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, and non‐breeding period.
Pre‐Breeding 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 Evaluation
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, 8, and 9. 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.
Vaccination
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 (BVDV), 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
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 two years of age and younger [2].
Leptospirosis
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 eliminate 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.
Campylobacteriosis
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.
Bovine Viral Diarrhea
Clinical manifestation of 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 non‐cytopathic BVDV [20, 21]. Semen from one bull with prolonged testicular BVDV infection was capable of infecting heifers through artificial insemination (AI); however, others studies evaluating transmission from bulls with
