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any weight that was lost during the breeding season. Body condition scoring or weighing bulls at this time will allow producers to make the necessary dietary changes to ensure that the bull battery enters the next breeding season at an appropriate body condition. The effect of body condition on breeding performance of mature bulls is not well documented; however, logic would suggest that avoiding overconditioning will reduce the effort to mount and the force on the hindlimbs during mating. Commonly, breeding bulls are fed to reach a body condition score of 6–7 prior to entering the breeding season [34]. Dietary energy excess has been shown to have detrimental effects on semen quality in yearling bulls. These effects are thought to be related to impaired thermoregulation of the testes due to increased scrotal fat deposition [35].
Housing during the Non‐Breeding Period
Group housing is generally recommended for bulls used in multisire breeding groups to reduce bull attrition due to injury during the breeding season by allowing the social hierarchy to be established and maintained in the absence of females. Some separation of bulls may be necessary depending on the uniformity of the bull battery. Young bulls less than two years of age may need to be grouped separately to avoid competition for feedstuffs and allow for continued growth and development. Bulls finishing the breeding season in poor body condition may likewise be grouped together or fed separately until their body condition improves. Housing recommendations in cold climates with significant snowfall may also include the addition of a shelter with bedding to avoid scrotal frostbite.
Parasite Control
Specific recommendations on parasite control will vary depending on the geographical region. Most bulls are dewormed once yearly prior to the breeding season. In warmer more humid climates, twice‐yearly deworming may be necessary, especially in younger bulls. Yearling bulls are at the highest risk for internal parasitism and may need to be dewormed immediately following the breeding season to remove the parasite burden accumulated while in the cow herd. Fly control is also important in bulls to eliminate fly strike in the scrotal region, which can lead to inflammation and impaired thermoregulation of the testes [34].
Biosecurity Concerns for Replacement Bulls
The introduction of replacement bulls into the cow herd represents a significant risk for disease introduction. The primary pathogens of interest in the USA include Brucella abortus, Campylobacter fetus subsp. venerealis, Tritrichomonas foetus subsp. venerealis, Leptospira species, Mycobacterium avium subsp. paratuberculosis, Mycobacterium bovis, BVDV, and BHV‐1.
In addition to diagnostic screening for potential pathogens, all replacement bulls should be quarantined for a minimum of 30 days before exposure to the cow herd. This period will allow viral shedding to cease in bulls with acute BVDV and BHV‐1. Producers should also be aware that no diagnostic test is 100% sensitive and therefore bulls should be purchased from herds with good health programs. Purchasing bulls from herds that are certified free for a specific pathogen provides additional security. Likewise, purchasing virgin bulls eliminates the risk for C. fetus and T. foetus.
Recommendations for Dairies Utilizing Natural Service
The genetic improvements associated with AI and their impact on production and profitability in the dairy industry are well established. However, utilization of natural service (NS) is still widespread for a variety of reasons [36]. Bulls may be utilized to improve estrus detection, resulting in increased conception rates, as a management tool to prevent dystocia in heifers, for the production of dairy beef, and to decrease the cost and labor associated with AI [36, 37]. The use of NS to improve reproductive efficiency in dairy herds with poor reproductive performance can improve profitability by decreasing the calving to conception interval, resulting in increased milk production per cow and reducing culling. These improvements may not be economically significant in herds with acceptable reproductive performance [37]. The most obvious disadvantages of NS are reduced genetic improvement, cost associated with bull purchase and replacement of bulls, disease introduction and transmission, and safety. The extent of the reduction in genetic improvement by utilizing NS depends on the quality of bulls utilized. Some dairies offset this loss by utilizing a combination of one or multiple attempts of AI followed by NS. Only heifers produced from AI are then retained for replacements.
The typical stocking rate of bulls on dairies ranges from 1 : 20 to 1 : 30 bulls to cows [38, 39]. This stocking rate can vary significantly between dairies, especially those that utilize AI followed by NS. In herds that utilize this combination, the bulls needed should be based on the actual number of cows that are eligible to become pregnant at the time of bull turnout. The length of the bull breeding season is also quite variable between dairies and is commonly impacted by seasonal breeding. Dairy bull attrition due to lameness is a common problem resulting from a combination of diet, housing and flooring, and injury from fighting. Lactating cow rations are higher in energy, protein, and calcium, which exceed the needs for bull maintenance. The excess energy leads to laminitis, while excessive calcium has been shown to induce skeletal lesions [38]. Rest and rotation of bulls during the breeding period is recommended to reduce the impact of diet and hard‐surface flooring on lameness. Whether bulls are rotated or not, the length of rest periods varies greatly among dairies. One report from California dairies utilizing NS found that bulls are never removed from the breeding group after initial introduction in 47% of the dairies surveyed [40]. Reports from Australia found that bulls are left in the breeding group from 1 to 14 days and then are rested for an average of 7 days before returning to the breeding group. The Australian dairies surveyed used bulls on average for 56 days following multiple attempts at AI [41]. Using young bulls, less than 2.5 years of age, is recommended to reduce the impact of social dominance on breeding and fighting and to improve human safety. Social dominance is thought to be less dramatic if bulls of similar age and size are utilized [36].
Other management considerations of paramount importance include the fertility of the bulls used and their potential to introduce disease into the herd. Lameness is common in dairy bulls, as discussed above, and it can impact fertility by reducing bull libido and physical ability to seek out and service cows. Chronic lameness will also reduce semen quality. Dairy bulls should be assessed frequently for lameness and should have breeding soundness examinations performed before introduction and, at least, annually thereafter [36]. The biosecurity risk that the dairy bull represents is similar to the concerns discussed above regarding beef bulls. Special considerations should be given for sexually transmitted diseases like trichomoniasis, campylobacteriosis, and host‐adapted leptospirosis when NS is utilized. The diagnosis, treatment, and prevention of these conditions is covered in their respective chapters. Likewise, a complete overview of biosecurity measures for the beef and dairy herds is covered in Chapter 28.
References
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