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.
Source: Modified from [123], © 2010, Kansas State University.
| 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.
Source: From [10], © 2004, Elsevier.
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.
Source: From [45], © 2012, Elsevier.
