The semen of Arabian stallions tended to withstand the freezing and thawing cycle better than the offerings of their Warmblood, Quarter Horse and Icelandic counterparts, according to researchers.
While this breed difference was identified in their study, it was by no means the biggest factor affecting the viability of thawed sperm.
It is well recognised that the quality of stallion semen after freezing and thawing differs among stallions.
However, it remains to be determined whether such differences are also affected by breed.
In their study, reported in the journal Animals, Professor Jörg Aurich and his colleagues at the University of Veterinary Medicine, Vienna, examined semen quality in 1012 frozen and thawed ejaculates from 134 stallions of five breeds.
The percentage of ejaculates acceptable for artificial insemination (AI) and the number of insemination doses per ejaculate was then calculated.
The study team found that sperm motility before freezing was the most important explanatory variable for the percentage of ejaculates with a post-thaw quality acceptable for AI. Thawed semen was classified as acceptable for insemination when progressive motility was 35% or higher.
Of the other variables studied, stallion age was the most important parameter. A decrease in the percentage of acceptable ejaculates was evident in stallions older than 9 years.
“The risk that ejaculates do not fulfil quality standards for cryopreservation thus increases with the age of the stallion,” they said.
“In stallions with high breeding values obtained through success in equestrian competitions, semen cryopreservation should therefore preferably be done at a younger age and not after the stallion has been retired from a career in equestrian sports.”
There were also more frozen–thawed ejaculates assessed as acceptable for use in AI programmes among Arab stallions compared to Warmbloods, Quarter Horses and Icelandic horses.
The percentage meeting the criteria was above the expected average among Arab stallions, below the average in Icelandic and Quarter Horse stallions and close to the average in Warmblood and Lipizzaner stallions.
“While the number of Icelandic horses and Lipizzaners in our study may be considered relatively small, breed differences between Arab, Quarter Horse and Warmblood stallions are based on a substantial number of animals and ejaculates.”
The researchers continued: “Differences in the percentage of acceptable cryopreserved ejaculates among horse breeds suggest a genetic basis for cryotolerance of equine semen.
“Total sperm count was the most important variable determining the number of semen doses obtained per acceptable frozen–thawed ejaculate.”
Aurich and his colleagues say the economic efficiency of semen cryopreservation is in part determined by the percentage of frozen–thawed ejaculates deemed acceptable for use in commercial AI programmes and by the number of semen doses obtained per acceptable ejaculate.
Stallions with good semen quality directly after collection are, on average, more likely to yield acceptable semen after cryopreservation than stallions with an already reduced quality of raw semen.
However, good sperm motility in raw semen does not always correlate with similar post-thaw semen quality.
The researchers said the conclusions of their study do not contradict the finding that there are individual “bad freezer” stallions despite an acceptable quality of raw semen.
“In agreement with previous reports, approximately 20% of the stallions in our study were consistent ‘bad freezers’,” they said.
The full study team comprised Jörg Aurich, Juliane Kuhl, Alexander Tichy and Christine Aurich.
Aurich, J.; Kuhl, J.; Tichy, A.; Aurich, C. Efficiency of Semen Cryopreservation in Stallions. Animals 2020, 10, 1033.