All you ever wanted to know about boar semen.

By Meritxell Donadeu DVM, MSc - Veterinary Manager, PIC Western Europe - This article answers 30 questions on the main points of boar semen.
calendar icon 4 December 2006
clock icon 12 minute read

1 What is in boar semen?

Boar semen is a suspension of sperm cells and secretions from the boar reproductive tract, including the accessory glands. The fluid portion of this suspension is known as seminal plasma and it helps to carry and protect the sperm cells. In boars, the semen also contains large quantities of gel.

2 How long does it take for boar sperm to be ‘produced’?

It takes about five weeks for a sperm cell to be produced, and a further two weeks to go through the epididymis [The epididymis is a long, convoluted tube just outside the upper surface of the testis, where the sperm migrate slowly while continuing to mature and gain the potential ability to fertilise the eggs. It is also the main storage for sperm]. It is interesting to note that sperm collected today started to be produced seven weeks ago.

3 What does normal boar sperm look like?

A normal sperm can be seen in Figure 1. It has a head and a tail and the entire sperm cell is covered by a membrane. . The head contains the genetic information (chromosomes). On top of the head is the acrosome. The acrosome is a thin sac that contains enzymes that help the sperm penetrate the egg during the fertilisation process. . The tail is composed of the neck, middle, principal and end pieces. The middle piece contains the mitochondrias which generate the energy required for sperm motility.

4 What abnormalities can a sperm have?

It can have several. For example: abnormal head (too big, too large, abnormal shape, etc.), abnormal tails (double tail, bent tail, coiled tail, etc.), acrosome defects (knobbed, incomplete, etc.), or citoplasmatic droplets. During the sperm production, a cytoplasma droplet is formed, that is usually lost when the sperm matures. Sometimes the droplet remains and it can be proximal or distal, depending if it is closer or further away from the sperm head. Some of these abnormalities can be seen in Figure 2.

5 How are these abnormalities produced?

Some abnormalities are produced during the spermatogenesis process - that is while the sperm is being produced in the testis - some during the passage through the epididymis, and some can be during, or after, ejaculation due to improper semen handling or storage. At each stage, different factors can produce abnormalities. For example, if a boar has a fever (high temperature) this can affect the spermatogenesis. Changes in temperature, pH, or osmotic pressure, during the semen dilutions process will also produce abnormalities.

6 What are the implications of sperm abnormalities?

It is assumed that the majority of the abnormal sperm will not be fertile, even if for some abnormalities it will depend on the degree of maturation of the sperm. For this reason, the number of sperm in a dose takes into consideration that a percentage of the sperm may have abnormalities (up to 30%).

7 What is the size of a boar sperm?

A normal sperm is about 45µm (that is about 0.0045 cm). To give you an idea, if you joined the sperm cells in a chain, head-to-tail, you could fit around 220 sperm cells in 1 cm.

8 If the sperm is so small, how does it reach the egg? Does the sperm have to swim all that way to the egg?

No! If you consider that the uterus (womb) has two horns up to 1.5 m in length (see Figure 3), it would be impossible for such a small sperm to swim up all that way. The sperm reaches the place of fertilisation (where the sperm and the egg meet) mainly due to the uterus’ contractions during the insemination - that is why stimulating the sows during insemination is so important. Once the sperm is in the place of fertilisation, then sperm motility is important as the tail helps the sperm to move into the egg and push their heads into the egg.

9 How many sperm reach the fertilisation site?

Only a very, very, small proportion of sperm reach the fertilisation site (far less than 1%). After the insemination, there is a rapid transport through the uterus and only about 10,000 sperm reach the sperm reservoir in the oviduct – the majority of the sperm are lost through reflux i.e. leakage out of the cervix, and phagocytosis where the sow’s body attacks foreign material. When ovulation happens, sperm are slowly released from the reservoir towards the site of fertilisation and less than 100 sperm are present where fertilisation occurs. (Ref: Brussow K-P et al. Physiological aspects of in vivo fertilisation in pigs, 2003, Proceedings AI Vets Meeting – Hungary, 7-12)

10 Is sperm motility related to fertility?

Studies done by Billy Flowers in North Carolina concluded that, if boar sperm motility is 60% or greater, there are no relationships between the percentage of motile sperm and farrowing rates and litter size. That is why PIC aim to deliver semen with motility over 70%.

11 Is there a single test that can accurately predict fertility of the individual ejaculates?

Unfortunately, no. At the moment, we can evaluate several components of sperm quality but not all of them, because the technology is not available or practical to implement. However, the aspects we are able to evaluate might not necessarily be linked to fertility if that component is not the limiting factor. For example, we can look at some elements like tails and heads and check if the shape is correct, but we cannot ensure that they are functional, or evaluate if the chromatin (genetic material) is in good condition. Think of a car, you can look at it and say the tyres look good or the shape is nice. You might even try the lights and see if they work but, until you switch on the engine and drive, you will not know if it can take you to your destination. But if you look at the car and it has no tyres, you already know that the car is no good to you. That is what is happening today with semen quality. We can look at the sperm and see if it is of bad quality, but we cannot necessarily ensure that it is of good quality. At PIC, we are looking at each ejaculate before and after dilution, to ensure that motility and morphology are good so that we can reject any ejaculate that looks bad, but we cannot predict fertility of that ejaculate.

12 How much sperm are in a boar ejaculate?

It will depend on several factors like age and frequency of collection, but between 20-60 billion.

