Technical information - selection

Bull Reproductive Soundness

by Alex Ashwood


The profitability of a business is highly dependant on efficient reproductive performance and the capacity of fertile bulls to successfully service cows for several seasons.

A deficiency in the bull’s breeding ability has a large impact on net income and the costs of production. Using sub fertile bulls leads to lower calving percentages, extended calving intervals, reduced genetic progress and the expense of carrying empty cows. Additionally, cow wastage may increase due to fat cows which can be difficult to get in calf. The impact of these factors are further accentuated if the beef operation relies on a limited breeding season of 60-90 days.

The actual service capacity of a bull is the result of a combination of the reproductive soundness, genetic potential and environmental (eg nutrition, climate, health) factors and the level of herd management. Subsequently, it is not possible to determine the "true" breeding soundness before a bull is used in the herd

"Breeding soundness evaluations assist in the assessment of bulls"

Both pre and post sale evaluations are necessary to determine the reproductive soundness of bulls. Ongoing assessment is also necessary on an annual basis 60-90 days prior to turning bulls out to allow time to replace ineffective bulls.


Heritabilities are moderate-high for most of the semen traits (Table 1) and high for testes size and scrotal circumference. This has major implications for bulls with poor semen traits and below average semen testes/scrotum measurements.


Characteristic Heritability
Scrotal Circumference 0.40 0.68
Volume 0.24 0.44
Concentration 0.36 0.52
Mobility 0.31 0.46
No. of sperm 0.38 0.54
No. of sperm/ejaculate 0.49 0.64

Note: Repeatability 0.41 - 0.64 (ie high)

Factors Affecting Reproductive Soundness

There are 7 main factors which can affect bull fertility and performance. These are:

  • Age of the bull
  • Capability of reproductive organs
  • Semen quality
  • Level of libido
  • Plane of nutrition
  • Structural soundness
  • Environment

Age of the bull

Puberty is related to age, body weight, testes size and feed management. Increasingly bulls are being purchased at a younger age (18-24 months) with the expectation that they can effectively service cows. Age and bodyweight at puberty can vary across bulls but scrotal circumference at puberty remains relatively constant.

Bulls exhibit sexual interest about 3 weeks prior to puberty and attain mating ability about 6-8 weeks after puberty. Certainly young bulls can breed but reproductive efficiency increases up to maturity.

Producers may believe that after a successful season that bulls are of sound breeding value for the next season. Unfortunately this can be incorrect. Mature bulls that do not settle their share of cows in a prescribed breeding period are contributing to reproductive inefficiency. Subsequently a breeding soundness examination should be carried out annually to ensure that the bull is basically sound.

Reproductive System

Figure 1 illustrates the various parts of the bulls reproductive tract. Sperm is produced continuously by the testes and stored in the epididymis. The prostrate gland, seminal vesicles and cowpeas gland secrete the fluid component of the semen. The seminal fluid contains substrates, buffers, inorganic ions (sodium, chlorine, calcium etc) and proteins. These proteins (known as fertility associated antigens) are particularly important since they bind to certain compounds in the female tract that increased the chances of fertilisation. Factors that affect the quality of sperm cells also impact on the efficacy of the seminal fluid.

Figure 1: the reproductive tract of the bull

Figure 1: the reproductive tract of the bull

The scrotum supports and encloses the testes. Its main function is to regulate testicular temperature through perspiration and muscular contraction that raises the testicles in cold weather and relaxation that lowers them in warm weather.

Inside the scrotum (Fig 1) and attached to each testicle is the epididymis, a 3 metre long tightly coiled tube made up of three sections (head, body, tail). The functions of the epididymis are concentration, storage, maturation and transportation of sperm cells. Immature sperm cells are immobile when they enter the epididymis and become mobile after maturation.

Testicle Size and Consistency

Testicular consistency refers to the firmness and resilience (or springiness) of the testicles and is a reasonable indicator of testicle function and semen quality. Soft testicles with low resilience are associated with high percentage of abnormal sperm and low reproductive performance. Normal testicle function and good semen quality are indicated by firm testicles with high resilience (tendency and return to normal shape after squeezing) Table 2.

