The Science Behind SC Measurement

 

The relationship of SC to fertility

Testicular shape is remarkably uniform among Bos taurus breeds. Bull testis growth is very rapid from weaning to 1 year of age and slows from 1 to 2 years of age.  At 2 years of age, bull testis growth is at approximately 90% of its full potential reached at 3 to 4 years of age1.

Currently, SC measurement is still the best tool for assessing testis weight. Scrotal circumference measurements are highly correlated to paired testis weight and paired testis weight is directly and highly correlated to daily sperm production and high semen quality traits2-5. There is also considerable evidence that SC measurements between 1 and 2 years of age are moderately to highly heritable6.

The most important reason for selection for large testis size in bulls is to select for early puberty in their female offspring. Nearly all heifers are bred at approximately 14 months of age to result in calving at 2 years of age.  On average, about 50% of heifers start cycling at 12 months of age and most should have their first cycle by 14 months. The first estrus may be subfertile and normal pregnancy rates do not occur until the 3rd estrus cycle after puberty. Therefore, selection for early puberty in heifers is advantageous for optimal reproductive success in this age group. The easiest way to select for early puberty in heifers is to select for large testes in their sires. A 1996 study showed that bulls with higher EPDs (expected progeny differences) for SC produced daughters reaching puberty at significantly earlier ages, even when bulls were used in crossbreeding7.

Scrotal circumference is a more accurate predictor of age at onset of puberty than either age, or weight, regardless of breed8.  Among breed groups, negative correlations greater than 0.9 have been observed between SC and bull age at puberty and age at puberty in half-sib heifers9,10. A Beef Research Progress Report11 indicated an extremely high correlation (-0.98) between breed averages for yearling SC and average age at puberty in heifers (larger SC was associated with earlier age at puberty in heifers).  In addition, a highly significant correlation of -0.91 was observed for scrotal circumference with age at puberty in heifers12. These very strong genetic relationships indicate that age at puberty and SC are essentially the same trait.

There is good evidence that heterosis in cattle for traits related to size and age at puberty in females and SC of males is due to dominance effects of genes12.  Furthermore, there were correlations of 0.66 and 0.97 between breed means for scrotal circumference and fertility of female offspring12,13.  Toelle13 reported strong genetic correlations between SC and age at first breeding (- 0.77), age at first calving (- 0.66) and pregnancy rate (0.66).  As age at puberty in females is favorably associated with subsequent reproduction14, selection for larger SC should improve the reproductive potential of the cowherd.

There is no question that small testes are undesirable. In histological studies of 14-month-old bulls the proportion of seminiferous tubules with normal seminiferous epithelium was significantly less in small testes (less than 32 cm SC) than in larger testes15.  The results of these studies may be confounded by the possibility that young bulls with small testes were those that had not yet completed puberty (late maturing animals).  However, in another study involving 2 to 3.5-year-old beef bulls, testes from bulls with less than 32 cm SC had increased tubular degenerative changes16.  Cates17 reported that yearling bulls with small testes do not catch up over time and will have small testes at two years of age as well.  Therefore, yearling bulls with below minimum recommended SC should be culled.

 

Effect of breed

 Significant genetic variation exists between breeds of beef cattle for age at puberty18.  In general, faster gaining breeds of larger mature size reached puberty at a greater weight than did slower gaining breeds of smaller size.  Breeds historically selected for milk production, e.g., Braunvieh, Gelbvieh, Red Poll, Pinzgauer and Simmental, reached puberty at significantly younger ages than did those breeds not selected for milk production, e.g., Charolais, Limousin and Hereford.  There are great differences between breeds of bulls in average testicular size at any given age.  In general, the large milk-producing breeds have an earlier onset of puberty and develop larger testes at an earlier age and at maturity than smaller breeds of cattle that have lower milk production.  Double-muscled breeds such as Piedmontese, Belgian Blue, Parthenaise, Blonde d’Aquataine and Limousin have a later onset of puberty and smaller average testis size at puberty and at maturity. Few breeders have made an effort to select for larger testis size in these latter breeds, consequently the breed averages for scrotal circumference are small.  However, some breeders have capitalized on the high heritability of testis size and have made remarkable progress selecting for that trait.

