How fertile is your bull?

Not knowing whether your bull is fertile, subfertile or infertile will have an impact on the length of the calving period, the percentage of barren cows and calf weaning weights. Current studies show that 1 in 5 bulls are subfertile, either on physical exam or semen evaluation or both. This most likely would increase to 1 in 4 bulls when libido and serving capacity are evaluated.

Subfertile bulls are not usually obvious, especially when several bulls are rotated around the cow groups for an extended period of time. A fertile bull has an average pregnancy rate of 60%, if he is in with the group for 12 weeks the expected in calf cows would be 98%. If the bull is subfertile and only has an average pregnancy rate of 30% then after a 12 week mating period the percentage of in calf cows would be 76%. This is much lower than the fertile bull but could be explained away by individual cow problems, illness in the group or external factors such as heat stress. However, if the bull had a pregnancy rate of 40% which is subfertile, after a 12 week mating period the percentage of in calf cows would be 86%. This is often considered an acceptable number, when considering the age of animals or problems at last calving so probably wouldn’t be investigated. If this reduced fertility is put into profit lost by a reduced number of calves it is more significant. The average cost of one finished steer currently is £1240, which is a potential loss of £14,880 for a subfertile bull with a pregnancy rate of 40%. Therefore, the value lost from reduced calves reared outweighs the cost of the bulls fertility test.

Just like in breeding cows the bull’s fertility is reduced by stress, systemic illness, lameness, back injuries, extreme body conditions and parasitic burdens. Any abnormalities or injuries to the penis, prepuce or testicles can temporarily or permanently make a bull infertile. This can change throughout and between mating periods, so it is important to examine the bull before each mating period.

Pre-breeding bull exams are a cost effective way of reducing the risk of using subfertile bulls, although they do not eliminate the potential for illness or trauma during the breeding season that can affect fertility. Close observation of the bulls during the mating period is also important, as it provides information on how he is performing physically and whether he is actually mating.

The bull breeding soundness exam (BBSE) consists of a physical exam and evaluation of the semen. Libido and serving capacity can be assessed but this is a more difficult process so is often advised only if there is still concern over the bull’s fertility. The physical exam should indicate any signs of systemic illness, lameness and under or over body condition.

Examination of the scrotum can reveal abnormalities of size or consistency, such as orchitis and epididymitis. For a bull over 24 months of age his scrotal circumference should typically be >34cm although there are some breed variations. Anything smaller and there is reduced testicular tissue meaning a smaller volume of semen is produced. If this is the only abnormality and the bull is with a small group of females then the overall fertility of the group shouldn’t be affected. However, if there are sperm abnormalities, or the bull is with more than 50 cows, fertility will be reduced. Examination of the prepuce can show abnormalities such as swelling of the prepuce due to trauma or tearing and prolapse. Both can lead to strictures or adhesions preventing the bull from fully extruding his penis. It is ideal to examine the fully extended penis to check for any deviations, haematomas and papillomas which can hinder serving ability. Most of these conditions can develop during the breeding season as well as before so it is important to keep an eye on the bull during breeding.

Next a semen sample is collected via electro-ejaculation. Colour, volume and consistency are noted before it’s examined under a microscope. This provides information on motility, percentage of normal and abnormal sperm and any abnormal cells such as white blood cells in the sample. Based on this, the vet will pass, fail or recommend to retest the bull if it’s possible the reason for subfertility is resolvable.

Infectious diseases are an important factor in fertility and can be tested for and vaccinated against. IBR, BVD and Leptospirosis can reduce fertility and cause abortions. It is important to include the bull in any disease control programmes and, if buying in, make sure his disease status is known. Unless the bull is naïve then there is the potential of him passing on venereal diseases, such as campylobacter, which reduce fertility of both cows and bulls.  

So to avoid any loss due to poor fertility in the bull, make sure his disease status is known, any lameness dealt with and a BBSE performed a few months before breeding season.

Please speak to your vet, or contact your nearest Westpoint practice, if you would like more information or to book a bull breeding soundness exam.

Written by Mia Ellis BSc (Hons) BVetMed MRCVS