During the third Midwest boar stud managers conference that was held in the U.S. in August 2008, I was able to relay the outcomes of an electronic survey conducted in 2007 by a team from several U.S. universities to obtain information on practices in the North American boar stud industry. The survey has highlighted some contentious points, including the continued pooling of semen from multiple boars to create each dose for artificial insemination.

Boar studs as separate segment

The background to the survey is that one of the key developments in pork production across North America within the past 20 years has been the emergence of boar studs as a separate segment of the industry, maintaining reservoirs of genetic resources for repopulating herds. Their most important function remains the frequent distribution of highly valued genetics in the form of boar semen, to farms at numerous remote locations, without spreading disease.

Today, the studs operate as isolated units in which high biosecurity and routine health monitoring are normal requirements.

From previous reports it seems that the U.S. has close to 120 boar studs and that they represent a total inventory of around 20,000 boars. On that basis, the replies to the latest survey came from centres holding about half of all boars used nationally for AI. Among these respondents, the most common size of U.S. studs supplying semen for pig AI was between 100-500 boars. Their housing is typically in individual stalls with evaporative and mechanical cooling systems and most of them feed the animals according to body conditions.

Training in semen collection tends to start when the boar is 8 to 9 months old and usually takes one to three weeks to complete. Most often, semen is collected by a stud on Monday and Thursday of each week. With the average age of boars being one to two years, the collections can yield from 51 billion to 150 billion sperm per week or enough for the preparation of between 21 and 40 doses. The typical routine for boars is to be rested for five to seven days between collections. Turnover of inventory is rather high, with boar replacement rates in the range of 21 to 70% per year. Most are replaced either for genetic improvement reasons or because their semen was of poor quality or they had structural problems with their feet and legs.

Quality testing

Samples of ejaculates are most likely to be evaluated for motility after being warmed in extender for up to five minutes; the evaluations take place under a microscope at a wide range of magnifications. Typically, the AI centre discards up to 10% of ejaculates on grounds of poor motility, abnormalities in sperm or bacterial contamination, although rates of rejection relate mostly to individual boars and to the season of the year.


All these findings became evident in the replies received from 44 boar studs located in five Canadian provinces and 15 U.S. states. Most respondents were from the American Midwest. Together, they had a combined inventory of about 10,000 boars. Their answers to our electronic questionnaire told us about the boar breeds kept for AI in North America and how they are managed, but also how semen doses are being processed.

Most studs packaged semen in tubes or bags with 2 to 4 billion fertile sperm cells in 60 to 80ml of extender. But, it emerged that over 60% of them have been preparing the doses from a mixture or pooling of ejaculates. There are at least two ejaculates and at times up to six for a pool in these cases.


Pooling semen discouraged

Is this a technique which should be encouraged? The argument against pooling of semen for AI was put forth at the Midwest conference by Dr George Foxcroft from Canada. Among stud management procedures that limit sow fertility, he said, is the industry practice of utilising average boars.

We should place more emphasis on using higher fertility sires with higher genetic value to produce the greatest amount of pork. This means, for example, more use of single-sire matings, since single-boar AI studies have shown 10% differences in farrowing rate and increases in litter size. Further, a loss of genetic merit and productivity occurs if high-fertility boars are diluted more than necessary. With AI, a similar limit on fertility can come from pooling semen. Although pooled semen is used to reduce the chances of poor fertility, it can result in overall average performance.

Dr Foxcroft also recommended making use of fewer sperm and smaller doses, remarking that the industry over-uses both sperm numbers and insemination volumes. His other suggestion was to move toward single service. This technology is enhanced by new ovulation induction protocols and hormone treatments which allow synchronised ovulation and good fertility with use of a single, fixed-time AI.


Dr Foxcroft reported that differences in fertility appear only with a large variation in semen motility. When motility ratings of over 70% were recorded in breeding trials, no fertility effects were found. Fertility following five to seven days in storage using a longer-term extender appears to show potential as a semen test. Some boars retain 10 to 20% motility benefits over others, but this seems unrelated to other test results and even some field fertility measures.

Rest for regeneration

Are we treating the boars correctly? Dr Billy Flowers and Dr Don Levis underlined the need to rest boars between collection days to allow regeneration of sperm numbers.

The time required for a sperm cell to reach the mature state for ejaculation can take up to five to seven weeks, they said. It is possible to accelerate the appearance of normal ejaculates by increasing collection frequency to twice per day for three to four days and then providing three days of rest. This can clear out abnormal sperm. In the case of continued poor ejaculates after this treatment, this indicates that a longer-term insult has occurred.

A panel of stud managers told the Midwest conference that centres in practice commonly use a 65% cut-off for motility for identifying problem boars or ejaculates following five to seven days of storage of the ejaculate. Bad ejaculates are treated by collecting boars twice daily for a few days and then resting for days. Most studs plan for 15 to 20% reduction in doses produced by season to compensate for heat stress and declines.

Training boars usually occurs as individuals, but in cases where a boar was not advancing in training, he was run in pairs and then would receive a lutalyse injection. To train these slow boars might require two to three events in a pen with people and then the boars would be collected once every week and then every time used for production.