It is a not uncommon experience to find that satisfactory reproductive performance is obtained in the herd during the artificial breeding period, but that the reproductive performance of the cows that fail to conceive during the artificial breeding period is poorer during the subsequent bull mating period.
There are several possible reasons for this difference. One reason is that of the cows that are not pregnant at the end of the artificial breeding period, there is likely to be a core of 'difficult to breed' cows. But another reason may be that bull management has not been optimal.
The breeding management of the herd sires must ensure that they detect the animals which come into heat within a herd of mainly pregnant cows, that they can competently serve these bulling cows and that they can achieve satisfactory pregnancy rates at these services.
It is essential that all bulls that are to be turned out with herd are reproductively sound. Several levels of bull examination are available.
A very basic breeding soundness examination will detect a large percentage of abnormal bulls. In the dairy farm situation, a basic veterinary examination that will allow the detection of the majority of bulls that are not suitable for use as herd sires consists of:
- Measurement of bulls’ scrotal circumference
- Clinical examination of the bulls’ breeding organs
- Assessment of the conformation of the bull (with particular emphasis on legs and feet).
A more detailed examination also includes the collection of semen from the bull and examination of the semen sample. An initial examination of the semen is done 'crush-side' and we can also forward samples of the semen to a semen testing laboratory for more detailed examination. The laboratory we use is accredited by the Australian Association of Cattle Veterinarians and undertakes a detailed examination of individual sperm to assess the percentage of normal and abnormal sperm.
In a more complete examination the ability of the bull to perform natural service can also be undertaken. This step is not commonly undertaken in dairy cattle, but the herd manager is asked to regularly monitor the serving performance of all the bulls in the bull team.
Such a system of testing will detect most of the infertile and sub-fertile bulls, but will not be able to differentiate between high and very high performing bulls. It is important that these 'dud' bulls be removed from the system because some of them are likely to be dominant bulls and, even though they are of reduced fertility, they may stop other bulls from serving some cows which are on heat.
Running with the herd
The bulls should be vaccinated against vibriosis before they are turned out with the herd. Further, they should have been run together so that they have formed their natural pecking order and they will not waste time early in the breeding season having to develop this. When bulls are turned out with the herd, it is important that they are closely observed to ensure that they are capable of performing service.
The number of bulls that will be required to run with the herd will depend on the herd size, and how many cows became pregnant during the artificial breeding (AB) period. Allowing a six weeks AB period, it is reasonable to assume that 60 to 65% of the cows will become pregnant to AI. Thus in a 200 cow herd, in many cases at least 120 cows will be pregnant after a six-week AB mating program. This means that there will be around 80 cows which will need to be serviced by the bulls - a reasonable recommendation is to have one bull per 30 not pregnant cows.
In my opinion, there should always be at least two bulls running with the herd as the jealousy motive will help to maintain their enthusiasm for their task.
Depending on herd size, one or more bulls can be held in reserve - these are commonly used on a rotational basis whereby one or more bulls is removed each week and replaced with fresh bulls.
Monitoring bull health
If bulls are run with the herd in hotter months of the year, the heat stress effect on bulls may be minimised by attention to good breeding management. This includes the provision of adequate supplies of cool water, access to shade, maintaining bulls in good, but not excessively obese, body condition and ensuring that bulls are not overworked during hot periods of the year.
It is necessary to monitor the bulls continued ability to serve during the mating season. Injuries that render a bull incapable of service are not an uncommon occurrence during the mating season – it is essential that these injured bulls be detected at an early stage so that they can be removed from the herd and replaced by a bull that is capable of performing service.
Lameness in bulls can be a particularly important issue. If bulls are run with the herd, and allowed into the concrete milking yard, they frequently suffer excess hoof wear and severe lameness can result. It is strongly recommended that bulls be drafted from the herd at milking time and not allowed into the concrete yards.
Good management of your bull team will maximise the chance of getting in calf those cows which failed to conceive to AB.