13 What is in the flatpack?

  • Semen: sperm and seminal plasma
  • Water
  • Extender or diluent: This protects the sperm providing nutrients and metabolic support, protects against temperature and pH changes, inhibits bacterial growth and increases the volume so that the doses can be produced. Usually they contain glucose, electrolytes, buffers and antibiotics.

14 Is the sperm in a flatpack ready to fertilise the egg?

No. It still has to undergo a process called capacitation (when the sperm gains the ability to fertilise the egg) that usually happens once the sperm is inside the sow and it takes about 6-8 hours.

15 How long can the sperm survive inside the sow?

Usually, good quality semen will survive within the sow reproductive tract for approximately 24 hours. Eggs only survive for about 12 hours, so it is better to have sperm already at the fertilisation site before ovulation.

16 At what age can young boars start to be collected?

Puberty begins at 51/2 months of age and sperm can be found in the ejaculate. Training can start once the boars are physiologically mature and produce an adequate quantity of good quality, mature sperm. That is at 6-7 months of age.

17 What is the shelf-life of a flatpack?

Usually 3-10 days, depending on the extender used, and assuming that the semen is stored properly. At PIC, we have taken the decision to use an extender to allow a 5 day shelf-life.

18 How many sperm are in a flatpack? How does PIC know?

PIC aim to have 2.3 billion sperm in each flatpack. To ensure this, the sperm concentration of each ejaculate is measured and the number of doses is produced accordingly. At PIC, we take great care to measure each ejaculate as accurately as possible and to mix the semen thoroughly, so that all flatpacks have an average of 2.3 billion sperm.

19 Why do I need to turn the flatpack twice a day?

If the flatpack is not turned, the sperm settles at the bottom. It needs to be resuspended so that the sperm can have access to nutrients and metabolites, which are re-distributed in the extender.

20 Why do I need to keep the flatpack away from sunlight?

UV light present in sunlight can damage and kill the sperm. That is why it is so important to transport the semen inside a box.

21 Why do I need to keep the semen at 17oC? What are the implications above and below this?

The ideal temperature to store boar semen with the current extenders is 17°C. If the semen is stored at higher temperatures, let’s say above 20°C, the sperm are not inhibited enough (therefore they use their nutrients and energy) so the shelf-life will be decreased. There is also an increased risk of bacterial growth. If the semen is stored below 15°C, the membranes that protect the sperm, including the acrosome, can be damaged (you might not notice this using a simple inspection under the microscope because motility can still remain unaffected).

22 How do you measure morphology and how often?

At PIC, we evaluate the morphology by looking with the microscope after each ejaculate is diluted, and once every four weeks in a more detailed way.

23 How does PIC ensure that the semen is of good quality?

We have a very strict quality control system that includes complying with the BPEX protocol. We have our internal PIC controls, but we also send samples to an independent lab for evaluation. Independent vets visit all our studs once a week to evaluate not only the health of the boars, but also that the semen produced is of the highest quality standard.

24 How does PIC pass new young boars for commercial semen sales?

All new boars have to pass strict quality control tests, including morphology, motility and ORT (a special test we use to evaluate the integrity of sperm membranes). They must have a number of consecutively passed samples prior to being approved.

25 Why is PIC GP semen coloured?

Is it harmful to the semen? We are using some dyes in the GP semen to differentiate it easily. (These dyes and anything that is in contact with the semen are non-toxic.) They are obtained from reputable manufacturers and, in addition, we toxicity-test each batch.

26 Is the semen bacteria-free?

No. The semen is not sterile and it might contain bacteria. The bacteria can come from the semen itself, from the boar, or from the environment. They do not usually produce disease in the sows but some, if they are in large quantity, might affect semen quality. That is why the majority of the extenders contain antibiotics, the semen is stored at 17°C, and great precaution is taken during the collection and processing of the semen to ensure that there is a minimum amount of bacteria.

27 How can I be reassured that the PIC semen will not spread disease?

At PIC, we go to great lengths to ensure that semen will not spread disease.

  • All boars come from PIC farms that are monitored by the strict PIC health programme. The number of sources is minimised at each stud and all sources are PRRS (blue ear) negative.
  • Every day the staff do a clinical inspection to check that all the boars are eating properly and are looking their best.
  • Boars’ temperatures are taken every day the boars are collected, to quickly identify any potential problem.
  • All our studs are EU approved; therefore having extensive serological testing, including Brucellosis, Classical Swine Fever and Aujeszky.
  • PRRS is monitored routinely by serology (at least once a month) and by PCR in semen (minimum twice a month). The PCR is an expensive test, but has the advantage that it detects virus so if there was a new infection it could be detected very quickly (serology looks for antibodies that can take up to 10- 14 days to be produced).

28 Can you pool semen?

Yes, you can, but PIC’s decision not to pool semen is based on minimising any associated health risks. Therefore one batch number on your flatpack equals one boar.


29 Why do we use fresh semen and not frozen semen?

There is the technology available for freezing boar semen, but the results are not as good as with fresh semen; the farrowing rate and the born alive are usually lower. There seems to be a big individual variation and the semen of some boars seems to perform better than others. The technology also requires more attention at farm level, so the use is still limited to some valuable genetic movements but is not used routinely at commercial level.

30 When will we have sexed semen available?

Sexed semen is becoming a reality in bull semen, but the problem is that the production process is slow and costly. If we consider the huge amounts of semen needed for the sow compared to the cow (see Stuart Revell article on page 10), it is still a long way from becoming a commercial reality for producers.

November 2006

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