Chapter 10 E. Heath King Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, Starkville, Mississippi, USA The purchase and maintenance cost of the bull battery represents a significant expense to the cow calf producer. Improper management of this investment can have negative economic consequences through both a reduction in herd productivity and the loss of bulls due to injury or death. A reduction in the size of the calf crop or loss of the calf crop is easily noticed by the producer, but many subfertile bulls are capable of inefficiently producing offspring and this may go unnoticed in some herds. This inefficiency leads to delayed conception, which has been estimated to cost 23–27 kg of weaning weight for every 21 days a cow remains open during the breeding season.1 In an attempt to compensate for poor bull management, producers will often increase the stocking density of bulls within their herd. The expense of purchasing and maintaining excess bulls can also limit the profitability of a herd. The management of breeding bulls will vary somewhat between herds, but the overall goal should be to provide a group of disease-free, structurally sound and physically fit, fertile bulls with good libido. Achievement of this goal requires an understanding of both the general and reproductive health requirements of breeding bulls and how these requirements change during the production phase of the bull. The production phase of a breeding bull can be divided into three time periods: the pre-breeding period, breeding period, nonbreeding period. The pre-breeding period begins 60–90 days prior to the start of the breeding season. Breeding soundness evaluations, vaccinations, and diagnostic testing for reproductive pathogens should all occur at the beginning of this period. Performing these procedures early gives the producer time to find replacements if necessary. Vaccination prior to the start of the breeding season should provide protection to the bull while reducing or eliminating his significance as a disease reservoir or vector. Vaccinating bulls at least 60 days prior to the breeding season should allow any viral shedding or transient negative effects on spermatogenesis to subside before breeding. Body condition can also be assessed at this time and minor adjustments to the diet made if necessary. Breeding soundness evaluations should be performed by a veterinarian on all bulls prior to the breeding season. Any bulls deemed unsatisfactory potential breeders, by the standards for breeding soundness of the Society for Theriogenology, should not be used for breeding and bulls placed in the deferred category should be reevaluated prior to use. Herds with two breeding seasons should have their bulls examined before each season. The complete breeding soundness evaluation of breeding bulls is covered in Chapters 7 and 8. Lameness and other foot problems are often first identified during the breeding soundness evaluation. Performing these examinations early will allow for treatment and corrective trimming prior to breeding, possibly salvaging bulls that may have been culled otherwise. As a general recommendation, breeding bulls should be vaccinated for the same pathogens as the mature cow herd. Vaccination should include protection against clostridial diseases, leptospirosis, campylobacteriosis, bovine viral diarrhea virus, and bovine herpesvirus 1. Vaccine efficacy and safety varies between vaccine types and products. The goal of this section is not to endorse specific products but to demonstrate the role vaccination plays in control of these pathogens and to address any negative impacts vaccination may have on bull fertility. Clostridial diseases are not transmitted directly between cattle and are not considered reproductive pathogens, but they are invariably fatal. Therefore, vaccination is solely aimed at protection of the bull. All clostridial vaccines are killed products and should be boostered according to the label directions. Annual revaccination is recommended especially in all cattle 2 years of age and younger.2 Leptospirosis is a common cause of abortion and infertility of cattle worldwide. More than 200 serovars of Leptospira have been identified each of which is supported within its own maintenance host. The severity and chronicity of disease caused by organisms of the genus Leptospira depends on whether infection is taking place in the maintenance host (host-adapted) for that specific serovar or an incidental host (non-host-adapted).3 Cattle are the maintenance host for serovar hardjo, which contains two genetically distinct types: Leptospira interrogans serovar hardjo type hardjoprajitno found primarily in the British Isles and Leptospira borgpetersenii serovar hardjo type hardjo-bovis found throughout the world including North America.4,5 Infection of cattle with a non-host-adapted serovar is characterized by acute disease, late term abortion, and transient renal shedding of the organism. Infection with the serovar hardjo is characterized by chronic infections, prolonged renal shedding, abortion, and infertility.4 