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15 Inability to Breed Due to Injury or Abnormality of the External Genitalia of Bulls
Herris Maxwell
Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, Auburn, AL, USA
Introduction
The inability to sire offspring is referred to as impotence and may result from abnormalities in sperm production and/or inability to copulate. Inability to copulate (impotentia coeundi) is further divided into inability to perform the copulatory act due to physical limitations and inability to copulate due to erection failure (impotentia erigendi). This chapter reviews important causes of impotence due to abnormalities of the genitalia of the bull. Abnormal spermatogenesis is covered in Chapters 3 and 5.
Impact of Impotence
Natural service typically utilizes bull to cow ratios of 1 : 15 to 1 : 50 [1]. The inability of the bull to breed in a single sire unit will always have a devastating effect on reproductive efficiency. Similarly, when a socially dominant bull in a multisire unit is unable to complete the copulatory act, the decrease in breeding efficiency may not be masked by the presence of additional subservient bulls. Although artificial insemination eliminates the requirement for coitus, most semen processed for the artificial insemination industry is collected by methods that mimic the copulatory act utilizing an artificial vagina (AV) as the bull mounts a teaser or phantom. While some impediments to the copulatory act may be overcome 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