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Testicular Development

Sexual development is associated with marked gonadal growth. Scrotal circumference (SC) is highly correlated with testicular weight (Figure 6.1) 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 a plateau (Figures 6.2 and 6.3). Although the overall pattern of testicular growth is somewhat 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, also differs considerably among breeds (Figures 6.2–6.6) [1–8]. These same differences can be observed within breeds between early‐ and late‐maturing bulls (Figure 6.7), emphasizing the effects of genetics on testicular growth [9–11].

Figure 6.1 Regression lines for paired testes weight (PTW) according to scrotal circumference (SC). Holstein¹ measurements obtained from mature Holstein bulls (n = 35); PTW = −1298.5 + (50.2 × SC) [116]. Holstein² measurements obtained from Holstein bulls (n = 47) 19 to 184 months old; PTW = −654.4 + (34 × SC) [117]. Angus and Hereford SC measurements obtained from Hereford (n = 199) and Angus (n = 136) bulls 11 to 30 months old; PTW = −722.28 + (36.53 × SC) [3]. Angus and Angus × Charolais SC measurements obtained from Angus and Angus × Charolais bulls (n = 111) 14–16 months‐old; PTW = −1274 + (54.04 × SC).

Source: L. Brito, unpublished results.

Figure 6.2 Top: regression curves for scrotal circumference (SC) according to age in Holstein, Jersey, Nelore, and Guzera bulls. Holstein measurements (n = 9614) obtained from bulls 6 to 77 months old; SC = −11.75 + [56.7 × (log AGE)] – [15.3 × (log AGE)²].

Source: ASB Global Inc., unpublished results.

Jersey measurements (n = 1038) obtained from bulls 7 to 75 months old; SC = −0.6814 + [40.26 × (log AGE)] – [10.27 × (log AGE)²] (ABS Global Inc., unpublished results). Nelore measurements obtained from bulls (n = 532) 7 to 43 months old; SC = 36.9/1 + [4.22^{–(0.11 × age)}] [118]. Guzera measurements (n = 7410) obtained from bulls 2 to 69 months old; SC = 35.96/1 + [2.86^{–(0.002 × age in days)}] [119]. Bottom: testicular length and width associations with SC between 3 and 16 months of age in Angus and Angus × Charolais bulls (n = 111) receiving adequate nutrition.

Source: L. Brito, unpublished results.

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).

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.

Breed	Growth trait	r _g	References
Angus	Yearling weight	0.24−0.68	[13, 15]
Sperm concentration	0.54	[16]
Sperm motility	0.36
Total sperm defects	−0.23
Composite	Yearling weight	0.40−0.43	[36]
Hereford	Weaning weight	0.08−0.86	[15, 18]
Yearling weight	0.30−0.52	[19–21]
Weaning–yearling ADG	0.22−0.35	[22, 24]
Sperm concentration	0.77
Sperm motility	0.34
Normal sperm	0.33
Hereford/Simmental	Sperm concentration	0.2	[14]
Sperm motility	0.11
Total sperm number	0.19
Limousin	Weaning weight	0.14	[26]
Nelore	Weaning weight	0.36	[27, 28]
Yearling weight	0.34
Longissimus muscle area	0.28
Backfat thickness	0.17
Red Angus	Yearling intramuscular fat	0.05	[31]
Yearling carcass marbling score	0.01
Various breeds	Birth–weaning ADG	0.02	[92, 122]
Yearling weight	0.10−0.63
Weaning weight	0.56
Weaning–yearling ADG	0.59

ADG, Average daily gain.

Figure 6.4 Mean scrotal circumference (SC) in registered yearling bulls according to year of birth. Measurements for Angus (unspecified number of bulls) and Limousin (n = 73 757; 1184–5200/year) bulls are adjusted to 365 days of age.

Sources: North American Limousin Foundation; American Angus Association.

Measurements for Charolais (n = 6984; 121–997/year) and Hereford (n = 5553; 360–536/year) bulls are unadjusted measurements obtained between 321 and 421 days of age.

Sources: Canadian Hereford Association; Canadian Charolais Association.

Heritability estimates for SC vary according to age. Studies have demonstrated that heritability estimates increase with age until approximately one year of age (or 15–18 months of age in B. indicus bulls), whereas estimates for two‐year‐old bulls are lower [4, 15, 18, 25, 27, 28, 30, 36]. 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 6.5). In order to adjust SC measurements to age 365 days, the Beef Improvement Program recommends use of the adjustment factors described in Table 6.2. Correlation coefficients between SC at one year of age and SC and paired testes weight at two years of age in Angus and Hereford 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 two year old [3].