Leptospira species are primarily shed in the urine of infected cattle. Transmission is possible via semen and venereal transmission of serovar hardjo is thought to be common.3,6 Vaccination of cattle against leptospirosis is a complex and controversial subject. Many pentavalent vaccines marked for prevention and control of leptospirosis contain antigens for hardjoprajitno not hardjo-bovis, the type most commonly isolated in North America.5 Challenge studies evaluating the efficacy of pentavalent vaccines for protection against hardjo-bovis have yielded conflicting data; however, these studies utilized different products and the time period from initial vaccination until administration of the booster differed greatly.7,8 Administration of a commercial monovalent hardjo-bovis vaccine has been shown to prevent renal colonization and shedding of hardjo-bovis following challenge.9 The subject is further complicated by the true incidence of infertility that can be attributed to hardjo-bovis. A study in California involving 207 first-lactation cows found that cows seropositive for serovar hardjo had a mean time from calving to conception 34 days longer than seronegative cattle.10 However, when beef cattle were vaccinated with a commercially available monovalent hardjo-bovis vaccine and administered oxytetracycline to eliminated the carrier state of hardjo-bovis, no improvements in pregnancy and calving rates were observed.11 The decision to vaccinate and the product used should be based on the prevalence of leptospirosis in the area and its potential impact on reproductive performance in the herd. All the commercial vaccines currently available are killed products which should be boostered according to the label and revaccination annually is recommended. Bovine venereal campylobacteriosis caused by Campylobacter fetus subsp. venerealis results in transient infertility in cattle and occasional abortion. The bull serves as an asymptomatic carrier transmitting the organism to cows during coitus. Vaccination of bulls with an oil adjuvanted killed bacterin can be both protective and curative.12–14 Exposure of 4- and 5-year-old vaccinated bulls to C. fetus-infected heifers resulted in transmission to only 1 of 17 unvaccinated naive heifers and carrier status was not established in any of the five bulls exposed.12 Therapeutic vaccination of experimentally infected 5-year-old bulls with a two-dose series of an oil adjuvanted killed bacterin was curative in 8 of 10 bulls.13 While prophylactic vaccination will not clear infection in all bulls, it is an important and inexpensive method of control. The efficacy of vaccines containing an aluminum hydroxide adjuvant is questionable.14,15 All bulls should be vaccinated and boostered according to the vaccine label and annual revaccination is recommended. Clinical manifestation of bovine viral diarrhea virus (BVDV) infection in cattle can vary from subclinical to fatal.16 BVDV is transmitted primarily by direct contact with bodily fluids of infected cattle. Venereal transmission via semen from acutely and persistently infected bulls is also well documented.17,18 Cattle that are born persistently infected with BVDV shed large amounts of the virus and are considered the major reservoir. Control of BVDV centers around elimination of these persistently infected individuals combined with vaccination and biosecurity measures to prevent exposure to BVDV.16 The bull is also capable of maintaining a prolonged testicular infection that persists beyond the initial viremia.19,20 These localized prolonged testicular infections can persist for at least 2.75 years and have been induced by vaccination of seronegative peripubertal bulls with a modified live vaccine containing noncytopathic BVDV.20,21 Semen from one bull with prolonged testicular BVDV infection was capable of infecting heifers through artificial insemination; however, others studies evaluating transmission from bulls with prolonged testicular infection have not resulted 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 noncytopathic BVDV can 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 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 artificial insemination 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 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 United States. 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 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 United States 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. 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 United States 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 multi-sire 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 multi-sire 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:
Management of Breeding Bull Batteries
Introduction
Pre-breeding period
Breeding soundness evaluation
Vaccination
Clostridial disease
Leptospirosis
Campylobacteriosis
Bovine viral diarrhea
Infectious bovine rhinotracheitis
Trichomoniasis
Breeding period
Stocking rate
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