Author: Richard Laven PhD BVetMed MRCVS
Reviewed:
Published: 2015
Natural mating has one perceived advantage over artificial insemination in that it can reduce the need for heat detection. Thus on dairy farms, bulls are often used alongside AI in an attempt to increase the rate at which cows get pregnant.
On seasonally-calving farms, particularly those using a New Zealand-style system, 'sweeper' bulls are often used at the end of the breeding season to 'mop-up' the cows that have failed to get pregnant during the period of AI. Finally, farms often use bulls to breed heifers. This avoids the need to bring individual heifers in for AI, which can be a significant issue when heifers are run on more remote areas of the farm.
Nevertheless, whatever the reason for using bulls, if they are not managed effectively then their value to the farm is significantly reduced.
The evidence that farms which use bulls do, on average, get an improvement in their fertility is not very strong. A US study which compared the fertility of herds which used bulls to those which only used AI found no difference between the herds in their reproductive performance. This shows that level of fertility obtained in herds which use bulls can be achieved relatively easily by herds using only AI.
A Californian study showed that using bulls could make fertility worse; they found that cows in AI pens had a higher risk of pregnancy than cows at the same stage of lactation which had been put in with a bull. They concluded that the best option was often to increase the number of opportunities for AI per cow rather than to put them with a bull.
In herds with poor fertility, it is likely that focusing on improving reproductive performance will be more effective than using a bull, particularly as the use of bulls often results in reproduction being managed less efficiently.
Natural mating is more expensive than using AI, particularly if the outcome wanted is a dairy heifer (principally due to the better genetic worth of AI heifers), but even if the outcome is a beef calf, the costs of AI are usually less than those associated with natural mating. This is because bulls are expensive to keep and otherwise non-productive - the costs of keeping a bull are commonly underestimated; they can be up to £1500 per year (based on average longevity of 4 working years, depreciation and fixed and variable costs).
So, given that using bulls probably doesn't improve fertility and they cost more than using AI, the main reason for using bulls is making herd reproductive management easier. This makes it even more essential that bulls are managed well as otherwise even this advantage can disappear.
Purchased bulls are a very large biosecurity risk, so it is essential that the bull's disease status is known before it comes on to the farm. Ideally bulls should only be purchased from farms with confirmed disease-free status, particularly BVD, IBR, campylobacter and leptospirosis. If you are purchasing bulls which have been used elsewhere, the disease history of their herd of origin and the herds they have been working in must be taken into account.
If bulls from herds free from disease are not available, then testing the bull prior to purchase is essential. The key diseases are 1) BVD - the bull must have a negative blood test for the virus (as this rules out the chance of it being persistently infected ('PI'), and Campylobacter - ideally you should buy a young unbred bull but if it is not it should be sheath wash negative and then should have its sheath washed with antibiotics on three consecutive days. Bulls which are not PI should be routinely vaccinated against BVD, starting at least 6 weeks before they are due to be used, if the herd they are to be used is not known to be free of the disease. Other diseases, such as Neospora, IBR, leptospirosis or Johne's disease may also be important. If you only buy bulls occasionally - get veterinary advice before purchase as a small amount of money spent on testing and vaccination can save you a large amount of money in the long run. If you regularly purchase bulls, sit down with your vet and develop a standardised testing programme to minimise disease risk
Using homebred bulls reduces, but does not eliminate, the risk of infectious disease causing a problem. Checking for BVD is still essential if your herd is not confirmed BVD-free as PI bulls have significantly lower fertility and their use can result in reproductive disasters.
If you can, choose a bull with good breeding values, paying particular attention to calving ease. One of the advantages of AI is the ability to choose bulls with breeding values that are far better than average. It's much more difficult (and much more expensive) to do so when choosing bulls for natural mating.
Complete infertility is unusual in bulls; most bulls will be able to get some cows pregnant over a breeding season. The key problem is sub-fertility where bulls are less fertile than they need to be to get optimal reproductive performance. The aim of a bull breeding soundness examination (BBSE) is to identify these sub-fertile bulls before they are used rather than after. The BBSE should, ideally, be undertaken before purchase alongside the investigation of the bull's disease status. It is important to remember that although a BBSE will identify most problem bulls, because it's a single snapshot it will not always identify every problem animal. A BBSE (or at the very least examination of the testes and scrotum) should be undertaken for every bull prior to first use, whether homebred or purchased. These examinations should be carried out by a trained professional, alongside a full physical examination of the bull.
