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From the Breeding Box


A question has been submitted by a fellow beef producer in an effort to more fully understand the genetic potential resulting from the use of science based, objective information in a breeding program.

QUESTION: I am a commercial producer from Oklahoma. I select for above average scrotal (but not too high) in my bulls as it is my understanding that his daughters will mature faster and be more fertile.

ANSWER: Congratulations, you have hit on one of the common wives' tales in the cattle business!

The scrotal circumference of a bull is important. A bull's scrotal circumference is the most easily measured estimate of his daily sperm production and it is one of the components of a bull breeding soundness exam. The size of the scrotum is also positively correlated with the percentage of normal sperm and motility of sperm in the ejaculate of a yearling bull. These relationships make the measurement of scrotal circumference and the selection of bulls with adequate scrotal circumference important.

Scrotal circumference is considered a "threshold trait." This means that once a bull's scrotal circumference exceeds a certain "threshold" (32 cm for yearling bulls) his fertility will be satisfactory. In fact, there is no evidence to support the idea that a yearling bull with a 40 cm scrotal measurement can breed more females or is more likely to have a higher pregnancy rate than a bull of the same age that has a 34 cm scrotal circumference. Hence, for the purposes of predicting a bull's breeding potential, meeting the minimum threshold is adequate.

A secondary and far less important byproduct of selection of males based on actual scrotal circumference or scrotal circumference EPD is the genetic correlation between a sire's yearling scrotal circumference and the age of puberty of his daughters. Simply put, bulls with larger testicles at 1 year of age tend to sire daughters that reach puberty (begin cycling) earlier. However, this facet of selection for larger scrotal circumference is of secondary importance for two reasons: 1) the magnitude of this effect is very small (<1 day earlier puberty for each cm of scrotal circumference), and 2) a relationship between scrotal circumference and daughters' age of puberty should NOT be construed to be a direct indicator of the daughters' potential "fertility."

Most cattlemen consider "fertility" to mean whether or not a cow becomes pregnant. None of the analyses performed by geneticists have attempted to relate scrotal circumference of the sire and "fertility" of the daughters based on that definition. Age at which a heifer reaches puberty is not "fertility" by most cattlemen's' definition. Therefore, it is important to differentiate between the effect that scrotal circumference of a sire may have on his daughter's age at puberty and the "fertility" of his daughters. Put very bluntly, there is NO evidence that scrotal circumference of the sire can be used to predict the ability of his daughters to become pregnant to a given natural service or AI breeding.

The "real world" recommendation here is to select bulls with larger scrotal circumference when possible. However, don't get so "crazy" about scrotal that you pass up the opportunity to use sires that are average or below average in scrotal (but above the minimum set for passing a pre-breeding exam) and have outstanding maternal, growth or carcass EPDs.

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