Figure 6.3 Scrotal circumference (± 90% confidence interval) in beef bulls of various breeds (n = approximately 300 000) according to age (top) and weight (bottom).

      Source: From [8], © 2014, Elsevier.

SC is a moderately heritable trait in cattle; yearling heritability estimates are 0.36–0.55 in Angus [12–16], 0.28 in Brahman [17], 0.40–0.71 in Hereford [14, 15,18–24], 0.67 in Holstein [25], 0.46 in Limousin [26], 0.39–0.60 in Nelore [27–30], 0.32 in Red Angus [31], and 0.48 in Simmental bulls [14]. Therefore direct selection can have a very significant impact on SC. For example, selection of Santa Gertrudis bulls based on minimum SC over a 10‐year period resulted in significant changes in average SC in one herd [32], whereas testicular weight at weaning was greater in the progeny sired by Limousin bulls with high expected progeny difference (EPD) for SC compared with progeny sired by bulls with average or low EPD [33]. Results from the Swedish performing station involving beef bulls of various breeds revealed an increase in SC of approximately 1 cm between 1997 and 2010 [34]. Similarly, comparison of data obtained from Canadian veterinary practices and testing stations between 2008 and 2011 from various beef breeds, with data compiled from the literature between 1972 and 1998, revealed that mean SC increased 0.8–2 cm in yearling bulls and 0.4–3.5 cm in two‐year‐old bulls [35]. Several studies have also demonstrated moderate to high phenotypic correlations between SC and growth traits and estimates of the genetic correlations with growth traits are generally positive (Table 6.1). Therefore the combination of direct selection for SC and indirect selection for growth traits is likely responsible for the general trend of increasing SC over the years in several breeds (Figure 6.4).

Table 6.1 Genetic correlations (r_g) between scrotal circumference, growth traits, and sperm production in bulls.