Figure 1

Bull Fertility Analysis

SCROTAL CIRCUMFERENCE

Measuring the scrotal circumference of young bulls is an accurate, repeatable method to assess current and future sperm-producing ability. The measurement gives an estimate of the weight of the testes, which is directly related to the level of sperm production. Scrotal measurement is also positively correlated with semen volume and quality. Table 2 contains minimum recommended scrotal measurements by breed and age. Bulls with adequate scrotal development for their age have a higher probability of becoming satisfactory breeders than bulls with smaller scrotal circumferences.

Scrotal circumference is of medium to high heritability. Fertility of the male offspring can be increased by selection for this trait. The scrotal circumference of a bull is also positively related to the fertility of his daughters. Heifers from sires with larger than average scrotal circumference tend to reach puberty earlier than those from bulls with smaller scrotal circumferences. Increased scrotal circumference in sires is also favourably correlated to their daughter's age at first breeding, pregnancy rate and days to rebreeding after calving. Due to low heritability, direct selection for female fertility traits has not been successful. The strong genetic relationship between scrotal circumference and female reproductive traits provides an alternative selection method.

Minimum Scrotal Circumference for Bulls

 Age (months) Breed
 Simmental Angus
Charolais
Maine Anjou
Hereford
Shorthorn
Limousin
Blonde d'Aquitaine
 12-24  33  32  31  30
 15-20  35  34  33  32
 21-30  36  35  34  33
 >30  37  36  35  34

 

Reproductive Organs

Figure 1 illustrates the various parts of the bull's reproductive tract. Sperm is produced continuously by the testes and stored in the epididymis. The prostate gland, seminal vesicles and cowper's glands secrete the fluid component of the semen. During mating, the penis is extruded from the sheath by the straightening of the S-shaped sigmoid flexure; sperm are transported up the vas deferens to the urethra and exit via the penis.

Deep body temperatures are too warm for proper sperm production. This is why testes are located outside the body core. As the environmental temperature changes, the testes are raised and lowered in the scrotum to maintain proper temperature for sperm production.

Abnormalities of the Reproductive Organs

Various conditions can affect the function of the reproductive tract. If the testicles cannot move because of fat pads, scar tissue or a small scrotum, proper temperature cannot be maintained and semen quality may suffer. Soft testicles indicate degeneration of tissue and poor semen quality. Very small testicles indicate unsatisfactory development of sperm-producing tissue. Severe frost-bite scabs, tumours or abscesses also indicate potential problems.

Infection and inflammation can occur in any of the reproductive organs. If the testicles become inflamed, the semen quality may be impaired long after the original condition has passed, since it takes approximately 60 days for new sperm to be produced and mature.

Common penile problems include:

  • spiral deviation
  • persistent frenulum
  • penile hair rings.

Spiral deviation, where the penis is twisted instead of straight, is the most common problem. Bulls with this defect produce fewer pregnancies than normal bulls. Bulls evaluated using an electroejaculator may display spiral deviations that will not occur under natural conditions.

A persistent frenulum is a heritable condition in which the tip of the penis remains attached to the sheath and cannot be extended. It can be surgically corrected.

Penile hair rings are most often seen on young bulls. A band of hair encircles the penis. If the condition remains untreated infection and scarring may result. Other conditions that can affect the penis include fractures, warts and scarring from previous injuries.