The ultimate goal of a bull development program is to raise physically sound bulls that meet or exceed minimum predictors of fertility established by the Society for Theriogenology.
Bull breeding soundness evaluations (BBSE) can determine whether bulls meet the standards and help to sort future sires with good potential to achieve high pregnancy rates in a short interval from those with poor potential and undesirable heritable traits.
What they won’t do is guarantee or rank fertility, says Dr. John Kastelic, a researcher in cattle reproductive health at the University of Calgary’s faculty of veterinary medicine. Nor does a routine BBSE evaluate libido, that is, a bull’s ability to identify cows in heat and his desire to breed, which makes it imperative to observe bulls to be sure they are identifying, mounting and breeding cows in estrus.
“The overall effect is to improve genetics for fertility and reproductive performance within the herd or breed,” he explains.
BBSEs look at overall reproductive development including testicular size and health, abnormalities of the scrotum, penis and seminal vesicles, the number of sperm with normal shape (morphology) and the percentage of sperm able to move (motility). Scrotal circumference (SC) is an important measurement because it’s highly correlated with testes weight (95 per cent), age at puberty (85 per cent), sperm output in young bulls (75 per cent) and semen quality (68 per cent). Furthermore, daughters of bulls with large SCs will reach puberty earlier than heifers from bulls with smaller scrotal measurements.
Some of the most common abnormalities include small testes, warts on the penis, and seminal vesiculitis (infection involving internal glands).
Physical attributes and conformation are evaluated because they are indicators of a bull’s overall ability to move and to mount a cow. This part of the evaluation takes in heritable defects, such as corkscrew claws and angularity of the hind legs, along with acquired foot problems like laminitis (founder) and interdigital fibroma (corns). General health of the joints, eyes, mouth, teeth and body condition are also considered.
The end result is a grade of sorts. Satisfactory means the bull met all minimum requirements for reproductive development, physical soundness and health. Unsatisfactory indicates that he failed to meet the minimum requirements in one or more categories, has a health or structural problem, and is unlikely to improve. The decision may be deferred if a bull doesn’t meet the minimum requirement in at least one category, but has a good chance of overcoming the shortfall in the near future.
The usual requirements are 70 per cent normal sperm (including no more than 20 per cent head defects) and 30 per cent motile sperm. The bare minimum SC standards are: 30 centimetres (cm) at 15 months; 31 cm at 15 to 18 months; 32 cm at 18 to 21 months; 33 cm at 21 to 24 months; and 34 cm over 24 months. Kastelic notes that higher standards are in use, especially for certain breeds.
One study of BBSEs done on more than 1,000 bulls showed that 63 per cent were rated satisfactory, 29 per cent were deferred and eight per cent were unsatisfactory.
By far, the most common reason for the unsatisfactory or deferred result was sperm morphology. This was the case for 52 per cent of the bulls that didn’t meet the satisfactory grade, followed by failure to meet the minimum SC standard (12.5 per cent) and failure to pass the physical evaluation (9.5 per cent). A combination of SC and morphology was the reason 11 per cent of the time.
Motility on its own was seldom the reason for failure, however, the failure percentage climbed by four percentage points when poor motility was combined with poor morphology.
Semen quality characteristics that have been shown to improve for four months after puberty, thereby moving a deferred decision into the satisfactory category, are sperm concentration, percentage of sperm with normal structure and progressively motile sperm.
In a group of 254 bulls (various breeds ranging in age from 12 to 15 months) only 40 per cent of the 12-month-old bulls were rated satisfactory on semen quality. The percentage increased to 55 per cent for 13-month bulls, 56 per cent for 14-month bulls and 73 per cent for 15-month bulls.
“This highlights the challenge of having yearling bulls deemed acceptable on a BBSE and the importance of early nutrition,” Kastelic says.
Another study of 200-day-old bulls of 13 beef breeds found that a calf with an SC greater than 23 cm had a 95 per cent probability of reaching 34 cm by a year of age, whereas, there was only a 54 per cent chance of meeting that mark if the SC was less than 23 cm at 200 days of age. The general recommendation, therefore, is to assess SC at weaning and castrate bulls with very small measurements because they have limited prospects of becoming future sires.
So, here again we see the importance of calfhood nutrition. True, SC is highly heritable (69 per cent), but without adequate nutrition during the first 30 weeks of life, a bull calf is unable to realize its genetic potential for testicular development and sperm production.