By S. Crowell and W.L. Flowers, North Carolina State University Department of Animal Science
One of the subjects for a previous column addressed the thermal environment to which extended semen is exposed after collection and extension (June 2017).In general, as extended boar semen cools, the metabolic activity of sperm and, thus, their motility begins to decrease significantly around 25 degrees Celsius and around 20 degrees Celsius the fluidity of their plasma membrane decreases due to the phospholipids of which it is composed becoming more rigid and less pliable. Because of this there are different ideas about how extended semen should be cooled after extension.
The two most common ways that this is done in commercial boar studs is either to let it cool to room temperature before it is placed in a semen storage unit or placing it in the semen storage unit immediately after being fully extended. Recently, a study was conducted to determine the effect of these two methods on selected semen quality estimates and fertility. Allowing extended semen to cool to room temperature first before placing it into the semen storage unit was designated as the “slow cooling method” while placing semen in the semen storage unit immediately after extension was designated as the “medium cooling method”.
For both of these situations, room temperature was 25 degrees Celsius and the temperature of the semen storage unit was maintained at 17-18 degrees Celsius. A third treatment was evaluated in which insemination doses were placed in an incubator maintained at 10 degrees Celsius for two hours immediately after extension and then moved to the semen storage unit. The purpose of this treatment was to mimic a situation in which extended semen initially was cooled to a temperature below its normal storage temperature. This was referred to as the “fast cooling method.” Each ejaculate (n=75) was extended with a five-day semen extender and the insemination doses were equally divided among the three cooling treatments. Semen quality estimates were evaluated during the first 24 hours after extension and then the insemination doses were used to breed sows (n=375 per treatment).
Temperature changes in the extended semen are shown in Figure 1. With the slow cooling method temperature changes resembled a “stair-step” pattern with the insemination doses not reaching their storage temperature until 12 hours after collection. With the medium cooling method, insemination doses cooled at a much quicker rate and reached their storage temperature between three and four hours after collection. Finally, with the fast cooling method, the temperature of insemination doses was 12 degrees Celsius by two hours after collection and took between one and two additional hours to warm up to 17 degrees Celsius when they were returned to the semen storage unit.
Changes in average motility (%) and curvilinear velocity (um/s) during the first 24 hours after extension are shown in Figures 2 and 3, respectively. Motility for insemination doses subjected to the slow and medium cooling methods were similar and higher for those observed in the fast cooling treatment. However, it is important to note that motility for the fast cooled insemination doses was still above 70% at the time of insemination. Curvilinear velocity followed a similar pattern for the most part with the fast cooled semen having the lowest values.
The only difference was that at 12 and 18 hours post extension medium-cooled insemination doses were higher than their slow-cooled counterparts. Farrowing rates and number born alive were similar between the slow (92 + 2%; 13.3 + 0.2 piglets) and medium (90 + 2%; 13.5 + 0.2 piglets) treatments. Both the slow and medium-cooled insemination doses resulted in superior reproductive performance compared with the fast-cooled insemination doses (84 + 3%; 12.6 + 0.3 piglets).
In summary, it appears that, at least when a five-day extender is used, either method — the two-step (slow cooling) or one-step (medium cooling) — result in similar and acceptable semen quality and fertility. However, situations in which insemination doses are first cooled below and then have to warm back up to their storage temperature should be avoided.