SCROTAL CIRCUMFERENCE OF YOUNG BEEF BULLS: RELATIONSHIP TO PAIRED TESTES WEIGHT, EFFECT OF BREED, AND PREDICTABILITY

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Abstract
The relationship between paired testes weight (PTW) and scrotal circumference (SC) was examined in 335, 1- and 2-yr-old Hereford and Angus bulls. Measurements were taken within 15 days of each other. The equation best describing these data is Y = − 722.28 + 36.53X (r = 0.95; SE = ± 57.6), where Y is PTW (including epididymides) in grams and X is SC in centimeters. Comparable equations having age in days or body weight in kilograms as the independent variable are Y = 70.46 + 0.76X (r = 0.80; SE = ± 108.2), and Y = 74.37 + 1.00X (r = 0.80; SE = ± 107.6), respectively. Differences in SC due to breed were examined in 3063, 1-yr-old beef bulls of nine breeds completing 140-day growth-performance tests. Data were adjusted for location-year and body weight for all breeds except Herefords, where age was also included. The average SC ± SE by breed were Simmental 37.7 ± 0.2 cm, Aberdeen Angus 35.6 ± 0.1 cm, Maine-Anjou 35.4 ± 0.2 cm, Charolais 34.7 ± 0.1 cm, horned Hereford 34.6 ± 0.1 cm, Shorthorn 34.2 ± 0.2 cm, polled Hereford 34.0 ± 0.2 cm, Blonde d'Aquitaine 32.4 ± 0.5 cm, and Limousin 32.1 ± 0.3 cm. Predictability of SC measurements at the end of growth performance tests from those taken at various times during the test varied from 0.44–0.68 at or near the beginning of the test period to 0.88–0.94 at 1 mo before the end. Although all correlations were highly significant (P < 0.01), only measurements taken within 1 mo of completing the growth-performance test were sufficiently predictable for culling 1-yr-old bulls. Correlation coefficients calculated between SC in 1-yr-old bulls and SC and PTW when they were 2 yr old were 0.76 and 0.65 (P < 0.01). These values suggest that SC in young beef bulls is a useful predictor of PTW. Also, this study showed that testis size of 1-yr-old bulls differed among breeds and that SC of 1-yr-old bulls can be used as an aid to select herd sires likely to have above-average testicular size, thereby increasing the probability of impregnating more females when under heavy breeding pressure. Key words; beef bulls, testis weight, predictability, breed