TABLE 2: testicular consistency score

Score Feature
1 Very firm
2 Firm
3 Moderate
4 Soft
5 Very soft

Bulls with testicle consistency scores of 2 or 3 generally produce good quality semen. Bulls with 4 or 5 scores are likely to produce poor quality semen. Bulls with very firm testicles (hard) may have suffered fibrosis and may have unsatisfactory semen.

Testicle size is the main factor determining the number of semen and volume of ejaculate. Since testicular development in bulls is a highly heritable characteristic (0.68) it is important to monitor the testicular development of bulls.

Scrotal Conformation

Scrotal circumference is the most accurate indicator of testicle size and its measurement is directly related to the total mass of sperm producing tissue, sperm cell normality, the onset of puberty in bulls and the fertility of female progeny. Table 3 shows the average scrotal circumference of Braham bulls at various ages.

TABLE 3: comparison by age of average scrotal circumference (cms) FOR pasture fed brahman bulls (personal communication., i tucker, qld)

18 - 23 24 - 26 27 - 30 31 - 36
28 - 30 32 34 36

(i) Several factors affect scrotal size (SS) measurements (eg nutrition, growth, environment).
(ii) Where available use SS-EBVs in preference to raw SS scores
(iii) Scrotal size can be 3 - 6 cms higher when bulls are fed concentrate rations

The appearance of the scrotum and its contents can vary widely (Figs 2 & 3). Lateral rotation of the testicular axis (Fig 2c) & incomplete separation of the scrotal septum (Fig 2d) whilst not functionally important often raise discussion amongst producers.

"Major faults and abnormalities reduce fertility"

Bulls with their testes close to the body (Fig 3a) should receive fertility checks whilst excessive elongation of the testes can result in serious injuries. Testicular bilateral hypoplasia (Fig 3b) (underdevelopment) with testes ½ - 1/3 normal size reduces fertility and is highly heritable and is subsequently not tolerated in bull selection. Fig 3d shows the underdevelopment of one testicle (unilateral hypoplasia)

Scrotal hernias (Fig 3c) are not common but easily diagnosed. This condition is heritable and these bulls should not be selected for breeding.



Scrotal conformation: (a) normal (elongated), (b) Normal (round), (c) north-south, and (d) Y-balls/cleavage.



Scrotal confirmation: (a) cold bath scrotum, (b) bilateral hypoplasia, (c) scrotal hernia, (d) unilateral hypoplasia

Penis and Prepuce

The penis and prepuce should be examined for any sores, lacerations, abscesses, scar tissue or adhesions. Penile hair rings and penile deviations and persistent frenulum (tied back penis) are occasionally found (Fig 4 & 5).

Spiral deviation, where the penis is twisted instead of straight is the most common structural fault of the penis. Bulls with this defect produce fewer pregnancies than normal bulls. Bulls may show spiral deviations when using a electro ejaculator which will not normally occur at matings.

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.



Persistent frenulum



Spiral deviation of the penis

Penile hair rings (band of hair encircles the penis) are most often seen in young bulls. If the condition remains untreated, infection and scarring may result. Warts on the tip of the penis are commonly found on young bulls.

Injuries to the penis usually occur during the breeding season. Old lacerations and adhesions can prevent full extension of the penis or cause pain during breeding reducing reproductive performance. The shape and anatomy of the sheath are heritable and selection against poor structure will result in immediate and longer term benefits. The sheath should be medium size with strong attachments. It is important to avoid extremes (ie too tight or too loose).

"Both sheath structure and sheath size are important"

A thick prepuce is often related to excessive membrane within sheath and is prone to injury. Prolapsed eversion of the sheath micosa and pendulous prepuces are conditions which make bulls more prone to injury and prolapsed.

Internal and External Examination

The purpose of an internal examination (rectal) is to detect any abnormalities (eg inflammation, adhesions, fibrosis) in the seminal vesicles, prostrate, ampullae. Rarely are there problems with the prostrate but infection can occur in the seminal vesicles and ampullae which characterised by enlargement of the seminal vesicles (seminal vesiculitis).

