Sexual development is associated with marked gonadal growth. Scrotal circumference (SC) is highly correlated with testicular weight (Figure 5.1) and is the most common endpoint evaluated to determine testicular development. The testicular growth curve in bulls shows an initial period of little growth followed by a rapid growth phase and then by a plateau (Figure 5.2). Although the overall pattern of testicular growth is fairly similar in all breeds, the characteristics of the growth curve are greatly affected by genetics. In general, the rapid growth phase is shorter and testicular growth plateaus sooner in bulls from breeds that mature faster (reach puberty earlier) than in bulls from late-maturing breeds, resulting in marked differences in the curve slope. This is especially evident when Bos taurus bulls are compared with Bos indicus bulls, which in general reach puberty later than the former. The asymptotic value of the testicular growth curve, namely adult testicular size (Figure 5.3), also differs considerably among breeds.^1–7 These same differences can be observed within breeds between early- and late-maturing bulls (Figure 5.4), emphasizing the effects of genetics on testicular growth.^8–10

SC is a moderately heritable trait in cattle; yearling heritability estimates are 0.36–0.55 in Angus,^11–15 0.28 in Brahman,¹⁶ 0.40–0.71 in Hereford,^{13,14,17–23} 0.67 in Holstein,²⁴ 0.46 in Limousin,²⁵ 0.39–0.60 in Nelore,^26–29 0.32 in Red Angus,³⁰ and 0.48 in Simmental bulls.¹³ 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,³¹ 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.³² 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 5.1). Therefore, either the combination of direct selection for SC and/or indirect selection for growth traits is likely responsible for the general trend of increasing SC over the years in certain breeds (Figure 5.5).

Heritability estimates for SC vary according to age. Studies have demonstrated that heritability estimates increase with age until approximately 1 year of age (or 15–18 months of age in Bos indicus bulls), whereas estimates for 2-year-old bulls are lower.^{4,14,17,24,26,27,29,33} Therefore, selection based on yearling SC is recommended over selection based on measurements obtained at other ages. Yearling SC is commonly recorded in performance evaluation programs for beef bulls, but age effects are very pronounced around this age since testicular growth is rapid during this stage of development (Figure 5.6). In order to adjust SC measurements to age 365 days, the Beef Improvement Program recommends use of the adjustment factors described in Table 5.2. Correlation coefficients between SC at l year of age and SC and paired-testes weight at 2 years of age in Angus and Hereford and bulls were 0.76 and 0.65, respectively, demonstrating that a bull with relatively small or large testes as a yearling will generally have comparable testes size as a 2 year old.³

Attempts to establish guidelines for selection of bulls at weaning based on the likelihood of attainment of certain minimum yearling SC have produced mixed results. In one study, it was recommended that the minimum SC in Angus and Simmental bulls 198–291 days old should be 23 or 25 cm to ensure an SC of 30 or 32 cm at 365 days of age, respectively; the same recommendations for Hereford bulls were 26 and 28 cm.⁷ In another study, differences between bulls that attained a minimum yearling SC of 34 cm and bulls that did not were observed for adjusted SC at 200 days of age (23.3 vs. 20.5 cm, respectively). Based on these results, it was suggested that SC at weaning could be used to select bulls for breeding and 23 cm was proposed as the minimum SC standard at 200 days.³⁴ However, this study included bulls from several breeds with known differences in patterns of testicular growth and mature size, while using a singular and very strict yearling SC minimum. SC at 240 days of age could be used as a tool to select bulls with a high probability of meeting the minimum requirements for SC at 365 days of age (i.e., Simmental 32 cm; Angus, Charolais, and Red Poll 31 cm; Hereford 30 cm; Limousin 29 cm); sensitivity and specificity analysis for determining cutoff values indicated that the probability of Charolais bulls with SC ≥24 cm, Simmental and Limousin bulls with SC ≥22 cm, and Angus, Hereford and Red Poll bulls with SC ≥21 cm attaining minimum requirements was greater than 80%. However, SC at weaning was not useful as a culling tool, since a large portion of bulls, irrespective of breed, met the minimum requirements at 365 days of age even when SC was below 21 cm at 240 days of age.³⁵

Although the heritability of semen traits is generally low, SC is positively associated with sperm production and semen quality and genetic correlations between SC and semen traits are generally favorable (Table 5.1). This suggests that direct selection for SC would be more effective in bringing about sperm production and semen quality improvement than direct selection pressure on semen traits themselves. In addition, several studies have reported an association between sire SC and daughter puberty. In Brahman and Hereford cattle, genetic correlations between SC and heifer ages at first detected ovulatory estrus, first breeding, and first calving were –0.32, –0.39 and –0.38, respectively.^16,36 In another study with beef cattle, favorable relationships between greater sire SC and ages at puberty and at first calving were demonstrated by negative correlation coefficients between the two traits.³⁷ In a population of composite beef cattle, the correlation coefficient among parental breed group means for SC and percentage of pubertal females at 452 days of age was 0.95, whereas the correlation with female age at puberty was –0.91.⁵ A significantly greater proportion of females had reached puberty at 11 and 13 months of age when sired by Limousin bulls with high SC EPD compared with females sired by bulls with low or average EPD.³²

Although sire SC is associated with daughter puberty, evaluation of the genetic correlation between SC and pregnancy rates has produced low estimates that in some cases are not different from zero.^28,30,38 A possible explanation for these observations is a nonlinear relationship between the traits. One study in Hereford cattle indicated that the effect of SC breeding values on heifer pregnancy exhibits a threshold relationship. As SC increases in value, there is a diminishing return for improved heifer pregnancy, suggesting that selection for a high SC breeding value may not be an advantage for increased heifer pregnancy over selection for a moderate SC breeding value.²² Although it would seem that the favorable genetic relationship between SC and age at puberty does not completely translate to heifer pregnancy, it is important to note that the experimental design might have confounded some of the referred results, since it is obvious that when the entire group of heifers reach puberty before exposure to breeding, those heifers reaching puberty at younger ages would have no advantage in conception over those reaching puberty at older ages. Moreover, end-of-season pregnancy rates were used in these studies as opposed to per-cycle pregnancy rates and the value of having heifers conceiving early rather than late in the season might have been lost.