Boar semen at ejaculation has a temperature of about 38°C. Normally the containers used in collection are pre-warmed to the same level. The ejaculate then is mixed with a similarly warm extender and the mixture cooled down to a storage mark of 17-18°C.

Wide experience over the years has demonstrated the potential for damaging the sperm cells through poor temperature control at any stage of this process. For example, a rapid fall in semen temperatures has often been noted at AI centres where ejaculates are carried some distance between the collection point and the laboratory. Even an apparently small reduction can cause a cold shock to cells that results in warning signs such as an abnormally high percentage of curled tails among the sperm examined under the microscope.

Another observation may be a tendency for the sperm cells in the sample to group or clump together, itself possibly an indication of their sub-standard fertilising capacity. Although such clumps can occur for a variety of reasons, one that is considered especially common is an exposure of the semen to fluctuating temperatures in the course of its transportation either within the AI facility or from boar stud to farm.

Note here that the amount of fluctuation does not need to be great before it is regarded as a risk to sperm quality. Many sources insist that the possibility for damage exists even where the semen has been exposed to temperatures varying by as little as 5 degrees Celsius.

Some laboratory tests undertaken recently in Sweden are worth reviewing in this context. They have supplied fresh evidence on probable semen effects if extremes of temperature occur during the transportation of the AI dose to the sow herd.

Such shipments conventionally are sent in boxes formed from polystyrene foam. This is an excellent insulator, of course. Extruded polystyrene is called the world's most popular choice for insulating homes and workplaces. Boxes made from it for semen delivery are water-resistant as well as lightweight. But the tests found that semen packed in a box of this type still cooled quickly in below-zero conditions resembling those of a Scandinavian winter, unless extra protection was provided.

These studies, conducted on behalf of Danish AI equipment supplier Jørgen Kruuse A/S and Swedish company Climator AB, had been designed to examine whether semen shipments could be protected effectively against damage from exceptionally low surrounding temperatures. First, the Danish investigators monitored the core temperature of doses packed in a regular Styrofoam box in a situation where the outdoor climate was freezing here, the exposure was either to a constant minus 3 degrees Celsius or to an unchanging -8°C.

Remember, the usual target of shipping boar semen is at a fixed 17-18°C to preserve its integrity. In both of the cold climates tested, a boxed but otherwise unprotected 6-dose shipment took only 90 minutes to drop in temperature from a starting mark of 25°C down to just 15°C. Increasing the pack size to 12 doses did not avoid a temperature fall, it was only less rapidthe 15°C low point now was reached in 4 hours when the surrounding environment was constantly -3°C.

"We were surprised at how quickly the semen temperature dropped, even at the higher of the levels tested," comments Claus Holmgaard at Kruuse's head office in Denmark. "You might expect something like that when the thermometer reads 8 degrees below zero, but a wintertime level of minus 3°C occurs quite commonly in Scandinavia. It is worth also pointing out that the ambient temperature around the semen was kept constant during the test period, instead of rising or falling gradually as would happen under normal transport conditions. A fluctuating temperature is likely to do as much harm to sperm than one that stays constantly cold."

Further measurements from the laboratory test indicated that severe changes in semen temperature during transport in winter could be avoided by increasing the level of protection. While polystyrene foam was retained for the shipment box, the 6 or 12 doses packed inside it were joined by sealed plastic tubes. Every tube contained a substance first used in other industries such as for lining the protective suits worn by firefighters and surrounding sensitive communications or computer equipment. The special mix of salts and water changes consistency with temperature, either absorbing warmth from the atmosphere or releasing it according to whether the environment is above or below 21°C.

The level of protection afforded to semen in cold conditions was shown to depend on various factors including the insulation of the box and the number of doses as well as the number of tubes provided. As an example from the studies at -3°C, however, putting 2 tubes with 6 doses in a standard polystyrene foam box extended the 25°C-to-15°C time from 90 minutes to 6 hours. Using 4 tubes made this 9 hours: more than long enough to deliver an AI consignment by road, even in the depths of winter Scandinavian-style.