Bull Testing Species: cattle

Bull Evaluation

Assessing bulls objectively and its impact on genetic gain and herd performance.

By Dr Andrew Marland BVSc (hons) Vet Cross

During a bull's reproductive lifetime, when joined under paddock conditions, he should sire between 150-350 calves. Given a commercial cow can only produce one calf per year for 6-8 seasons, breeding bulls have a major impact on genetic and performance gain within a commercial beef herd.

Producers have two equally important areas to consider when evaluating bulls for the breeding season:

• Evaluation/selection of new sires to introduce into a herd
• Regular evaluation/assessment of herd sires already selected to ensure they continue to remain fertile during their breeding life

Evaluation of New Herd Sires

When selecting new sires into a herd we expect them to:

• Be fertile
• Improve the performance and genetic gain of our herd in relation to the production system we already have in place.

Selecting fertile bulls:

Definition: Fertile bulls 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.

VBBSE (Veterinary Bull Breeding Soundness Evaluation)

Performed pre-sale a VBBSE is not an absolute guarantee that a bull will be fertile but currently gives buyers the best probability that bulls that pass the criteria set in a VBBSE will be functionally sound, seek oestus females and mate them repeatedly, impregnate them with good quality semen and be free of transmissible disease.

VBBSE objective testing includes the following elements:

• General physical evaluation to assess normal function
• Examination of the reproductive tract to assess reproductive normality
• Collection and assessment of a semen sample
• Assessment of sperm morphology where appropriate
• Serving assessment

NB: Tests to objectively determine bull fertility are not designed to identify the MOST fertile bull but rather establish a baseline, above which a bull can be regarded as having a high probability of being fertile

Objective measurements for selecting bulls that improve genetic gain and performance:

EBVs (Estimated Breeding Values), eye muscle areas, intramuscular fat measurements, gene star ratings, dam inter calving intervals, dam udder scores, scrotal circumfrences, semen morphology results, $$ export index's.... the list is long. It can be confusing and stressful enough buying bulls at auction without added confusion over all the supplied information.

Bull buyers these days are getting bombarded with different objective measurements trying to inform us of a bulls breeding potential. Buyers must be cautious as the depth of science and research behind many of these objective measurements is not consistent, nor is there benefit to many buyers.

Importantly:

No one objective measurement should be looked at on its own

• Many objective measurements can not be used to compare bulls of different breeds
• Many objective measurements cannot really be used to compare bulls from different ages or from a different stud

Estimated Breeding Values:

So how do we select bulls objectively for performance and genetic gain?

• Select a stud breeder who you have confidence in... Chances are they will have done much of the hard selection work for you. Talk to them before the sale and inspect bulls/breeding herd before the sale
• Determine where you need genetic and performance gain and set your EBV and other objective parameters around your targets
• Before the sale go through your sale catalogue and strike off bulls that don't fit your breeding program. To improve accuracy of EBV's look at the reference sire/dam results
• Consider shopping locally if possible. Bulls born and bred in our local area are used to our conditions and are good representatives of how their offspring will likely perform here also

Objective assessment of Working Herd Sires:

Herd bulls have already been selected for the genetic and performance potential they can bring to a herd. They must be objectively assessed to maintain their fertility so we achieve our 5 - 6 seasons out of them.

Physical assessment

Observe for lameness, eye problems, joint swelling, over grown claws or other abnormalities that may affect a bull's reproductive function.

Condition score assessment

If we expect bulls to go out and serve 50 cows in 9 weeks they must be in forward condition before they go out into the breeding herd. Forward store conditioned bulls have better serving capacity, libido and are more fertile.

Vaccination status

Vaccination for Three Day Sickness and Vibriosis is essential. These vaccinations must be given annually after a 2 shot initial course. Other vaccainations to consider include Botulism, pestivirus and 7 in 1.

Annual reproductive and semen assessment

Conducting annual veterinary BBSE assessments of bulls prior to mating is a quick and economic test to detect bull fertility problems before the breeding season to avoid economic losses from bull sub-fertility.

Annual morphological examination of semen samples in mature working bulls:

Morphological testing primarily reflects the physiological and pathological status of the testicles and of the transportation and maturation process of the epididymis (where sperm is stored). Bulls may be producing adequate sperm BUT the sperm are not fertile due to poor development acquired due to injury or illness.

In conclusion:

As your business growth is reliant on your breeding bulls performance and herd growth it would make good sense to have bulls tested regularly to ensure they continue to remain fertile during their breeding life.