 

Effects of nutrition

 Most bull calves that are intended for yearling bull sales are fed high-energy diets in the post-weaning period to maximize rates of gain in body weight. There is a great deal of literature on the effect of postweaning diet on pubertal development and scrotal circumference. Much of this literature is equivocal about whether high energy intakes increase testis size at a year of age and the literature does not indicate that sperm output, or achievement of good semen quality, is advanced by high energy diets compared to moderate dietary intakes. Larger scrotal circumferences in groups of bulls with greater energy intake do not necessarily imply greater paired testes weight as fat within the scrotal wall and inside the scrotum can increase scrotal circumference19. Nutritional level in the postweaning period likely has no effect on the potential testis size of yearling and older bulls20.

On the other hand, preweaning nutrition of the nursing calf appears to have a very strong effect on future testis size20-22. The Sertoli cells within the testis that support germinal cell development (germinal cells become sperm) multiply during the preweaning period and reach their lifetime number at 5 to 6 months of age.  The number of germinal cells that a Sertoli cell can support and nourish is finite. Germinal cell development and multiplication occurs mainly after weaning. When every Sertoli cell is “full”, maximum testis size is reached. Therefore the number of Sertoli cells that a calf has at weaning determines potential adult testis mass.

 

Effect of months of age on yearling SC

Age in months has an important effect on SC of yearling bulls. After 2 years of age SC changes very slowly and therefore the exact number of months of age beyond 2 years is usually of little importance. A considerable amount of SC data for yearling bulls that was corrected to 365 days of age, and data for 2-year-old bulls corrected to 24 months of age, has been available over the past several decades23,24.

Bulls sold as yearlings often vary in age from 11-15 months and since SC changes quickly at this age it is difficult to directly compare testis size in bulls of different ages. The expected 12-month SC in individual animals can be determined by multiplying the expected change in SC/day by the difference of actual age and 365 days.  However, the rate of growth of the testes is not linear, with more rapid growth between 11 and 12 months of age and significant slowing in growth between 15 and 16 months of age17. Therefore, age must be taken into consideration to choose the appropriate factor for change in SC/day. Fortunately, the rate of change in SC is similar for bulls regardless of breed2.

The table below shows the rate of change in SC per month for any breed of bull. For example, in a 14 mo old bull measuring 36 cm SC would have changed 1.1 cm from 12 to 13 mo and 0.8 cm from 13 to 14 months.  Thus at 12 months the bull would have been 36 – (1.1 + 0.8) = 34 cm. An 11 month old contemporary bull with a SC of 33.5 cm would be expected to gain 1.5 cm by the time he was 12 months old then equaling 35 cm. Therefore the 11-month-old bull would have the superior SC measurement.

 

Estimated increase in scrotal circumference (SC) per month of age for 10 breeds of beef bulls (Angus, Hereford, Shorthorn, Galloway, Red Poll, Charolais, Simmental, Maine Anjou, Limousin, and Salers).

  Increase in SC
Age in Months Daily (cm) Monthly (cm)
10-11 0.062 1.9
11-12 0.050 1.5
12-13 0.038 1.1
13-14 0.026 0.8
14-15 0.014 0.4

 

The following table shows the minimum recommended SC for beef cattle in Canada. For further information on SC for age and breed please consult your local veterinarian, or breed association.

Suggested Minimum Scrotal Circumference (cm)

Age (months)

Simmental

Angus

Charolais

Gelbvieh

Pinzgauer

B. Swiss/ Braunvieh

Hereford

Shorthorn

Maine Anjou

Red Poll

S. Devon

Holstein

Speckle Park

Salers

Tarentaise

Limousin

Blond d’Aquitane

Galloway

 Texas Longhorn
12 32 31 30 29 28
13 33 32 31 30 29
14 34 33 32 31 30
15 34.5 33.5 32.5 31.5 30.5
16-20 35 34 33 32 31
21-30 35 35 34 33 32

                       

