BACKGROUND
This paper is a narrative review addressing animal welfare assessment for bulls kept in artificial insemination centers (AICs), a production setting for which the authors state there is currently a lack of summarized welfare guidance. The review is motivated by the idea that animal welfare is multidimensional and should aim to provide the “five freedoms,” while also recognizing that compromised welfare may affect reproduction, growth, health, behavior, and longevity. The authors emphasize that bull fertility has broader implications because reproduction underpins meat and milk production, and improved reproductive efficiency may help reduce greenhouse gas emissions per kg of meat or milk produced. They note that AI bulls are kept under unusual conditions compared with natural breeding systems: many males of different ages are held in relatively static environments, without access to females, and semen collection requires close human-animal interaction. The review therefore focuses on stress as a central consequence of poor welfare and on reproduction efficiency as a practical outcome linked to welfare. The Welfare Quality assessment framework for cattle, with its 4 principles and 12 criteria, is used as the main conceptual structure. The authors also mention the Five Domains Model, especially for future consideration of human-animal interaction and positive welfare states.
METHODS
The authors describe the work as a review and explain that potentially relevant articles were identified using the main search engines. Abstracts were screened, and 80 articles were ultimately included and reviewed. Search terms included keywords such as animal welfare, bull welfare, poor welfare, and bull reproduction. Because of limited literature specifically on AI bulls, the review also drew from evidence in other cattle production groups, females of the same species, and, when needed, other species in comparable production systems. The paper does not present a formal systematic review method, risk-of-bias assessment, or meta-analysis. Instead, it synthesizes available evidence into resource-based, animal-based, and management-based welfare requirements, following the classification of Blokhius et al. and the Welfare Quality approach.
KEY RESULTS
The central conclusion is that multiple welfare domains are likely to affect AI bull fertility and semen production, although direct bull-specific evidence remains sparse. Under resource-based requirements, the authors highlight housing, flooring, bedding, temperature, air quality, and access to food and water. For space, they cite UK cattle welfare recommendations stating that accommodation for a single adult bull of average size should include a sleeping area of at least 16 m2, while bulls weighing over one ton should have at least 1 m2 for every 60 kg of live weight. If the bull is not regularly and routinely exercised outside the pen, the exercise area should be at least twice as large as the sleeping area. The review notes that slatted floors, small space allowances, and hard or slippery surfaces may negatively affect health and behavior, while better bedding or rubber matting can reduce discomfort.
Thermal comfort is presented as especially important because sperm production is temperature sensitive. The authors state that testicular temperature should be between two and six degrees Celsius lower than body temperature for fertile sperm production. They further cite an optimal environmental temperature for sperm production of 15 to 18 degrees C during the whole period of spermatogenesis of 65 to 70 days, while also noting that spermatogenesis takes approximately 60 days, so heat-related sperm changes may only appear after several weeks. The review discusses temperature humidity index as a useful indicator of heat stress and notes that Bos taurus bulls are more sensitive to high ambient temperatures than Bos indicus bulls, with crossbred Bos taurus x Bos indicus bulls recovering more rapidly after heat stress than Bos taurus bulls.
Regarding feeding and development, the review argues that adequate nutrition is essential not only for maintenance but also for puberty and later fertility. It summarizes prior work showing that feed restriction in young bulls altered the onset of puberty in relation to plasma insulin-like growth factor-I and IGF-binding proteins. The authors also discuss insulin-like factor 3 as a potential biomarker of testis status, especially Leydig cell status and number, and note that similar findings have been reported in bulls. More broadly, chronic stress is linked to reduced feed intake, negative energy balance, weight loss, and reduced growth, while obesity may also impair fertility.
Under animal-based requirements, the paper stresses that pain-free locomotion and sound hoof health are crucial because lameness can make bulls reluctant to mount for semen collection. Suggested health indicators include physical findings such as injuries, swelling, abscesses, and hair loss, along with physiological markers such as cortisol, reduced feed intake, immunosuppression, and depressed reproductive parameters. The authors also propose semen quality as an indirect welfare indicator and refer to work suggesting that seminal microbiota may differ among healthy bulls and may be correlated with fertility.
Behavior and human-animal interaction are emphasized as practical welfare domains in AICs. Bulls have genetically influenced sexual development, and stress may impair libido and mating ability. The review notes that if a bull does not respond to sexual stimulation within 5 min, the stimulus should be changed. It also cites semen collection practices showing that allowing three false mounts followed by a final fourth mount for semen collection can maximize sperm harvest, and that false mounting combined with active restraint can increase sperm concentration per ejaculate. Positive human handling is proposed as beneficial, based partly on evidence from heifers showing reduced heart rate and calmer behavior during gentle interactions.
CLINICAL IMPLICATIONS
The paper’s main practical message is that AIC bulls need a welfare protocol tailored to their specific biology and management conditions rather than reliance on dairy cow or general cattle standards alone. For clinicians, theriogenologists, herd veterinarians, and AIC managers, the review suggests that monitoring reproductive outcomes, semen quality, hoof health, thermal environment, feeding management, and behavioral responses may help identify welfare problems early. It also argues that management should aim not only to reduce negative stressors but also to promote positive human-animal interactions. Because bulls are often entered into intensive semen collection before 15 months old, the authors call for more research across the whole life course, from calf selection through the end of productive life. Overall, the review supports development of a bull-specific Welfare Quality-style assessment system that integrates physiology, behavior, management, and reproductive performance.