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Include Scrotal Circumference In Your Bull Selection

published: January 26th 2009
by: Dr. Glen Selk
source: Oklahoma State University Extension

           Scrotal circumference is a trait that commercial cow calf operators should include in their bull selection criteria.  Studies conducted by Cates in 1975 on 1944 bulls indicated that the probability of the beef bull having satisfactory semen quality increased greatly as scrotal circumference increased from 30 to 38 cm.
     Scrotal circumference is highly correlated to total sperm output and moderately correlated to normal sperm morphology.  Bulls with larger testes will produce more normal sperm cells.  Testes size (and scrotal circumference) has a very high heritability estimate (.68).  This means it is an easy trait to select for and rapid progress can be made in selecting bulls that will produce more normal sperm cells.  Selecting bulls with large testes also has a fortuitous spin-off in improving female fertility.  Work at Colorado State University has shown that female relatives of bulls with larger testes reach puberty at a younger age.  Through selecting bulls for testes size as well as growth traits, Colorado State has reduced the age of puberty whereby 80% of the heifers reach puberty by 10-12 months of age and nearly 100% by breeding age at 14-15 months.  There is a positive relationship between scrotal circumference and yearling weights so growth traits are not compromised when selecting for both fertility and growth rate.  The table below illustrates the minimum scrotal circumference (SC) that will allow a bull to pass a breeding soundness exam.  However, bull buyers should not be anxious to accept the minimum, but rather select bulls that are average or above average for the breed and the age of the bull.  Therefore the "Good" column of the following table is a better guideline for bull selection.
 

Recommended   Scrotal Circumference
 
  Age         Minimum SC (cm)  Good

 < 15 months      30                  >34
15 - 18 months   31                  >36
18 - 21 months   32                  >37
21 - 24 months   33                  >38
> 24 months       34                  >39
            There is great variation in scrotal circumference between breeds of bulls.  Measurement of thousands of English and European bulls have shown that yearling bulls should have scrotal circumference of at least 30 cm and by 20 months of age have a scrotal circumference of at least 32 cm.  Brahman breed bulls will have smaller testes at the younger ages and they will reach maturity at an older age.  The Brahman breed bulls will have adult scrotal circumferences similar to other beef breeds.