References

  1. Coulter GH. Testicular development: its significance in young beef bulls. Proc 8thTech Conf AI Reprod NAAB, 1980, p106.
  2. Almquist JO, Branas RF., Barber KA. Postpuberal changes in semen production of Charolais bulls ejaculated at high frequency and the relation between testicular measurements and sperm output.  J Anim Sci 42: 670-676, 1976.
  3. Coulter GH, Foote RH. Bovine testicular measurements as indicators of reproductive performance and their relationship to productive traits in cattle: A review. Theriogenology 11: 297, 1979.
  4. Moser DW, Bertrand JK, Benyshek LL, McCann MA, Kiser TE. Effects of selection for scrotal circumference in Limousin bulls on reproductive and growth traits of progeny. J Anim Sci 74: 2052-2057, 1996.
  5. Smith MF, Morris, DL, Amoss MS, Parish NR, Williams JD, Wiltbank JN. Relationships among fertility, scrotal circumference, seminal quality, and libido in Santa Gertrudis bulls. Theriogenology 16: 379-397, 1981.
  6. Coulter GH, Rounsaville TR, Foote RH. Heritability of testicular size and consistency in Holstein bulls. J Anim Sci 43:9, 1976.
  7. Moser DW, Bertrand JK, Benyshek LL, McCann MA, Kiser TE. Effects of selection for scrotal circumference in Limousin bulls on reproductive and growth traits of progeny. J Anim Sci 74: 2052-2057, 1996.
  8. Lunstra DD, Ford JJ, Echternkamp SE. Puberty in beef bulls: hormone concentrations, growth, testicular development, sperm production and sexual aggressiveness in bulls of different breeds. J Anim Sci 46: 1054-1062, 1978.
  9. Brinks JS, McInerney MJ, Chenowith PJ.Relationship of age of puberty in heifers to reproductive traits in young bulls.  Proc West Sect Am Soc Anim Sci 29: 28, 1978.
  10. King RG, Kress DD, Anderson DC, Doornbos DE, Burfening J. Genetic parameters in Herefords for puberty in heifers and scrotal circumference in bulls. Proc West Sec Anim Sci 34: 11, 1983.
  11. Lunstra DD. Testicular development and onset of puberty in beef bulls. Beef Research Progress Rep. No. 1. U.S. Meat Res. Cent. ARM-NC-21. p28. Clay Center, NE, 1982.
  12. Gregory KE, Lunstra DD, Cundiff LV, Koch RM. Breed effects and heterosis in advanced generations of composite populations for puberty and scrotal traits of beef cattle. J Anim Sci 69: 2795-2807, 1991.
  13. Toelle VD, Robison OW. Estimates of genetic correlations between testicular measurements and female reproductive traits in cattle. J Anim Sci 60: 89-100, 1985.
  14. Werre JF, Brinks JS. Relationships of age at puberty with growth and subsequent productivity in beef heifers.  Proc West Sect Amer Soc Anim Sci 37: 300, 1986.
  15. Madrid N, Ott RS, Rao Veeramachaneni DN, Parrett DF, Vanderwert W, Wilms CL. Scrotal circumference, seminal characteristics, and testicular lesions of yearling Angus bulls. Am J Vet Res 49: 579-585, 1988.
  16. Rao Veeramachaneni DN, Ott RS, Heath EH, McEntee K, Bolt DJ. Pathophysiology of the testes and epididymides, seminal characteristics, and endocrine profiles. Am J Vet Res 47: 1988-1999, 1986.
  17. Cates WF, Nicholson HH, Crow GH, Janzen ED. Testicular development in record of performance bulls. Proc Soc Theriogenology, Spokane, pp 16-30, 1981.
  18. Gregory KE, Lunstra DD, Cundiff LV, Koch RM. Breed effects and heterosis in advanced generations of composite populations for puberty and scrotal traits of beef cattle. J Anim Sci 69: 2795-2807, 1991.
  19. Seidel GE Jr, Pickett BW, Wilsey CO, Seidel SM. Effect of high level of nutrition on reproductive characteristics of Angus bulls. 9thInternational Congress on Animal Reproduction and Artificial Insemination, III.  Symposia  (free comm) 359, 1980. Abstr. Madrid.
  20. Bagu ET, Madgwick S, Duggavathi R, Bartlewski PM, Barrett DMW, Huchkowsky S, Cook SJ, Rawlings NC. Effects of treatment with LH or FSH from 4 to 8 weeks of age on the attainment of puberty in bull calves. Theriogenology 62: 861-873,2004.
  21. Brito LFC, Barth AD, Rawlings NC, Wilde RE, Crews DH, Mir PS, Kastelic JP. Effect of improved nutrition during calfhood on serum metabolic hormones, gonadotropins and testosterone concentrations, and on testicular development in bulls. Domestic Animal Endocrinology, 33(4):460-9, 2007.
  22. Lunstra DD, Gregory KE, Cundiff LV. Heritability estimates and adjustment factors for the effects of bull age and age of dam on yearling testicular size in breeds of bulls. Theriogenology 30: 127-136, 1988.
  23. Bull Breeding Soundness Evaluation. 2ndedition. Barth AD ed. The Western Canadian Association of Bovine Practitioners, Saskatoon, 2000.
  24. García-Guerra A, Barth AD. Increase in average testis size of Canadian beef bulls. Can Vet J 54:485-490, 2013.