The BBSE involves several stages:
1) Examination of the testes and associated structures to check for any abnormalities or unevenness. This will identify a range of diseases which may permanently or temporarily alter the function of the testes. The testes should be even in size and consistency and move freely within the scrotum with no differences in outline between the left and right sides.
2) Measurement of scrotal circumference. There is a very clear link between this simple measure and the risk of sub-fertility. Ideally a year-old bull should have a scrotal circumference of at least 32 cm (preferably more).
1) Examination of the prepuce and penis. The penis should be felt to ensure that it can move freely within the prepuce (sheath) and that there are no swellings or growths on either the prepuce or penis. It is not possible to examine the penis fully in the standing conscious bull; thus it is important to check the penis when is extruded during the testing of mounting ability (or when using an electro-ejaculator to collect semen).
2) Examination of the internal reproductive organs, particularly the prostate and seminal vesicles , to ensure they are normal is also normally undertaken as part of the BBSE
3) Examination of the sperm - the method of collection depends on facilities and bull behaviour. The best samples are those collected in an artificial vagina from a bull that is mounting a cow that is in oestrus; collecting samples by electro-ejaculation can often be quicker and safer but the quality of the sample is less (though it is usually adequate). Sperm motility is a quick bull-side test that uses a low-power microscope and measures the proportion of moving sperm and the proportion of motile sperm moving in a forward direction. (100% lack of motility in an electro-ejaculated sample should be confirmed by sampling on a separate occasion). The sperm should then be stained and examined under a more powerful microscope - this needs to be done in a laboratory and is best done by experienced staff.
4) Libido and mounting tests. Fertile bulls should show interest in and mount cows in oestrus. During these examinations penile deviation and other abnormalities such as corkscrew penis can be observed as well as causes of failure to mount e.g. musculo-skeletal conditions and , stance and gait abnormalities (such as excessively straight hocks, so-called 'post-leg' syndrome) and joint disease.
A full examination is ideal but may not always be practicable. Semen collection, particularly via an artificial vagina, requires a reasonable standard of facilities and a compliant bull. There is little point testing semen if there are gross abnormalities of the penis, testes or associated structures or scrotal circumference is too small, as in most cases semen quality will be reduced and even if it's apparently normal fertility may still be reduced.
Even when using a bull it is still worthwhile recording matings. If matings aren't recorded, cows which aren't mated will be missed and will only be identified when presented for pregnancy diagnosis and found to be not pregnant. The lack of mating records can also restrict treatment as non-pregnant cows running with the bull could be <4 weeks pregnant and so cannot be treated with an injection of prostaglandin or synchronised with a program that includes prostaglandin.
To perform effectively, bulls need to be in optimal health.
Nutrition is crucial - aim to keep the body condition score of bulls between 2.5 and 3 (neither too fat nor too thin). Avoid rapid dietary change; if there are major changes to the diet change it over a period of 3-4 weeks. Many lactation rations will be too high in energy density for bulls, which will result in over-conditioned animals
Parasite control is important (particularly lungworm as bulls can introduce infection into naive herds) - worm all purchased bulls before they come on farm, and if any bull loses body condition worm them again, alongside checking for other diseases.
Lameness is a key problem in bulls - a bull's feet and legs should be closely examined before purchase to ensure that bulls with a high risk of lameness are not bought. Bulls should be regularly mobility scored and if they have a score of 2 or more should be closely examined as soon as possible.
Bull health should be regularly monitored - any infection which causes a fever can reduce both bull libido and sperm quality (the latter can be affected for up to 2 months)
The simple answer is always more than you think need; if you have just enough bulls and one goes wrong (for example, it goes lame or injures its penis) then the impact on your fertility will be much greater than if you have an extra bull.
For sweeper bulls and bulls used with groups of heifers, the recommended number of females per male is summarised in Table 1
|
Age of bull |
Recommended numbers |
|
<2 years old |
15-20 cows |
|
2 years old |
20-30 cows |
|
3 years old |
30-40 cows |
Table 1: Recommended number of cows/heifers per bull. Based on bulls running loose with females
No matter how many females you have in a group it is essential that you never rely on only one bull. So if you have a group of 30 heifers, they need at least two bulls, even if they are both 3 years old. If you are using a sweeper bull, it is important to estimate the proportion of the herd that will not be pregnant at the start of the bull mating period as that will determine the number of bulls you need - for example in a 200 cow herd if only 40% are pregnant at the start of bull mating you will need 5-6 bulls, but if 60% are pregnant then you will only need 3.
