Unless the bull has an obvious physical ailment that is stopping it from being able to serve cows (lame, sick, etc.) it is impossible to determine his fertility just by looking at him. Being fertile the previous year does not guarantee that he will be fertile the following year.
Various studies have consistently shown that between 20 – 25% of bulls that undergo routine breeding soundness tests are found to be subfertile or infertile (Penny 2010).
Infertility is rare but subfertility is far more common; these bulls are still able to serve and impregnate cows, but they are not achieving a conception rate equal to a fertile bull. These subfertile bulls are often hard to discover without a BBSE, particularly if the farm runs more than one bull, as the other bulls will continue to serve the cows and effectively ‘cover up’ the subfertile bull’s shortcomings. Poor fertility figures, a poor conception rate or a spread-out block calving pattern are all outcomes of using a subfertile bull. These subfertile bulls will typically achieve a 40% conception rate, whereas a fertile bull will achieve at least 60%.