by the collection of semen using electroejaculation, others may not be easily managed.
Physiology of Erection in the Ruminant
The unique and interesting features contributing to extension and erection of the penis of the bull are well described [2, 3]. A brief summary of the process follows.
The fibroelastic ruminant penis is encased in the rigid tunica albuginea which encloses the erectile tissues of the corpus cavernosum penis (CCP). In the non‐erect state the penis is maintained within the prepuce and sheath by the traction of the retractor penis muscles, which insert on the ventral surface of the mid‐shaft of the penis at the distal bend of the sigmoid flexure. The proximal terminus of the CCP is the bulb of the penis, which divides laterally into two crura which are covered by the ischiocavernosus muscles at they attach to the bony pelvis at the ischiatic arch.
At the initiation of sexual arousal, the retractor penis muscles relax in synchrony with increased blood flow from the deep arteries of the penis into the crura, and the CCP fills with blood. The loss of muscle tension from the retractor muscles coupled with complete filling of the CCP initiates partial straightening of the sigmoid flexure. Contractions of the ischiocavernosus muscles compress the blood‐filled penile crura against the pelvis and occlude all vascular flow to and from the CCP, resulting in a closed hydraulic system. Further rhythmic contractions of the ischiocavernosus muscles increase the pressure of the blood contained within the crura and the CCP to produce the force necessary for full erection and extension of the penis. The pressure generated in the CCP far exceeds the arterial pressure of the circulatory system, reaching 14 000 mmHg (1.87 MPa) or higher at the time of peak erection in the bull [4].
Coitus
The breeding act requires orchestration of interrelated simple and complex events. The bull must identify the female in estrus, approach, extend the penis, mount appropriately, achieve full erection, adjust his stance and posture, blindly search for the vulva with the penis, achieve intromission, make the breeding lunge, ejaculate, and dismount. The physical requirements are obvious and structural soundness is a necessary component of the successful breeding act.
In cattle the coital act occurs rapidly, with time from mounting to intromission and ejaculation generally less than 10 seconds and the time from intromission to ejaculation 1–2 seconds [5, 6]. Some non‐productive false mounts are expected, but excessive numbers may be associated with pathology of the axial or appendicular skeleton, problems with the erection process, impediments to penile protrusion and extension, or lack of innervation of the glans penis.
Following a successful breeding attempt the bull has a brief refractory period, the length of which varies among bulls and is likely affected by age, physical condition, and the presence of other bulls and females exhibiting estrus. For young physically sound bulls the refractory period may be only a few minutes and most bulls can be expected to copulate multiple times per day.
Abnormalities of the Prepuce
The prepuce is an invagination of abdominal skin that contains the free portion of the non‐erect penis and covers the body of the penis behind the glans. The preputial orifice is located caudal to the umbilicus and is surrounded by a tuft of long hair that serves to divert urine away from the skin surrounding the orifice following micturition.
The penile attachment of this invagination of skin occurs caudal to the glans penis and in the non‐aroused state penile retraction results in the presence of a fornix in the preputial cavity adjacent to the free portion of the penis. When erect, the free portion of the penis extends beyond the preputial orifice and the hairless reflection of skin within the preputial cavity extends with the penis and covers the shaft. The subcutaneous tissues of the prepuce must be freely moveable to allow extension and retraction of the penis and the preputial reflection. The specialized connective surrounding the tunica albuginea are referred to as the elastic layers of the prepuce.
Congenital and Developmental Anomalies
Congenital Preputial Stenosis
Congenital preputial stenosis will prevent extension of the penis, but stenosis secondary to injury or laceration of the preputial tissues is more common.
Incomplete Separation of the Penile and Preputial Epithelium
Prior to puberty, the epithelium of the free portion of the juvenile penis and the integumentary epithelium of the penile portion of internal lamina of the prepuce are joined by an interdigitating attachment [7–9]. Exposure to the trophic effects of androgens as bulls approach puberty leads to an increase in penile size and to development of the sigmoid flexure characteristic of ruminants. Simultaneous with the increase in penile size, the epithelial attachments between the preputial skin and the epithelial surface of the free portion of the penis weaken and begin separating. Separation may be facilitated by mounting activity and early attempts at erection during sexual role playing by young bulls. Until the separation is complete at 8–11 months of age, extension of the penis is not possible [7, 8, 10].
Normal separation is sometimes delayed and may result in an inability to fully extend the penis at the time of a breeding soundness examination or during an observed breeding attempt (Figure 15.1a). The attachment will often spontaneously regress in affected bulls if allowed adequate time to mature, but assisted separation by gentle application of manual traction to the penis is sometimes advocated in cases where partial separation has already occurred (Figure 15.1b and c).
Figure 15.1 (a) Failure of separation of the epithelium of the free portion of the penis and the penile portion of the prepuce. (b) Note the adherence between the preputial and penile skin. Application of gentle traction to the penis to facilitate adherence breakdown. (c) Penile and preputial epithelium following breakdown of the epithelial adherence.
Source: Courtesy of Robert L. Carson and Dwight Wolfe.
Young bulls at times tear the epithelial attachment prematurely, and hemorrhage into the space between the penile and preputial epithelium results in a localized hematoma. Such injuries may go unnoticed and are often self‐limiting [10]. If bacteria gain access to the damaged tissues and associated hematoma, abscess formation and fibrosis may compromise the ability to extend the penis. Case management should emphasize isolation from other animals to discourage sexual role‐playing during the healing period, accompanied by broad‐spectrum antibiotic treatment to minimize complications. Drainage of an abscess in this location is difficult and must be done with care to minimize loss of the elastic layers of the prepuce.
Persistent Frenulum
The frenulum is a thin band of tissue found in juvenile bulls that extends from the midline of the ventrum of the tip of the penis to the attachment of the preputial epithelium near the base of the free portion. Like the epithelial connection between the penis and prepuce, this band of tissue normally separates under the influence of androgen exposure as the bull matures. Unlike the epithelial tissues joining the
