Factors Affecting Fertility with Cooled Semen

With the use of cooled shipped semen on the rise, breeders want to maximize the effects of this new technology as best they can. However, there are a variety of factors that can affect fertility with cooled semen, said Dickson Varner, DVM, MS, Dipl. ACT, professor in equine reproduction in the College of Veterinary Medicine at Texas A&M University (TAMU), at his presentation at the Bluegrass Equine Reproduction Symposium. The symposium was sponsored by Hagyard-Davidson-McGee Associates veterinary clinic in Lexington, Ky.

Varner said that with stallions being judged primarily on their pedigree, performance record, and conformation, with little consideration being given to semen quality and fertility, the industry might be inadvertently promoting subfertile stallions. There are many factors that affect semen quality and the fertility of the stallion; factors ranging from how the semen is handled and processed to the ability of the semen to withstand cooling, storage, and shipping. Varner discussed several factors in detail, including composition of extender; pH and osmolality (or osmotic pressure) of extender; extender antibiotics; semen dilution ratio; indications and techniques for centrifugation of semen; and selection of shipping containers.

Varner cautions that sperm are quite susceptible to many environmental factors, such as temperature, light, and a variety of common chemicals (i.e. soap and disinfectants).  He recommends keeping the semen warm immediately following collection (35-38°F) until it is mixed with a suitable extender.

Proper preparation of the artificial vagina (AV) is important. Varner recommends the use of sterile HR lubricant or Priority Care lubricant, as opposed to KY jelly for lubricating the artificial vagina, because the KY jelly contains a chlorhexadine base that could jeopordize sperm viability.  He also advises that the volume of AV lubricant be minimized to avoid osmotic- or pH-related damage to the sperm.  Putting a filter in the AV when collecting the semen effectively separates the gel and gel-free portions of the semen, thereby increasing the sperm harvest. Varner suggests using disposable baby bottle liners in the collection receptacle to reduce the risk of chemical or bacterial contamination of the ejaculated semen. In addition, cleaning of the AV prior to collection should be done with hot water only, followed by rinses with deionized water and 70% isopropyl or ethyl alcohol. Use of soaps and disinfectants (i.e. Nolvasan) can permeate the rubber of the AV and damage the semen.  Varner’s preferred AV is the Missouri-model.  He indicates that it is most easily assembled, cleaned, and stored.  In addition, it is the most reliable in his hands for prompt collection of semen, especially from stallions that are difficult to collect

Varner then went on to discuss the dangers of certain antibiotics that are included in the semen extender. It has been found through research at TAMU that a combination of potassium penicillin K and amikacin sulfate optimizes spermatazoal motility, while providing the broadest spectrum of antibacterial activity for semen stored at 41°F (5°C) for 24 hours. The antibiotic, Polymixin B sulfate, was found to be consistently detrimental to semen quality.  According to Varner, it is important to remember that semen from different stallions will sometimes react differently to antibiotics.  The semen might need to be tested with several antibiotics to determine the right antibiotic for semen of a particular stallion.  As for extender osmolality, Varner’s research indicates that a slightly hypertonic environment  (having a higher osmotic pressure) improves semen quality following storage.  Extenders are formulated and prepared at Texas A&M with this in mind.  Varner underscores that the pH of the semen extender is also an important factor. TAMU researchers have found that a lower pH of 6.6-6.8 is the best range for semen extender. Commercial extenders are typically not within the osmotic and pH ranges preferred by Varner so the TAMU staff prepares a specially formulated extender to meet these requirements. 
In addition, some semen might not survive the cooling and storage process because of excessive seminal plasma or toxic factors in the seminal plasma.  Based on studies done at TAMU, when the semen from certain stallions does not survive the cooling and storage process, semen quality might be improved with centrifugation and partial or complete removal of seminal plasma prior to cooling and storage. Removal of essentially all seminal plasma and extending the sperm in a milk-based extender containing modified Tyrode’s medium might benefit stallions that experience  poor semen viability following cooled storage. However, these steps are not necessary for semen from stallions whose ejaculates tolerate cooling and storage with standard processing, according to Varner.

Another important factor for maintaining semen quality is the type of container that is used. There are several different commercial containers available in the United States. Varner cites the Equitainer I and Equitainer II as being the most popular passive-cooling containers. A passive-cooling container will cool the semen at progressively slower rates (as the internal temperature drops, the semen is cooled at a slower rate). However, with these containers, environmental temperature plays a role on cooling rates and storage temperature. Varner says that based on comparative studies, most of the commercially available equine semen transport containers will maintain a similar progressive spermatozoal motility in ambient temperatures of 71.6°F (22°C) to 98.6°F (37°F). However, if the environmental temperature drops to -4°F (-20°C) for six hours or more, motility might be reduced with some containers. If this is a possibility, Varner recommends the Equitainer I or II.

There are more factors that affect semen quality, such as semen dilution ratio, insemination technique, insemination volume, and timing of insemination; however, these were unable to be discussed within the time allotted for Varner’s presentation. More information on semen quality and artificial insemination can be found at www.TheHorse.com by browsing the Breeding/Reproduction category.


About the Author

Sarah Evers Conrad

Sarah Evers Conrad has a bachelor’s of arts in journalism and equine science from Western Kentucky University. As a lifelong horse lover and equestrian, Conrad started her career at The Horse: Your Guide to Equine Health Care magazine. She has also worked for the United States Equestrian Federation as the managing editor of Equestrian magazine and director of e-communications and served as content manager/travel writer for a Caribbean travel agency. When she isn’t freelancing, Conrad spends her free time enjoying her family, reading, practicing photography, traveling, crocheting, and being around animals in her Lexington, Kentucky, home.

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