With studies showing one in four bulls is either sub-fertile or infertile, farmers are urged to investigate the virility of their stock bulls.
Causes of bull sub-fertility can be divided into four categories: poor libido, inferior semen quality, presence of infectious disease, and inability or difficulty to serve.
According to SAC Consulting vet Catriona Ritchie, poor libido is often seen when a bull carries too much condition and, as a result, is unable or too lazy to seek out cows which are in heat.
She says: “When fat gets laid down around the scrotum it raises the temperature of the testicles, which adversely affects semen quality. It is therefore not uncommon to find a recently bought young bull has poor semen quality, but once he loses some of his condition his fertility often improves.”
However, Ms Ritchie says sperm production takes six to eight weeks, so it could be the next season before the bull is suitable to use.
There can also be problems with the development or functioning of the penis – such as with cases of corkscrew, bent or short penis – which is why it is important to make sure a bull can serve by monitoring it closely.
“Warts are not uncommon on the penis of a young bull and they can interfere with service if large or painful. The penis can also get damaged during service and fracture, usually causing a swelling along the lower abdomen.
“Occasionally the prepuce may be too narrow, have warts or be swollen due to infection which could prevent the penis from being extruded.”
Another cause of a bull’s inability to serve cows is pain caused by lameness, pelvic or back problems.
“This might make it reluctant to seek cows in heat, or it may want to serve them but be in too much pain to do so.”
Poor semen quality can be due to infection in the testes, sperm storage reservoirs or in the internal glands which produce fluid to transport sperm. An infection causing the bull to have a high body temperature can also affect semen production. Sperm defects can be due to a developmental defect of the testes or occasionally a genetic defect.
“Examination of the scrotum, its contents and the internal sex glands may detect where the problem lies. If only one testicle is in the scrotum or only one is functional, the bull will only have half the serving capacity. A retained testicle can be a hereditary problem so they should not be bred from,” says Ms Ritchie.
If semen is poor quality because of an infection, it will sometimes improve with time.
However, infectious diseases, such as BVD and campylobacter, carried by the bull can affect conception rates.
“If a bull is a persistently infected [PI] animal it will shed BVD virus in its semen as well as from its nose etc.
“PI bulls can have poorer semen quality but the presence of the virus in semen causes inflammation in the female reproductive tract which severely affects conception rates and causes high levels of embryo loss in naive females, resulting in returns.
“When a bull is first exposed to BVD it will become transiently infected and semen quality can be adversely affected. The bull will develop antibodies and clear the infection in a week or two, but the virus tends to remain for longer in semen – a couple of months is possible.”
Ms Ritchie says campylobacter bacteria can be transmitted by bulls during service, resulting in returns which are often irregular and longer than three weeks. It can also cause abortions and is it not uncommon for it to be first picked up in a herd through testing of abortion material.