TABLE 4: bull breeding soundness evaluation

Motility - min is 30% (Preferably 60 - 70%)
Rapid swirling >70% Very good
Slower swirling 50 - 69% Satisfactory
Generalised oscillation 30 - 49% Fair
Sporadic oscillation <30% Poor

Note: It takes approximately 60 days for a sperm cell to develop and a further 10 days for it to mature thus semen quality tests assess the function of the tests 60 - 80 days prior to the sampling

An external examination involves the palpation of the testes, epididymis and scrotum. The upper portion of the epididymis should be soft pliable and free from any lumps or enlargements.

Semen Quality

Sperm cell concentration, motility and morphology evaluations are the basis of scoring semen (Table 4).

- Concentration (the number of normal sperm cells present in each cc of ejaculate) and volume (the number of cc’s of ejaculate) represent the total seminal output. Some bulls do not respond well to electrical stimulus and the collection can be inconclusive

- Motility of individual sperm is an important factor in determining the breeding soundness of bulls. Whilst >30 per cent motility is the acceptable standard (Society of Theriogenology), the figure of 60-70 per cent provides a more confident value since active moving sperm cells are essential for fertilisation. Semen with high motility has vigorous swirls whilst poor activity represents limited motility.

- Morphology or the shape of the sperm cells is important since normal cells are necessary for fertilisation. Minimal values are set at 70-75 per cent normal with abnormal cells less than 25 per cent. Sperm cell abnormalities can either be primary or secondary conditions.

- Colour is also an indication of semen quality. The semen should be milky-creamy in appearance and free from contaminants such as blood, pus or urine.

Semen tests for young bulls may be inconclusive and subsequently not totally reliable indicator of semen quality. Semen quality improves dramatically up to 4 months following puberty and bull maturity (4-6 years).

"Semen quality tests are important snapshots at a particular point of time"

Semen quality tests need to be considered in conjunction with other aspects of breeding soundness examinations, ie visual assessment of mounting, internal and external examination and monitoring pregnancies.

Unless there is trauma, fever, stress or inflammation, testes function is relatively stable. Where function is compromised, semen quality may be affected immediately and subject to the degree of damage may persist for several months.

Libido (mating ability)

Libido, or sex drive, is important in the bulls ability to service a large number of cows. There is no practical way to estimate a bulls potential mating ability except to observe the bull servicing cows.

"Up to 20 per cent of reproductive failures are due to low mating ability"

Semen production, scrotal size or hormone levels do not relate well to mating performance. It is possible to get good semen bulls with a low desire to mate and vice versa.

Irrespective of whether formal evaluation tests are conducted for reproductive soundness, producers would be wise to test a new bull with a group of females (ie trial group post sale). These observations should be supported by heat observations and monitoring and pregnancy testing.


Nutrition is important for the development and maintenance of the reproductive system. Balanced rations (ie energy, protein, fibre, minerals) are necessary for semen health, semen production and physical activity.

"Feeding practices can have a profound effect on reproductive performance"

Underfeeding causes delays in puberty of young bulls and reduced libido and sperm quality of mature bulls. Overfeeding, on the other hand, can cause irreversible testicular damage & low libido. Excessive fat deposits in the scrotum interferes with temperature regulation although the degree of fatness to cause deteterious effects is yet to be fully determined. Studies in Canada have shown that high carbohydrate diets with low levels of exercise reduces the fertility of bulls. The study showed that bulls with 4mm of fat compared to 10mm had 60 per cent more sperm cells and also had higher quality semen.

"Young bulls should be well grown but not in a fat condition"

Fertility can be negatively affected by certain feedstuffs (eg excessive gossypol from white cotton). Certainly cottonseed byproducts can be fed to bulls but they should be fed at low to moderate rates (ie 1-2kg/day for a limited period of time (3-4 months). Fertilisers (eg MAP/DAP) are not recommended for cattle feeding as a source of phosphorus due to the presence of heavy metals. The presence of cadmium has also been shown to produce adverse effects on sperm quality. Apparently the toxic effect induces membrane impairment, lower maturity and reduced fertility.

High levels of soluble carbohydrates (eg fast grains) greatly affects the breeding soundness of bulls. It is suggested that producers when obtaining a bull acquire information on the type of diet & level of feeding.