Make sure that you regularly observe the bulls with the cows. By doing this you will identify potential problems more quickly - such as lack of libido or penis damage. If a problem occurs replace the bull immediately. Ideally, use a one-week-on, one-week- off rota; this will reduce the risk of over-working. This is especially important for younger and unproven bulls.
If you have multiple bulls on a farm, then ensure that they are even in size, and ideally make sure each working group comes from the same source. Remember that bulls may not show aggression to each other when initially kept together; but may not show, but if they are separated and then re-introduced (such as after a period of lameness) aggression can occur even though it didn't before
At all times ensure staff safety as your first priority. Make sure they are all aware of the dangers associated with bulls and that they have been trained to deal with them. Get rid of aggressive bulls as they are a danger to staff, to other bulls and even the cows. Nose rings may help handling but are not an alternative to getting rid of dangerous bulls.
Ensure you choose the right bulls for your cows - farms with bulls often have increased rates of injury to cows. These can be reduced by careful bull selection and using younger bulls.
Management of bulls at pasture: Avoid bringing bulls that are at pasture with cows onto the concrete collecting yard. If possible, try and train bulls to remain in the field when the cows are brought in for milking. Putting bulls onto concrete increases the risk of lameness, particularly if they try to mount cows on the collecting yard.
If you have bulls at pasture, ensure that your infrastructure (such as gates and fences) is up to withstanding the impact of a bull, particularly if you are going to separate them. Try to avoid keeping bulls near groups of cattle that you don't want them to serve (such as yearling heifers).
Management of bulls in housing: If your bulls are housed avoiding concrete is difficult, particularly if your cows are kept in cubicle yards rather than loose housing. Ensure that any area where the bull does its mating is as clean and dry as possible and that the floor has good grip and is not slippery as otherwise leg and foot injuries are likely to be common.
If your cows are kept in a cubicle yard, the cubicles are unlikely to be suitable for bulls because of their different size and weight. They are also likely to damage them. So even if the bull serves the cows in cubicles, which is not ideal, ensure that they have a separate bullpen for sleeping / feeding. Ideally have a separate area where cows can be brought to the bull for mating and then separated from the bull easily with as little contact between staff and bull as possible.
The bullpen needs to be properly designed. It should have railing topped walls which are >1.5m high, and secure gates which are child proof up to 1m. The floor should be slip resistant with an easily cleaned bedding area which should be at least 16 m2. There should also be a feeding area with a head yoke for restraint which should also allow the bull to be foot trimmed if necessary. If the bulls are not put in with the cows for service they need a separate exercise area, which should be at least twice the size of the bedding area. If bulls are separated from cows, ensure that the bulls have sight and sound of other stock when they are in the bullpen; this can be achieved by having the bullpen next to the cow housing. One potential alternative to housing bulls, which may be better for their welfare, in a bullpen is to keep them with the dry cows, if the dry cows are loose housed
Just as you have to for bulls at pasture, you need to ensure that your infrastructure is robust enough and large enough to safely handle bulls. Races and crushes need to be capable of taking the largest of your bulls without having to push and cajole them through.
Using natural mating can aid the management of reproduction on some farms. However it is not a panacea and without a focussed approach and efficient bull management, using bulls will cost you more and result in reduced fertility.
It is essential to check the disease status of all purchased bulls, and to ensure that a thorough physical examination is undertaken before purchase, ideally alongside a thorough breeding soundness examination.
However, even if you purchase healthy fit bulls, this is only half the job. You need to regularly monitor bull health and performance to ensure that any problems which arise are identified before they have had a major impact on fertility. Observing and recording matings is crucial for achieving this goal.
Bulls are dangerous and need careful management. They need facilities designed for bulls not for cows, particularly when they are housed. If you are building new facilities, design for bulls, design them with a safety first. Individual bullpens may seem a simple solution but they need to be easy and safe to use and keep clean and the bulls need to be able to see and hear other cattle.
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