BRAHMAN NEWS MARCH 2014 ISSUE #182 PAGE 25
by Paul Williams TBTS TECHNICAL OFFICER
The Australian Cattle Veterinarians (ACV) has recently released revised standards for minimum scrotal circumference to pass a Veterinary Bull Breeding Soundness Evaluation (VBBSE).
The revised standards have resulted from recent advances in understanding as to what constitutes normal scrotal size in the Australian herd based on the "Bull Power" project. This project involved analysis of ~260,000 scrotal circumference measurements that had been recorded with BREEDPLAN between 2000 and 2012 across 13 common breeds of Australian beef bulls, including Brahman. Scrotal circumference measurements were mostly taken when bulls were within 250–750 kg live weight and 300–750 days of age.
Measuring scrotal circumference can indicate the likelihood that a bull has reached puberty, and whether testicular development is within the normal range. Scrotal circumference assessment is an important part of a VBBSE, which is used to assess whether bulls have normal reproductive function prior to making selection and management decisions.
Scrotal circumference is:
Previous VBBSE standards listed simple minimum cut-off points for scrotal circumference based on breed and age. Historically, bulls with a scrotal circumference greater than the cut off were scored with a tick for the scrotal circumference component of the VBBSE, and those below with a cross.
In recent times, the VBBSE has moved from a pass/fail model to a description of risk with a T (Tick), Q (Qualified) and X (Cross) system being used to acknowledge that there are shades of grey and that assessment of risk is not always absolute.
The main findings from the recent analyses include:
There is no clear point at which a scrotal circumference indicates an increased risk to fertility. Thus, scrotal circumference needs to be interpreted along with other elements of the physical examination, and with crush side semen and morphology results if available. Bulls with a scrotal circumference that is not within "normal limits" should be viewed with suspicion unless it can be shown by other means that this does not pose a risk.
Details of normal scrotal circumference in the Australian Brahman bull population are provided in Figure 1. As a general principle, it is recommended that the minimum acceptable scrotal circumference is the bottom 5% value at any weight.
It should be noted that the minimum recommended is "bare minimum". For example, bulls close to the minimum threshold are more likely to suffer from some of the problems associated with small scrotal circumference than bulls well above the threshold. Bull breeders may consider setting a higher scrotal circumference minimum for sale bulls than what is indicated by normal range.
Table 1 shows the data from Figure 1 in tabular form and provides a ready reckoner to the minimum acceptable scrotal circumference at a given live weight in Brahman bulls:
Acknowledging that it is not always possible to obtain live weights for bulls, Table 2 provides recommended minimum scrotal circumference thresholds for Brahman bulls in situations where live weight is not available. Again, these are the "bare minimum".
The above standards do not indicate that either puberty or sexual maturity has been reached; rather, they indicate normal minimum levels of testicular development as reflected in scrotal circumference. Separate evaluation of crush-side semen and or sperm morphology are required to confirm attainment of puberty and sexual maturity, with most bulls reaching puberty in a narrow range of 27 – 30 cm.
Figure 2 shows the percentage of bulls within each weight category that did not meet the recommended scrotal size standard.
Australia Cattle Veterinarians, Veterinary Bull Breeding Soundness Evaluation, 2013
Correct method for measurement of scrotal size. (From Ott, RS (1986) Breeding soundness examination of bulls. In Morrow, DA (ed), Current Therapy in Theriogenology 2, WB Saunders, Philadelphia, p131)
Table 1: Minimum Acceptable Scrotal Circumference by Live Weight in Brahman Bulls
|live weight||Scrotal Circumference|
Table 2: Minimum Acceptable Scrotal Circumference by Age in Brahman Bulls (Where Live Weight is Unavailable)
|12 months||20 cm|
|15 months||23 cm|
|18 months||25 cm|
|21 months||27 cm|
|24 months||29 cm|
|27 months||30 cm|
Figure 1: Details of normal scrotal circumference in the Australian Brahman bull population are provided in Figure 1
Figure 2: Percentage of Brahman bulls with scrotal size below the minimum recommended
Correct application of a Barth tape.