Bovine Reproduction. Группа авторов

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      Source: L. Brito, unpublished results.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      Source: ASB Global Inc., unpublished results.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obtained from bulls 7 to 75 months old; SC = −0.6814 + [40.26 × (log AGE)] – [10.27 × (log AGE)²] (ABS Global Inc., unpublished results).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      Source: L. Brito, unpublished results.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).

      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.

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