The average scrotal circumferences for the performance tested bulls were 34.7 cm independent of breed and age. The average scrotal circumference, adjusted to 365 days of age for the different breeds, was always above the minimum SC threshold values set for approval at the performance testing station. The averages of the scrotal circumference in our study were, in general, higher than those in earlier studies, complied by Barth . One reason for this might be that the minimum threshold values used at the performance testing station in Sweden are higher than the Canadian recommendations for minimum scrotal circumference values, which probably have led to a more pronounced selection towards larger scrotal circumferences in our study. However, the majority of the studies on which Barth (2000) based his compilation were performed between 20 to 40 years ago; hence the SC measurements of those studies might not be considered completely relevant in comparison with scrotal circumference measurements found today. In addition, Barth  emphasizes that ultimately bulls should be selected for a scrotal circumference that is in line with the average or higher, instead of just reaching the set minimum SC threshold values (usually based on the mean minus 2 standard deviations), in the same way as selections for growth rate and other traits are made, to hasten breeding progress.
Only a few bulls, 11 out of 1332 during the studied 13 batches of bulls, were culled at the final evaluation due to too low scrotal circumference. However, the bulls that were culled earlier due to too low scrotal circumference before the final evaluation are not included, which makes the total number of rejected bulls somewhat higher. Since it only concerns very few bulls altogether during the whole time period, according to the veterinarian in charge (Lilja-Ambuhm M, personal communication, 2010), the total number of bulls that did not fulfill the minimum threshold values set for the SC at the station was still very low.
Our findings concur with previous observations that an increased age results in a higher body weight and also to some extent in a higher scrotal circumference , since a certain correlation can be seen among these parameters. Apart from age and weight, other factors can also influence the scrotal circumference measurements of the bulls in the different batches, such as ambient temperature at the time of examination . Throughout the years, the final evaluation has taken place in February and March in the semi-outdoor barn at Gismestad. Recordings of the prevailing temperatures in the barn at the time for the final evaluation are, unfortunately, not available and therefore the influence of a possible suboptimal ambient temperature cannot be assessed in our study.
The results from our study showed, however, that there has been a gradual increase in the scrotal circumference of 0.06-0.07 cm/year for all breeds during the studied period (1997-2010). In total, the increase in the average scrotal circumference (independent of breed and age) among young Swedish performance tested beef bulls was almost 1 cm during these 13 years. Although the positive trends over time were similar for all studied breeds in absolute figures, the increase was only statistically significant for the Charolais breed, which was numerically over-represented compared to the Hereford, Angus and Simmental breeds.
Barth (personal communication, 2010) recently made a similar investigation regarding performance tested Angus-bulls in Canada and concluded that there had been an increase in the SC of approximately 1 cm during the last 15 years, which is accordance with our results. This indicates that despite using only the set minimum SC threshold values for selection purposes, a certain increase in the scrotal circumference can be reached over time. In Canada, however, some breeders have taken into account the high heritability of scrotal circumference, and used the average values for the breed in question when making their sire selections instead of the minimum scrotal circumference values, thereby achieving considerable breeding progress by their strategy (Barth 2000).
In many countries electro ejaculation is routinely used when conducting BBSE but this technique is prohibited on unanaesthetized animals in Sweden and in many other European countries . Collection of semen using an artificial vagina is by far the most common method used in Swedish dairy bulls. However, this method is considered too difficult, time consuming and sometimes dangerous for routine semen collection from untrained beef bulls under field conditions. Consequently, semen from yearling beef sires is not evaluated at all before the bulls are offered for breeding purposes. Breeding for a larger SC ought, in the long run, to result in a larger proportion of bulls having reached puberty and a normal sperm quality at the time for their final evaluation at about 365 days of age, than the 48% that Persson and co-workers reported earlier  based on assessments of semen samples obtained from post mortem examination of genital organs from bulls at the performance testing station during 1999-2003. A larger proportion of sexually mature bulls at the time of the final evaluation at approximately 12 months of age would facilitate the application of a complete BBSE procedure, whereby a semen sample collected by transrectal massage could be routinely included . It is generally accepted that sires with above-average SC produce female offspring that reach puberty earlier (0.75-10 days) and have greater lifetime reproductive potential. A possibility to select for bulls with larger testicles could be to start to use a certain scrotal circumference value for each bull relative to all the other bulls at the performance testing station within the same breed and batch in line with the value that is currently calculated for the growth rate of the individual bull.
Our study showed that the average scrotal circumference measurements for the performance tested beef bulls in Sweden were somewhat higher than those internationally set and that very few bulls did not reach the minimal SC requirements. However, the vast majority of beef bulls used for breeding purposes has not been subjected to any BBSE at all. Thus, information about scrotal circumference measurements is lacking for the majority of the beef bulls used for breeding in Sweden today. This is unfortunate and needs to be more carefully addressed by the breeding organizations in Sweden and also by the insurance companies that, at present, apply no minimum SC threshold values before insuring beef bulls.