Introduction
A major objective in a cow-calf operation is to produce one calf from each cow annually. The degree to which producers meet this goal influences their net income. A key component of efficient calf production is a high fertility level in each breeding animal in the herd. Since individual bulls service many females, a deficiency in the breeding ability of one bull has a larger impact on herd productivity than fertility problems in a single female. Using a subfertile bull may lead to longer calving intervals, a lower number of calves produced and increased costs from wintering open females. All of these results cause serious economic loss to the cow-calf producer.
Bull fertility is often measured by the percentage of cycling females exposed to the bull and impregnated during a specific time period (usually 60–90 days). Actual fertility levels are the result of a combination of genetic potential and environmental factors including nutrition, health, and cow and bull management. It is not feasible to determine true fertility before a bull is used in the herd. Nevertheless, a bull can be evaluated for breeding soundness and this information used to assess his potential fertility. A few bulls may be sterile, but most have fertility levels ranging from very high to very low.
Puberty
Puberty is defined as the age at which a bull is first able to produce an ejaculate containing 50 million sperm with a minimum of 10% motility (capable of motion). It is related to age, body weight and testes weight. Age and body weight at puberty vary across breeds (Table 1), but scrotal circumference at puberty (an indicator of testes weight) remains constant at 28–29 cm. Bulls exhibit first sexual interest about 3 weeks prior to puberty and attain mating ability about 6 weeks after puberty. Although bulls that have reached puberty can breed, reproductive capacity increases as the bull continues to mature. The reproductive development and performance of young bulls has gained attention as beef breeders attempt to accelerate improvement of economic traits in cattle and reduce costs by using younger sires.
Table 1
Factors Affecting Bull Fertility
There are 5 general factors that can affect bull fertility:
• structural soundness
• capability of the reproductive organs
• quality of semen
• level of libido
• plane of nutrition.
All are important and a deficiency in any category will negatively affect fertility.
Structural Soundness
General health and structural soundness are important aspects of fertility. Poor health can affect libido, mating ability, and semen production and quality. Structural soundness, including functional feet, legs and associated joints, is critical for the bull to effectively travel the breeding pasture and service females in heat. Any disease that impairs the mobility of the bull hinders reproductive performance.
A conformational problem commonly associated with poor breeding performance is extreme straightness of the rear leg (post-legged). A moderate angle in the hock joints is necessary for the bull to thrust properly after mounting. Proper angulation in the leg joints also helps to absorb the shocks produced during each step and increases the productive life-span of the bull.
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.
Figure 1
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.
