Bull soundness - reproduction

Scrotal size and tone

Scrotal size (SS) is important for two reasons:

  • It must satisfy the current mating load of the bull.
  • It affects the fertility of the bull’s daughters.

Are the testicles big enough for the current mating load?

A minimum scrotal circumference of 32–34 cm is required, but aim for 35 cm and above. This applies for most Bos taurus breeds, at a working age of 18–24 months, and with average mating loads of 30–40 cows over a period of 3 months.

Breeds vary a little—but don’t use this as an excuse!

Note: Bos indicus breeds have slightly longer, smaller-diameter testicles.

Is bigger better?

If you are breeding replacement heifers—yes (see below).

If you do not keep heifers and do not use high mating loads, there appears to be little benefit in going much over the minimum. It seems wise, however, to have a little in reserve, so a couple of extra centimetres is a good safeguard. There is some evidence that bulls with larger SS get higher pregnancy rates early in the joining.

How about yearlings?

Most British breeds reach puberty when SS is around 30 cm, but again it seems wise to use bulls which have the minimum of 32 cm.

Size varies with condition. SS increases 1–2 cm when bulls are in good condition compared to when in store condition.

Testicle tone

Size isn’t, of course, the only criterion. The testes need to be ‘firm and springy’ and free of lumps. Soft testes may indicate degenerating semen-producing tissue. It is also wise to check the head and tail of the epididymis for any swelling or hardening.

Big testicles are a benefit if you keep your heifers

Scrotal size is of greater importance here, because bulls with large SS sire daughters with better fertility.

The female fertility link is thought to be some mix of earlier puberty and earlier conception.

Look for bulls with large SS. If BREEDPLAN fertility Estimated Breeding Values (EBVs) are available, they offer the best way of maximising this beneficial link with female fertility.

Scrotal size is an indicator of puberty (British breeds reach puberty at around 30 cm SS). It has been well documented that bulls showing early puberty will have sisters also showing early puberty.

A more far-reaching question, however, is whether a single SS measurement will predict this early puberty and also continue with a positive link with later female fertility (beyond puberty, to subsequent rejoinings)?

Fortunately, there does appear to be a positive correlation, although this is still not certain. Two important Australian research findings in this area are discussed below, and there is supporting overseas work.

  • An analysis was done on five BREEDPLAN herds in Victoria, NSW and Queensland, with 10 000 calving records and 3000 scrotal measurements. This showed a favourable genetic link between SS and the female fertility trait ‘days to calving’ (DC).

    ‘Days to calving’ is the time elapsing between the introduction of a bull to a group of females and the dates at which they calve. This is identifying some mix of early puberty (for maiden heifers), early conception and return to oestrus, and gestation length.

    Bulls with larger SS had female relatives with fewer days to calving (the genetic correlation was –0.3).

  • In a large CSIRO research herd in Queensland, 20 years of calving records were analysed and cows drafted into high-fertility and low-fertility lines. The high line not only continued its higher fertility, but also produced sons that had bigger testicles than did the sons in the low line—the reverse proof!

When to measure scrotal size

Measurements in the herds described above were taken at a variety of times from yearling age through to 20 months. The favourable link with female fertility held in all situations.

Given present knowledge, however, it seems best to measure reasonably close to puberty to give the strongest link. In British breeds on good feed, this will be around 10–13 months. For later maturing breeds/strains or for bulls on poorer feed, measurements should probably be made at around 18 months.

These measurements are used to produce the BREEDPLAN fertility EBVs. As previously discussed, these EBVs are more valuable in identifying the female fertility link than is a single scrotal measurement at sale time.

This is a developing area of cattle genetics, so it is wise to keep up with developments as new knowledge is gained.

Semen quality

Most bulls in temperate areas with large, firm testes and no obvious abnormalities in the epididymis should have sound semen. Where scrotal tone and size can be checked in yearling and 2-year-old bulls, semen testing is often not recommended (given the currently available semen tests).

Some bull breeders will, however, complement their other fertility examinations with a semen test. This allows even greater confidence in the bull’s fertility. If a serving capability test is not done, collection of semen for testing also allows a thorough examination of the penis and sheath.

If a bull has been sick, through disease or diet change, a semen test may be worthwhile before using the bull. Your veterinarian can give you the best advice in this case.

Single-sire mating groups are, of course, particularly sensitive to bull fertility.

Serving capability

Sound structure and testicles are of course no use without libido or the will to serve. Testicle size and libido are separate traits with no genetic link—bulls with large testicles do not necessarily perform any better in serving tests.

Serving capacity test

This test counts the number of serves a bull performs in a set time with restrained cows. This is used to rank bulls as high, medium or low servers, and to predict the number of cows they can be mated to. When correctly performed by experienced operators, this test has generally given useful results for mature Bos taurus bulls. Yearling bulls are difficult to test repeatedly, as are Bos indicus. The test is a valuable technique for detecting arthritis and joint problems. Older bulls with these problems may start to exhibit pain or discomfort after performing several serves in a short period of time.

The Australian Association of Cattle Veterinarians Code of Practice for Serving Capacity Testing must be observed at all times.

Serving capability test

This shorter, simpler test gives a bull only a couple of test serves. There is no ranking, but capability is proven. It is also useful for picking up problems apparent after only one or two serves (e.g. broken penis, swellings, adhesions).

Serving capability = libido + structure/mobility

Further information

This publication is adapted from NSW Agriculture’s Better Bull Buying manual. Other publications adapted from this manual include:

Bull health
Bull management
Bull soundness - structural
Yearling bulls - tapping their immense potential