Bull Testing

31st August 2018

Many aspects of Veterinary Science are cut and dry.  Pregnancy testing for example is either pregnant or non-pregnant.  Unfortunately Bull Breeding Soundness Evaluations (BBSE) are not such black and white situations.

A Bull Breeding Soundness Evaluation involves a number of tests or components (some of which may not be carried out in all instances) that are designed to assess health, locomotor and reproductive functions in order to make a judgement on both the current and future fertility of a bull. The end result is an assessment of the ‘risk level’ of a bull failure occurring rather than a definitive guarantee of fertility. 

We tell our clients that performing BBSEs is all about degrees of confidence.  The greater the number of components of a BBSE that we carry out, the greater the degree of confidence we can tell you with that a bull should be able to perform (NB - following confidence levels are approximate and are used to demonstrate a point)

If we look at the bull in a paddock and see he is not lame, has good conformation and has testicles then we may say we have 50% confidence he is going to do the job based on what we are able to assess. Upon measurement and palpation of his testicles, eliminating bulls with palpable lumps or small scrotal circumference, we may become 60% confident he can do the job. With an internal palpation of his accessory sex glands that may rise to 65% confidence.

Examination of the penis via electro-ejaculation is used to ensure that a bull is able to achieve an erection and to identify and treat/cull any bulls with signs of trauma, warts, adhesions or infections of the penis or prepuce, thus raising our confidence to 70%.

If we then proceed to collect a semen sample we are able to visually assess the sperm density and volume produced. Further examination under a microscope then allows us to make an estimation of the percentage of viable sperm cells within the sample and to check for white blood cells which may be indicative of an infection somewhere in the reproductive tract. At this point our confidence levels could be 80-85%.

Crush side semen analysis is used as a screening test for morphology analysis (assessing the number of normal/abnormal sperm).  At this stage one can make an educated assessment that the bull is fertile or for greater confidence we can submit a sample to a Semen Morphology Laboratory.  Under higher magnification, the Laboratory can identify morphological changes within the sperm heads (which are unable to be visualised at a crush side examination). Therefore, a more detailed assessment can be undertaken and bull with a positive morphology report may increase our degree of confidence to 90%.

This is the extent to which we carry out most of the BBSEs that we perform in the region. If greater confidence is desired, the next step is to perform a serving capacity test which enables us to assess a bull’s libido and ability to gain intromission to the cow. Often a serving capacity test may be carried out on individual bulls suspected of poor fertility based on preg-testing results, especially if initial semen testing and morphology does not show any abnormalities to explain the cause. Another common indication for a serving capacity test is to assess the impact that penile issues detected during semen collection may have on serving ability (i.e. spiral deviations, preputial scarring, etc). If a bull is able to serve an adequate number of cows within the given time frame our confidence in his potential fertility may increase to 95+%.

It is important to note that a BBSE provides a snapshot of a bull’s reproductive status at a single point in time. Anything that upsets the sperm development cycle has the potential to cause a transient or even permanent period of subfertility or infertility. Events that may lead to this include disease (especially even mild bouts of acidosis in times of drought feeding), stress, transport and environmental and nutritional changes. A full semen production cycle takes approximately 70 days therefore it may take this amount of time for normal sperm to be produced following the above insults. Therefore, it is advised to minimise the stress to bulls as much as possible in the couple of months leading up to joining and if purchased bulls are moved to completely different environments we would recommend performing another BBSE on them prior to joining.

Another common cause of bull breakdowns are physical injuries, either to the musculoskeletal system or the external sexual organs. Bulls fighting are the most common reason for injury and keeping joining percentages to 2-2.5% will help to minimise this during the mating period.

During the joining period we strongly recommend regular bull inspections to ensure they are in good health and suffering no lameness or injuries and that they are still showing good libido and are able to achieve intromission. If any concerns are held about a particular bull it is recommended to replace him with a spare if possible or rotate the bulls between the cow mobs to ensure all are covered by a working bull.