In addition to balanced intakes of protein and energy (ie energy to protein ratio), adequate intakes of mineral (trace and macro) are necessary for reproductive performance eg zinc deficiency causes lower fertility due to reduced sperm quality. Selenium deficiency in bulls decreases the development and maturation process for sperm (spermatogenesis). Calcium and phosphorus are necessary for skeletal development and the latter essential for efficient metabolism of feed nutrients. Zinc, copper and manganese are also needed for skeletal development and hoof integrity for walking and mounting.

The administration of trace minerals and inclusion of key macro minerals can stimulate mineral dependent reproductive process where there are mineral deficiencies in certain diets and environments.

The best approach to mineral supplementation is to determine the levels of intake & potential shortfalls then provide strategically timed intakes of balanced mineral supplements prior to the breeding season. Imbalances of minerals or excessive levels of intake have an antagonistic effect eg zinc and copper become antagonistic if either mineral is oversupplied and extreme ratios of phosphorus and calcium can limit the absorption of these minerals.

Exercise is important for bulls fed on high grain-low fibre diets. Physical activity strengthens the feet, legs & back ie physical attributes essential for walking and servicing cows. Prolonged periods of lying down also negatively impact on semen quality.

Structural soundness

The bull needs to be able to effectively source and service females and any factor that reduces walking and mounting ability impacts negatively on the bulls breeding performance.

The structural soundness of feet and legs is paramount if the bulls are expected to seek and service cows. Structural defects of feet and legs can seriously impact on mating ability and longevity of the bull. A conformation problem commonly associated with poor breeding performance is extreme straightness of the rear legs. A moderate angle in the hock is necessary for the bull to thrust properly after mounting. Proper angulation in the leg joints also helps to absorb the shocks produced when walking, and increases the life span of the bull. Most structural faults (eg sickle hock, cow hock, post legged) are heritable (0.12) and subsequently defects can be passed onto both female and bull progeny.

In the preparation of bull for sale, overfed bulls may have been foundered (acute and sub acute laminitis) to some degree in their growing phase. The effects of laminitis (Brahman Journal June 2001) may not be fully expressed for some time after purchase (ie swollen joints and deformed feet). The long term effects however is reduced walking, servicing capacity, lameness and foot problem, lower reproductive efficiency and reduced bull longevity.


Degeneration of the testes may occur at any time and can be caused by prolonged hot weather with high humidity levels, extreme cold weather with severe frosts, poor blood circulation, age, trauma and stress, bacterial diseases and general ill health. A general sign of degeneration is either excessive swelling or a decrease in testicular size.

"External and internal heat can be detrimental to semen quality and bull fertility"

In locations experiencing severe winters (eg Tablelands NSW) a bulls scrotum may be damaged. Low temperatures & frostbite can damage the epididymis permanently & prevent the passage of semen.

Excessive heat and humidity can reduce semen quality due to inadequate thermo regulation of the testicles. Respiratory problems also have a negative effect on breeding ability.

Sources of internal heat include ill health, infections and disease (eg 3 day sickness) which can reduce bull libido, mating ability, semen production and quality.

Studies to evaluate the important environment and physiological status on semen quality (volume of ejaculate, sperm concentration, sperm maturity and morphology) found that semen characteristics improved up to 6 years of age. Cooler temperatures and lower humidity improved all semen quality traits in young bulls and most traits in mature bulls. Several factors were involved in these seasonal effects including feed quality and intake.

Reproductive Outcomes

The importance of bull reproductive soundness cannot be underestimated. Besides the potential economic losses due to reproductive failure, the bull is responsible for 50 per cent of the genetic base of the calf drop and the genetic improvement of the herd. This means the bull to cow ratio of 30-40+ cows to 1 bull is an underestimation of the importance reproductively of sound bulls.

Reproductive unsoundness is caused by a variety of factors including poor health, disease, poor nutrition, unsuitable environments and genetics.

It is subsequently essential that bulls undergo breeding soundness assessment at purchase and pre and post seasonal matings. Breeding soundness assessment by qualified practioners in conjunction with visual evaluation can assist in the elimination of bulls with a less than satisfactory breeding potential and allow the selection of bulls that will improve profitability and productivity through effective reproductive performance.

"Selecting fertile and active bulls improves profit, reduces wastage and helps beat the rising costs of production"