in transmission [20, 22]. While the exact risk of transmission from these bulls appears to be low, further studies are needed before the use of vaccines containing non‐cytopathic BVDV can be recommended in breeding bulls [14, 20]. Current recommendation are to vaccinate all bulls with a vaccine containing cytopathic modified live BVDV at least 28 days prior to breeding and booster according to the product label recommendations [14].
Infectious Bovine Rhinotracheitis
Bovine herpesvirus (BHV)‐1 is a common cause of abortion, conjunctivitis, and respiratory disease of cattle. Genital infections also occur, resulting in balanoposthitis of bulls and pustular vulvovaginitis of cows. Cattle that become infected often develop latent infection that can reactivate later, complicating BHV‐1 control in the herd. The virus is shed by the mucous membranes of the respiratory tract, conjunctiva, and genital tract and transmission occurs primarily through direct contact. Venereal transmission occurs during natural mating and through AI with contaminated semen [23].
Vaccine products containing killed or modified live BHV‐1 are available for cattle. In general, the protection induced by modified live viral products is more rapid and of longer duration than killed viral products. However, modified live BHV‐1 vaccine products are capable of establishing latency that can be reactivated under stress and have been reported to cause abortion if used inappropriately in pregnant cows [24–26]. Most modified live products available for use in pregnant cows carry a label which states that the vaccine may be used in pregnant cows and calves nursing pregnant cows only if those cattle were vaccinated with the same product within the last 12 months. In order to err on the side of caution, it is my opinion that bulls should be vaccinated with the same BHV‐1 vaccine product as the mature cow herd. Vaccine products containing modified live BHV‐1 should not be administered to bulls unless the cow herd received the same vaccine product in the previous 12 months.
Trichomoniasis
Trichomoniasis caused by the protozoan Tritrichomonas foetus subsp. venerealis is a venereal disease resulting in endometritis, embryonic death, abortion, and pyometra in infected cattle. The bull serves as an asymptomatic carrier [27]. Prevention of bovine trichomoniasis is centered on biosecurity practices to avoid introduction of infected individuals into the herd. Only one T. foetus vaccine is currently available in the USA. Administration of this killed vaccine to bulls aged 55–72 months prevented colonization of the preputial epithelium following intrapreputial challenge [28]. Within affected herds, vaccination of bulls and cows may reduce the economic impact of this pathogen [14].
Breeding Period
The length of the breeding period is generally determined by the producer and can vary from 65 to 365 days. A controlled calving/breeding season has obvious economic advantages and simplifies the overall health management of the herd; however, in many herds especially in the southern USA the bull is housed with the cow herd year round. The following discussion on the stocking rate of bulls will assume that the herd has a controlled breeding season of 65–90 days.
Stocking Rate
The correct stocking rate of bulls, or more specifically the bull to cow ratio that will result in the most efficient and successful servicing of females, is not known. Typical bull to cow ratios in the USA ranges from 1 : 20 to 1 : 30 for mature bulls [29]. Young bulls less than 3 years of age can be placed with one cow per month of age; for example, a 15‐month‐old bull can be expected to service 15 cows during the breeding season [30]. Assuming that a bull is fertile and has passed a breeding soundness examination, other factors that will influence the number of females he can successfully service during the breeding season include his libido or “sex drive” and his social ranking, both of which are difficult to quantify and predict. The assessment and measurement of bull libido can be attempted by performing a servicing capacity test; however, no standardized testing method exists and results vary depending on age of the bulls tested, breed, and the methodology of the test [31]. An Australian trial of 1100 servicing capacity tests found that measurements of sexual behavior were not consistently correlated to the calf output of the tested bulls and that the main value of a servicing capacity test may be to identify bulls that are unable to mount and/or gain intromission [32].
Social dominance occurs in multisire herds where the presence of a dominant bull suppresses mating activity of other bulls. Dominance is generally expressed by older, more senior bulls and the effects may become more pronounced as the number of females assigned to each bull is reduced [29, 33]. The impact of social dominance on individual bull performance was demonstrated by a series of trials from northern Australia which examined the effect of the bull to cow ratio in multisire breeding groups on reproductive performance. Reducing the percentage of bulls to cows from 6 to 2.5% had no impact on conception patterns between groups. Variation in calf output between bulls was noted in both groups but was significantly reduced when fewer bulls were utilized. The area of movement of individual bulls within the pasture expanded as the percentage of bulls to cows decreased, suggesting that the effects of social dominance were reduced by increasing the breeding pressure on the bull battery. Interestingly, bull attrition due to injury was also reduced [33].
Recently, workers from Auburn University published the following formula for bull to cow ratio:
where B is number of bulls required to breed the given number (T) of cows in the breeding group and N is the individual bull's fair complement of cows as a single‐sire breeding group. N is calculated for bulls less than three years of age by assigning one cow per month of age. Scrotal circumference is used to calculate N for bulls greater than three years of age, where one cow is assigned for each centimeter of scrotal circumference.
This formula assumes that after the first bull each additional bull added to a multisire herd can only be expected to reliably service half the number of females that he would service in a single‐sire breeding group [30]. Utilizing this formula for a 200‐cow breeding group will demonstrate the inefficiency that many people believe exists in a multisire breeding group. The calculated requirements for this group would be nine adult bulls with 40‐cm scrotal circumferences; however, if this same group was divided into single‐sire breeding groups of 40 cows each, only five bulls would be needed to service the entire herd. Clearly, single‐sire breeding groups would be a more efficient use of bulls in this situation, but the added expense of fencing required for single‐sire breeding groups and the risk of bull injury/illness going unnoticed make producers hesitant to adopt this management strategy.
Scientific recommendations on bull stocking rates are difficult because variations in libido exist between bulls but are hard to predict and the effects social dominance on breeding performance are not well understood. While the effects of social dominance and breeding overlap may reduce the efficiency of multisire breeding groups in terms of the number of bulls needed compared with single‐sire breeding groups, these effects have been reportedly reduced by increasing the breeding pressure on the bulls [33]. Clearly, more research is needed in this area so that more accurate recommendations can be made to producers, allowing them to avoid purchasing and maintaining excess bulls. Until further research is published, the Auburn formula provides a good guide when making recommendations to producers and is not likely to result in inadequate bull power.
Producer Observation
Libido is not assessed during the routine breeding soundness examinations of bulls, so it is up to the producer to ensure that the bull is actively servicing cows. Producers should take care to observe that intromission is achieved when a female is mounted and make note of the date when the individual was serviced. Breeding injuries and lameness are best identified
