Breed, Age, And Seasonal Impact On Quality Of Frozen-Thawed Stallion Semen

By Sara Mastellar
SDSU Equine Instructor

Like many other livestock species, horses are seasonal breeders. Their greatest reproductive activity occurs in the late spring and summer. However, some producers aim to breed for foals early in January or February. The advantage to this is being able to compete horses at an older age within their age group, a practice commonly used within the racing industry. Equine breeders who wish to produce early foals, must have an effective protocol in inducing estrus in mares, but also understand the importance of having high quality semen.

Frozen-Thawed Semen Quality: Factors to consider

A 2016 study conducted by J. Kuhl, F. Schott, K. Deichsel, & C. Aurich, at the Centre for Artificial Insemination and Embryo Transfer in Vienna, Austria, evaluated the effects of age, breed, and season on the quality of frozen-thawed semen in stallions. A total of 898 ejaculates were collected from 180 stallions during various months throughout the year, and were then frozen in liquid nitrogen using a standard protocol. The ejaculates were grouped based on age and breed of the stallion. Ages ranged from 2-14+ years, and breeds included: Warmbloods, Quarter Horses, Arabs, Standardbreds, Lipizzaners, Icelandic horses, and others.

The quality of the sperm was assessed using computer-assisted semen analysis. Spermatozoa were termed progressively motile if they had a curvilinear velocity of greater than or equal to 10 ųm/s, a distance straight line of greater than or equal to 6 ųm, and a radius of greater than or equal to 15 ųm. Once thawed semen was deemed acceptable for insemination when it had a progressively motile percentage of greater than or equal to 35%. A standard insemination dose consisted of 300x106 motile spermatozoa. Statistical analyses were used to determine differences in sperm quality due to age, breed, and season.

Age

The study showed that age had a significant effect on the percentage of viable ejaculate (Table 1). The age comparison also showed that the number of insemination doses per acceptable ejaculate was relatively the same between different ages.

Table 1. Effect of stallion age on percentage of frozen-thawed acceptable ejaculates.

* Acceptable ejaculates were defined as having a progressive motility of ≥ 35%

Breed

The percentage of acceptable ejaculate varied greatly among the different breeds (Table 2). The breed of the stallion also impacted the number of insemination doses per ejaculate.

Table 2. Effect of stallion breed on percentage of frozen-thawed acceptable ejaculates.

Season

Acceptable ejaculates varied greatly among seasons as well (Figure 1). Unlike breed, seasons did not have an impact on the number of insemination doses per ejaculate. A possible explanation as to why winter yields the lowest acceptable ejaculates can be correlated to the amounts of hormones being produced. According to Aurich (2016), season influences cooled-shipped and frozen-thawed stallion semen. During the winter stallions have been shown to produce less gonadotrophin, testosterone, and estradiol, which are necessary in maintaining spermatozoa production. Decreased testicular size and sexual behavior were also reduced in the winter months.