BY THE AUSTRALIAN CATTLE VETS (ACV)

BRAHMAN NEWS JUNE 2007 Issue #155

GENERAL

A BBSE can include evaluation of the Scrotum, other Physical attributes, Crush-side Semen assessment, laboratory evaluation of Sperm Morphology, and in many situations, Serving assessment. All components of a BBSE are important. Semen evaluation is critical because it is a bull’s genes that are really the objective in his purchase. Other aspects of a BBSE relate to a bull’s ability and willingness to deliver the semen.

Whether there is a requirement or not, semen may also have been submitted for Sperm Morphology assessment. If a bull fails such an assessment and, there is evidence of a defect that may persist and impair fertility, this must also be declared.

SCROTUM AND PHYSICAL

ACV standards for Scrotum are that the scrotal contents are normal on palpation, and that scrotal size is within normal ranges for the breed, age, and nutrition of the bull.

The standards for Physical require compulsory assessment of eight components. If a bull is noted to fail on any of these or any other component of Physical he fails this category.

THE STANDARDS FOR CRUSH-SIDE SEMEN AND SPERM MORPHOLOGY

Crush-side Semen evaluation and laboratory evaluation of Sperm Morphology are strong indicators of whether a bull is fertile or sub-fertile. If a physically-normal bull is rated as having passed these evaluations, he is expected to be fertile, ie, if mated, he can impregnate (pregnant at day 42 of gestation) by natural service at least 60% and 90% of 50 normal, cycling, disease-free females within 3 and 9 weeks, respectively.

The ACV has 2 pass levels for semen evaluation. The highest level –Tick – indicates that the bull is fertile with respect to the category assessed and has 60% or higher progressively forward motile sperm. Further, it is very likely that the semen of this bull will be fertile after freezing. The lower level – P – was introduced to indicate that a bull may be fertile under natural mating, but it is likely his semen is not suitable for freezing.

The P level is given to bulls with between 30 and 59% motile sperm in Crush-side Semen evaluation. Bulls have to pass thresholds for eight Sperm Morphology categories and 70% or higher normal sperm, as assessed by an accredited sperm morphologist, to gain a tick for this assessment catagory. For a P-level pass, this includes having between 50% and 69% normal sperm.

It is possible that physically-normal, P-level bulls, and especially those close to the lower threshold, may be shown to be less than fully fertile when mated. Such bulls are acceptable under multiple-sire mating, but veterinary advice should be considered before single-sire mating of such bulls.

A FAIL ON CRUSH-SIDE SEMEN OR SPERM MORPHOLOGY

The word morphology means shape. Motility means movement. Research shows that when there is a problem with sperm formation and maturation in the testes, there is an increase in the proportion of defective sperm produced, which results in the bull either being infertile or sub-fertile. Defective sperm have reduced forward motility, and abnormal shape. The selected motility and morphology thresholds are based on research which has investigated the points at which fertilization and embryo survival are compromised.

CHANGES IN SEMEN QUALITY IN YOUNG BULLS

Most bulls reach puberty between 6 months and two years of age. At puberty, a bull is sub-fertile, ie, he can achieve pregnancies, but not at the rate of fertile bulls. After puberty, semen quantity and quality increase until the bull is sexually mature.

Puberty to sexual maturity is a variable time but will usually take at least several months in high-growth bulls. Early in the process, a full range of sperm abnormalities are evident with all decreasing over time. The most persistent abnormality is proximal cytoplasmic droplets (and to a lesser extent pyriform heads), and for this reason this is usually seen as an indicator of immaturity in young bulls (NOT in sexually-mature bulls).

High-energy diets are known to reduce semen quality, especially in young bulls. This is because fat laid down in the scrotum may counter the temperature control mechanisms of the bull, and if testicular temperature rises, semen production is impaired. 

CHANGES IN SEMEN QUALITY IN SEXUALLY-MATURE BULLS

Once bulls are sexually mature and producing satisfactory quality ejaculates, testes function is relatively stable, ie, if the bull is tested 2 months prior to sale and passes a semen evaluation, then, if managed under the same conditions without experiencing conditions that cause problems with function of the testes, there is a high probability (based on research done in northern Australia) that if tested again on the day of sale it would produce an ejaculate containing a similar proportion of normal sperm. 

If a bull experiences trauma, fever, inflammation, fat insulation or stress, that are sufficient to impair sperm formation and maturation, semen quality may be affected within days and may persist for months.

It takes about 60 days for a sperm cell to develop and then a further 10 days for it to mature and be available for ejaculation. Therefore, a sperm quality assessment reflects function of the testes in the preceding 70-80 days. Conversely, if a bull experiences problems such as trauma, fever, inflammation, fat insulation or stress, which are sufficient to impair sperm formation, he may produce defective ejaculates for at least 2 months.

If a bull has passed a semen evaluation, and does suffer an event that causes temporary sub-fertility, it is most likely that fertility will be restored once the bull is returned to sound stable management. 

When a bull fails on a single examination, it cannot be determined if the abnormal testicular function is permanent, resolving or getting worse. The only way to determine this is to retest the bull. For problems that are apparent at examination, the interval between first and second test can be as short as 3 weeks if the problem is easily treated (eg, case of acute footrot), but generally a period of up to 70 days (10 weeks) is recommended as this will allow for a new generation of sperm to be produced and available for ejaculation.

In some bulls, particularly over fat bulls, the recovery period may be longer. Semen quality will usually improve once bulls start working.