Having selected replacement bulls, there are a number of management practices that can be used to gain the most from them.
In multiple sire herds, dominance by one bull may be such that a large proportion of calves may be sired by a small percentage of bulls. This can be avoided by using bulls of similar age and similar libido in the one mating group. Dominant bulls may endeavor to keep less dominant bulls away from the cows, yet they may not serve the cows themselves. High libido bulls often have a way of serving the females on heat and going before they are threatened. Therefore, all bulls used on property should be assessed.
The older the bull becomes the more prone he is to subfertility or even infertility either dueto some damage to the reproductive organs or physical problems, for example arthritiswhich may interfere with service. Bulls also become more difficult to handle.The longer it is possible to keep a bull in the herd, the more calves his purchase price can bespread over and a lower bull cost per calf results. It must be remembered however, thatwhile a bull is in the herd his own genetics is not improving and the benefits from keepingbulls longer, in terms of cost per calf, must be balanced against the opportunity to replacethat bull with one that is genetically better.Therefore to retain a bull in the herd to an old age, the bull must be fertile and be equal to,or better than, the most recent bull replacements for all genetic traits used in selection.
Bulls can transmit venereal disease such as vibriosis and trichomoniasis. Both cause either delayed conceptions or abortions. Only vibriosis can be prevented by vaccination and it is recommended that bulls be given complete vaccinations prior to first mating. All bulls should be vaccinated annually prior to mating or time of peak conceptions. Heifers may also be vaccinated for vibriosis with a similar vaccination program to that of bulls. Both diseases tend to be more prevalent in older bulls, although this is not always the case. If breeders are vaccinated for leptospirosis, the bulls should be vaccinated also to reduce the effect of infection at or during the mating period. Infection of a bull during a short mating season can have a serious effect on the herd conception and subsequent branding rates. Other vaccinations may be worthy of consideration. These include tick fever, ephemeral fever (3-day sickness), mucosal disease and vaccination with the 5-in-1 package against the clostridial organisms such as blackleg and tetanus.
Bulls should be mated according to their ability and the paddock characteristics, as there isno set percentage. Whilst a figure of 5% is often quoted, work at Mt Bundy Station in theNorthern Territory found that high pregnancy rates could be achieved with a lowerpercentage providing infertile bulls were identified and culled. Field trials are indicating thatbull percentages of less than 3% can be used, provided that the bulls are examined with afull bull soundness examination prior to mating.The size of the paddock and the distribution of watering points also have an effect on bullpercentages independent of the fertility of the bull.41As the number of bulls increase in any one multiple sire group, the percentage of calves siredper bull decreases. As the number of bulls decrease e.g. 2-7 bulls per mating group, themaximum percentage of calves per bull has ranged from 24 to 94%.
Regular replacement maintains continued genetic progress. Whether they be purchased bulls or home bred, bulls should be given an annual screening prior to the peak mating period for conditions affecting their ability to produce offspring.
Not only should buying fat bulls be avoided, but bulls should also be maintained in good forward store working order (condition score 3 to 4) to achieve the best performance. It is interesting to note that working bulls can suffer some weight loss with little or no impact on libido. Over-fed and over-fat bulls often demonstrate depressed libido and can be at risk of contracting metabolic disease if required to work and with poor feed quality in the pasture. The relocation of bulls from one site (at purchase) to another (your property) can not only result in depressed bull fertility due to a change in nutrition, but also exposure to a range of new diseases that can affect the bull’s fertility. Therefore, we recommend that bulls be relocated in advance of when they will be used in the breeding program.
Ask the vendor, where you buy your bulls, for their genetic information. If it is not available, endeavour to obtain that information as it will influence your future returns. Moreover, ask for the AACV Bull Breeding Soundness Evaluation certificate which will provide a basis for insurance and any subsequent changes in the bulls’ fertility.