collec- tions required for adequate cleanout prior to cryopreservation. DSO represents the number of sperm collected per day after the extrago- nadal sperm reserves (epididymal tail, deferent ducts, ampullae) have been cleared of sperm build up which occurs with sexual rest (Squires & Pickett, 2011 ; Thompson et al., 2004 ). Sperm production in a normal stallion is a reflection of testicular volume, thus testicular measurements and calculations can be used to calculate expected DSO (Love et al., 1991 ; Thompson et al., 1979 ) and compare to total sperm count in serial ejaculates (Table 1 ). The pre- freezing prepara- tory period is essential in maximising the number of sperm available
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