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Semen Freezing

Why Freeze?

  • At Stallion AI Services, we pride ourselves in delivering the highest possible service to you and your stallion. Our laboratory boasts the latest technology in equine semen freezing and processing including our world leading freezing extender that enables us to specialise in the freezing of sub-fertile stallions. DEFRA approved for both UK and worldwide export markets, our services enable you to preserve your stallions breeding potential and provides the opportunity for you to partake and capture the worldwide frozen semen market.
    • Multi-award winning semen collection centre
    • State of the art facilities
    • Specialists in freezing of sub-fertile stallions and epididymal semen extractions
    • Technologies which push the scientific boundaries in equine semen freezing
  • Stallion AI Services laboratory carries out the most robust and sophisticated post thaw semen assessment in the UK, being the only equine semen laboratory using fluorescent microscopy, to ensure that the semen frozen from your stallion is of the highest possible quality.

Collection of Semen

Semen is collected using one of several models of an artificial vagina (AV). The AV is lubricated thoroughly using a non-spermicidal sterile gel. The semen is collected in a sterile bottle designed to separate the sperm rich fraction from the often-copious gel fraction. A typical stallion will produce between 30-70ml sperm rich semen. The semen is quickly diluted with a transport medium, which is a cocktail of a series of chemicals, which maintains the viability of sperm for up to 7 hours from the collection of sperm. This remarkable solution is capable of holding the semen at room temperature, and therefore the necessity of chilling and incubating is removed. The sperm are checked for motility at this stage.

The laboratory at Stallion AI Services is equipped with the latest high tech apparatus for the correct processing, freezing and evaluation of semen from your valuable stallion.

Centrifugation of the ejaculate

Sperm are then spun down on to a liquid cushion to prevent damage during centrifugation; this is the process of reducing the volume of semen resulting in an increased density of sperm. The semen is centrifuged at 1000 times the force of gravity for 20 minutes. Once spun down, the band of cells suspended on the liquid cushion can be harvested ready for the next stage of processing.

Photometric equipment is used to accurately calculate the exact number of sperm in the ejaculate. When this has been established the semen can be diluted with a freezing extender to the correct concentration for insemination. 900 million sperm will eventually be used to inseminate into the mare.

Filling and Printing of straws

The diluted semen will be packaged into 0.5 ml plastic straws. Each straw is printed with information relating to the stallion. This information includes the ministry licence number, the stallion’s name, date of semen collection and the stallion’s registration number. The printed straws are then placed in a machine that will fill and seal each one. The machine will automatically fill each straw with exactly 0.5ml of semen. Each straw contains 150 million sperm cells and 6 of these straws will make up the single insemination dose.


The straws containing the semen are then placed into the digit cool semen freezer. This Cryogenic machine uses liquid nitrogen to freeze the semen. A computer linked to the freezing machine is pre-programmed to cool and freeze the semen at a specific rate down to a chilly –140C. Once at this temperature the straws can be removed from the digit cool and plunged into liquid nitrogen at –196C. At this temperature the sperm are in suspended animation. The sperm will remain viable indefinitely at this temperature and will not degrade or perish. The semen can be used to inseminate mares many years after it was collected from the stallion. Random straws are removed for post thaw quality control.


After freezing, 2 straws from each ejaculate are tested by one of the centre’s experienced technicians. They are thawed at 37C for 30 seconds and pooled in a tube. The motility is assessed visually and must have at least 35% progressive motility. The semen is then stained with propidium iodide (PI) and carboxyflourescein diacetate (CFD). The PI stains the dead sperm cells red and the CFD stains live cells green. After counting 200 cells, the live percentage is calculated using a formula. A morphology assessment is done by looking at 100 live cells and noting the number of normal ones. The number of live, normal sperm post-thaw in each dose can then be calculated to form a clear objective view of the end product, which is to be inseminated into the mare. To pass the post-thaw quality tests at least 260 million cells must be live and normal, of the 800 million in each dose of 4mls.


Stallion AI Services provide over twenty years of industry knowledge within equine reproduction. We are a DEFRA approved distribution centre for worldwide export, currently storing semen from over 1000 individual stallions.

We have recently invested £38,000 in two band new state of the art nitrogen storage tanks joining our existing two tanks each storing >265,000 straws. Our nitrogen storage tanks are split in two different fire proof rooms; totalling two for the storage of UK eligible semen and two for export eligible semen.