THE SCROTAL CIRCUMFERENCE BANDWAGON
By Dr Bill Beal *
Not since the invention of a hip height stick has anything preoccupied Angus breeders as much as their current infatuation with scrotal circumference.
The most discouraging part of this ‘fad’ is that most of the attention has been focused on the wrong message related to scrotal circumference.
In practice, a bull’s scrotal circumference is the most easily measured estimate of his daily sperm production. As such, it has appropriately been designated as 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 are what make the measurement of scrotal circumference and the selection of bulls with adequate scrotal circumference important in a breeding program.
A secondary and far less important byproduct of selection of males based on actual scrotal circumference or scrotal circumference EPD is the correlation between a sire’s yearling scrotal circumference and the age of puberty of his daughters. Simply put, bulls with larger testicles at one 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 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’.
The point to be made is that by far the best use of actual scrotal circumference measurements on a bull are to insure that he has adequately sized testes to make him a satisfactory breeder. The scrotal circumference EPD is also the best indicator of a bull’s genetic potential to sire sons with larger or smaller testicles than the breed average. And finally, the bull’s actual scrotal circumference and scrotal EPD have a direct bearing on one reproductive trait in their daughters: age at which they reach puberty. The following paragraphs provide the information that explain these concepts and present a justifiable strategy that includes selection of sires based on scrotal circumference or scrotal circumference EPD.
SCROTAL CIRCUMFERENCE AND BULL BREEDING SOUNDNESS
To be a successful breeder a bull must detect and successfully breed females in heat which includes delivering an adequate number of motile, morphologically-normal sperm to achieve acceptance pregnancy rates. The Breeding Soundness Exam (BSE) devised by the Society for Theriogenology is designed to identify bulls which are most likely to be satisfactory breeders by measuring their scrotal circumference, estimating the motility of ejaculated sperm, and evaluating the percentage sperm in the ejaculate that appear normal in shape. To be classified as a good potential breeder a yearling bull must be in good general health and exceed a threshold for each of these traits (> 30 cm scrotal circumference; > 50% sperm motility; > 70% normal sperm).
Scrotal circumference is a component of the BSE because it is easy to measure and it is highly correlated (.72 – .92) with daily sperm output in 12- to 18-month-old bulls. Furthermore, the measurement of scrotal circumference in 18-month-old Hereford and Angus bulls explained 45% of the variation in sperm output when the same bulls were re-examined at 6 years of age. Finally, the scrotal circumference measurements of yearling bulls are positively correlated with sperm motility and percent normal sperm. Hence, scrotal circumference of a young bull directly and indirectly impacts his likelihood of passing all three components of the BSE.
Despite the positive relationships between scrotal circumference and semen traits, a consistent relationship between scrotal size and reproductive efficiency of bulls has been difficult to demonstrate. Researchers at the US Meat Animal Research Center reported that pregnancy rates in groups of 15-20 heifers single-sire mated for 45 days to yearling bulls with a small (< 28 cm; 54.6%) or moderate (28 – 32 cm; 63.6%) scrotal circumference were lower than pregnancy rates for groups mated to bulls with a larger (> 32 cm; 81.8%) scrotal circumference. On the other hand, several other studies have failed to demonstrate higher pregnancy rates for groups of cows bred by bulls with scrotal circumferences that differed, but were all above the minimum threshold recognized by the BSE.
The ‘common sense’ position on assessing a bull’s breeding potential is to consider the importance of a bull’s scrotal circumference as a ‘threshold point’. If the bull exceeds the recognized minimums set forth by the BSE, then there is every indication he will be a satisfactory semen producer and successful breeder. Conversely, 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.
One situation in which selecting bulls with a larger actual scrotal circumference and a higher scrotal circumference EPD could be recommended is for purebred breeders who are attempting to increase the average size of testes in their herd. Scrotal circumference is a moderately heritable trait (.26 – .53) and selection of sires with higher scrotal circumference EPDs will facilitate a genetic change in scrotal circumference for the herd. In herds in which the average yearling scrotal circumference is greater than 36 cm and few bulls fall below the 30 cm “threshold”, selection for larger testicles is probably unnecessary. However, if bulls are well-fed, but numerous bulls in the herd fail the BSE due to small scrotal measurements, selection of sires with larger scrotal circumferences and higher scrotal EPDs is recommended.
In total, breeding decisions about individual bulls involving scrotal circumference involve two different approaches. If the bull’s potential fertility is the issue, then the actual scrotal measurement of the bull is of the most importance. In contrast, if the interest is in selecting sires that will increase testes size in the herd, then selection based on scrotal circumference EPD is a more valuable tool for predicting genetic change.
