FAQ Stallions and Semen

by Jamie Anderson

At what age can I start breeding from my colt/stallion?

Colts can become fertile from 6-12 months old, but should not be used for breeding until they are at least 2, but preferably 3 years old.

2 year old colts are generally not mentally mature enough to cope with the rigours of breeding. If over bred, 2 year olds can often lose all drive to breed, and could possibly be put off breeding for their rest of their lives. If you must breed from your 2 year old, try not to breed more than 8-10 times in the season. Any more than that, and you risk ending your stallion’s breeding career before it has even begun!

What is “motility” that my vet/AI tech refers to when looking at semen samples?
We had a semen sample arrive that was only 20% progressive; does that mean my mare won’t get in foal?
We had a semen sample arrive that was 80% progressive; does that mean my mare will definitely get in foal?

Motility is a subjective (estimated by eye) measure of the percentage of sperm cells in a sample that are moving. A motile sperm cell could be swimming in a straight line, swimming backwards, swimming in a circle, or gently twitching along at a very low speed. This is also referred to as total motility or overall motility and although it is a useful indicator of the ratio of living:dead sperm cells, it is not a very useful indicator of potential fertility.

The important type of motility is referred to as progressive motility. Progressively motile sperm cells are those sperm that are swimming forwards in a relatively straight line. It is these cells which are capable of fertilising the egg.

It is easy to become obsessed with the percentage of progressively motile cells in a semen sample/dose. However, the percentage is not the important factor. For fresh semen, and chilled semen at the time of insemination it is the number of progressively motile sperm cells (not the percentage) that is important. The optimum number of progressive cells is 500 million, and it is this figure that is important.

To work out how many progressive cells are in an insemination dose, you must use the following formula:

Volume of dose x Concentration x % progressive motility = total progressive cells

We want the total number of progressive cells to be 500 million or more, so:

Example one: an optimal dose

20ml x 50 million sperm/ml x 50% progressive motility = 500 million prog sperm


Example two: an optimal dose despite “only” 25% of the cells being progressive

40ml x 50 million sperm/ml x 25% progressive motility = 500 million prog sperm


Example three: a sub-optimal dose despite 75% of the cells being progressive

20ml x 25 million sperm/ml x 75% progressive motility = 375 million prog sperm


All three factors (volume, concentration, and progressive motility) all have important roles to play in establishing whether or not you have an adequate insemination dose. Don’t put too much importance on progressive motility!

What is a semen extender and why is it used?

A semen extender is a liquid solution generally based on milk products that help the sperm cells survive for extended periods outside of their usual environment (that being the stallion’s or mare’s reproductive tract). Sperm cells are not designed to be outside in a lab (or any other) environment, and so need to be fed and protected. Raw, unextended, semen will not survive for more than a couple of hours outside of the mare.

The main functions of the extender are:

Dilution & protection: The fluid part of semen (i.e. the liquid that the sperm cells are ejaculated in) is called seminal plasma. Seminal plasma is toxic to sperm cells, and so needs to be diluted to stop it killing the sperm cells before they get into the mare. Fatty molecules in the extender also protect the sperm from the seminal plasma, as well as protecting them from other stresses such as cooling that is required for shipping chilled semen.

Nutrition & buffering: In an AI programme the sperm cells are being kept alive for longer than they are designed to live for, and so require an external food source. This is provided for them in the form of sugars in the extender. One of the by products of sperm cells metabolising the sugars in the extender, is acids. These acids can build up over time and damage the sperm cells. Buffers in the extender oppose the harmful action of the acids, and prevent the liquid environment from becoming acidic and killing the sperm cells. The extender also contains salts and other ions to help the sperm cells maintain a specific fluid balance, to stop them from shrivelling up or exploding!

Antibiotic & antifungal: most semen extenders contain broad spectrum antibiotics (and some contain antifungal agents) to help control the bacterial (and fungal) content of AI dose. This helps to prevent damage to the sperm cells in transit, and also helps to prevent bacterial (and fungal) infection in the mare after she is inseminated.

Past Issues

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