Figure 6.5 Mean scrotal circumference (SC) in yearling bulls. Charolais: n = 246–2622/age

(source: Canadian Charolais Association).

Chianina, Marchigiana, and Romagnola: n = 455, 415, and 425/age, respectively [118]. Hereford: n = 77–2510/age.

Source: Canadian Hereford Association.

Holstein: n = 162–1004/age.

Source: ABS Global Inc.

Simmental¹: n = 129 to 2276/age.

Source: Canadian Simmental Association.

Simmental²: n = 120 to 2022/age.

Source: American Simmental Association.

Standard deviations are all between 2 and 3 cm.

Sources: [1, 2, 118].

Table 6.2 Age adjustment factors for scrotal circumference (SC) at 365 days of age according to breed.

Breed	Age adjustment factor
Angus	0.0374
Charolais	0.0505
Gelbvieh	0.0505
Hereford	0.0425
Limousin	0.059
Red Angus	0.0324
Simmental	0.0543

365‐day SC = actual SC + [(365 – age) × age adjustment factor].

Figure 6.6 Weighted mean scrotal circumference (SC) reported for two‐year‐old bulls of various breeds: Simmental (n = 763), Angus (including Red Angus; n = 5046), Charolais (n = 1286), Hereford (including horned and polled; n = 8183), Brangus (n = 1312), Shorthorn (n = 271), Braford (n = 1210), Limousin (n = 229), Nelore (n = 6464).

Sources: [4, 35,119–121].

Figure 6.7 Mean (± SEM) scrotal circumference (SC) according to age (top) and age at puberty (bottom) in early‐ and late‐maturing Nelore bulls (n = 6 per group). Arrows indicate mean age at puberty (ejaculate containing ≥50 × 10⁶ sperm with ≥10% motile sperm; downward arrow indicates early; upward arrow indicates late). Early‐maturing bulls had greater body weight (not shown) and SC than late‐maturing bulls during the entire experimental period. In addition, early‐maturing bulls were lighter and had smaller SC at puberty than late‐maturing bulls, indicating that sexual precocity is not related to attainment of a threshold body or testicular development earlier, but that these thresholds are lower in early‐maturing bulls. G, A, and G*A indicate group, age, and group‐by‐age effects, respectively. Means with superscript asterisks indicate differences between groups within age.

Figure 6.8 Mean (± SEM) scrotal circumference (SC) and testicular ultrasonogram pixel intensity (TPI) according to age at puberty in Angus and Angus × Charolais bulls (Year 1, n = 37; Year 2, n = 39; Year 3, n = 43; Year 4, n = 33). TPI, determined on a scale of 1 (black) to 255 (white), started to increase 16–12 weeks before puberty and reached maximum values four weeks before or at puberty. These results indicate that a certain developmental stage of the testicular parenchyma must be reached before puberty and that the composition of the parenchyma remains consistent after puberty. Overall, TPI was greater (P < 0.0001) in Angus × Charolais than in Angus bulls. TPI means with superscript asterisks indicate overall change (P < 0.05) with age.

Figure 6.9 Top: Mean sperm viability and morphology according to age in Angus and Angus × Charolais bulls (n = 39). Bottom: Proportion of pubertal (ejaculate containing ≥50 × 10⁶ sperm with ≥10% motile sperm) and mature (ejaculate containing ≥30% motile and ≥70% morphologically normal sperm) bulls according to age. The interval between puberty and maturity was approximately 50 days.

Figure 6.10 Prevalence of proximal droplets in ejaculates from bulls of various breeds according to age (n = 7284).

Figure 6.11 Mean (± SEM) scrotal circumference (SC), testicular vascular cone diameter (TVCD), and fat thickness (TVCF) in Angus and Angus × Charolais bulls in two years (top: n = 37; bottom: n = 33). Testicular vascular cone diameter increased with age following testicular development, whereas vascular cone fat thickness increased similar to a pattern observed for body backfat. Means with superscript asterisks indicate last significant (P < 0.05) change with age.

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 [37]. 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 [38].

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 6.1). 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 [17, 39]. 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 [40]. 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 [33].

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 [29, 31, 41]. A possible explanation for these observations is a non‐linear 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 [23]. 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.

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