SCROTAL CIRCUMFERENCE AND DAUGHTER’S AGE AT PUBERTY
It was demonstrated in 1982 that scrotal circumference was the best indicator of the onset of puberty in beef bulls. Several other studies indicated that there was also a strong favorable relationship between scrotal circumference in bulls and age at puberty of their daughters. In fact, the negative correlation between scrotal circumference and age at puberty in daughters is so high, -.71 to –1.07, that scrotal circumference and age at puberty are considered to be the same trait.
Despite the strong relationship between scrotal circumference and daughters’ age at puberty, a difference in actual scrotal circumference of 1 cm between two bulls corresponds to less than one day difference in the average age at puberty of their daughters (0.8 to 0.9 d/cm). In other words, daughters of a bull with a 40 cm scrotum would be expected to reach puberty only 7 days earlier than daughters of a bull with a 32 cm scrotum. One study with Limousin bulls indicated that scrotal EPD may be a better indicator of age at puberty than actual scrotal measurements. The age at puberty differed by 25 days between two groups of daughters sired by Limousin bulls that were 2.5 standard deviations above or below the breed average for scrotal circumference EPD.
In theory selecting bulls with larger testicles to produce daughters that reach puberty earlier could affect a cow’s lifetime productivity. The basic premise of using scrotal EPD as an indicator of daughters’ lifetime productivity is the following:
- heifers of bulls with larger testicles reach puberty earlier in their first calving season
- heifers that calve early as 2-year-olds are less likely to be “open” and are more likely to calve earlier throughout their lifetime
- cows that calve early throughout their lifetime will wean older, heavier calves
Each of these is a logical concept supported by controlled research studies, but in practice these relationships may be hard to demonstrate. For example, in the Limousin study in which groups of heifers from High- or Low-scrotal circumference sires differed in age at puberty by 25 days, there was no difference in the pregnancy rates of those two groups of heifers at the end of the breeding season. Furthermore, after controlling for differences in management between farms, scrotal circumference of 667 sires from bull test stations in Ontario, Canada were shown to be unrelated to the age at first calving of their 3,025 daughters. The Canadian researchers stated that “farm level factors are currently more important in determining age at first breeding in heifers than selecting for early maturing sires by use of scrotal circumference measurements.”
The most often quoted data relating scrotal circumference of the bull and reproductive traits of his daughters indicates that for each centimeter larger a bull’s scrotal circumference:
- his daughters may reach puberty 0.8 days earlier
- his daughters are more likely to calve for the first time 0.8 days earlier
- his daughters’ calf weaning weights are likely to be 0.75 lb. heavier
Hence, while the genetic relationship between scrotal circumference and age at puberty is “real”, the impact of selecting for scrotal circumference on when a bull’s daughters are bred and calve or on their potential lifetime productivity is small.
A final note about the relationship between scrotal circumference and the “F-word”, fertility.
Most cattlemen consider “fertility” to mean whether or not a cow becomes pregnant. Note that none of the analyses performed by geneticists attempts to relate scrotal circumference of the sire and “fertility” of the daughters based on that definition. Age at which a heifer reaches puberty or the date at which she has her first or second calf 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 or calving date 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.
USE SCROTAL CIRCUMFERENCE MEASURES WISELY
Scrotal circumference measurements and EPDs can be valuable tools in making sire selection decisions if they are used appropriately. It is clear that the actual scrotal circumference measurement of a bull to be used for breeding is an excellent measure for assessing potential sperm output. The scrotal circumference is not predictive of how many females a bull can breed or how high his pregnancy rate will be, however, selecting bulls that exceed the minimum threshold based on age of the bull is the best insurance a breeder has against using a bull who will not produce an adequate number of sperm cells to be a successful breeder.
Scrotal EPDs are most useful for breeders that desire to increase testes size in their herd. Like EPDs for growth traits, the scrotal EPD is the best indicator of a bull’s genetic potential for testicular size and age at puberty. It is also encouraging to recognize that larger scrotal circumference has a small but positive impact on age at puberty among a bull’s daughters. While this is probably not an adequate reason alone to impose selection pressure on scrotal circumference, it is reassuring to know that selection of bulls with adequate scrotal size yields daughters that are likely to reach puberty well in advance of the time they need to be bred to calve by 24 months of age.
Scrotal circumference is one of 14 traits for which the Angus breed has calculated an EPD. Its importance depends on the objectives of the breeder. Placing added selection pressure on scrotal circumference, or any of the other traits, limits the genetic progress that will be made in any other trait. Hence, it is important to understand what selection to increase scrotal circumference CAN and CANNOT do for animals in the herd. Then, wise decisions can be made on the amount of selection pressure that needs to be placed on scrotal circumference.
Dr Bill Beal lectures at Virginia Tech University, USA.