INADEQUATE SEXUAL INTEREST AND AROUSAL; SLOW OR VARIABLE RESPONSE

Common complaints concerning inadequate sexual interest and arousal or slow or variable response include slow starting novice breeders, slow or sour experienced stallions, and specific aversions or preferences.¹ Certain genetic lines tend to be quiet or shy breeders under domestic in-hand conditions. A large portion of inadequate libido in stallions is believed to be the result of domestic rearing, training, or breeding conditions. Most conspicuously, stallion sexual behavior can be suppressed by adverse experience.² For example, stallions that have been disciplined for showing sexual interest in mares during their performance career, or are discouraged from showing spontaneous erection and masturbation, or are injudiciously or roughly handled during breeding under halter are at risk of libido and sexual behavior problems. When exposed to a mare for teasing, such stallions may simply stand quietly, may appear anxious and confused, or may savage the mare. Most stallions with experience-related libido problems respond well to behavior therapy alone or in combination with anxiolytic medication.² These stallions typically respond well to extended exposure to mares, either in direct contact (turned out together) or in adjacent paddocks or stalls. Some extremely inhibited stallions may not respond in the presence of humans. Once they are responding at pasture, they may require gradual introduction to human handling during sexual interaction with mares. Confident, respectful, patient, positive reinforcement–based handling greatly facilitates the transition to breeding in-hand. These stallions appear to respond favorably to reassurance for even small increments of improvement. Tolerance of minor misbehavior, rather than punishment, is often the most effective initial retraining strategy with low libido stallions. Once these stallions understand and gain confidence with the domestic breeding process, necessary breeding manners can be imposed without suppressing arousal. Some of these stallions appear to demonstrate specific preferences or aversions, which can be for certain mares, colors or breeds of mares, handlers, locations, time of day, etc. The anxiolytic diazepam (0.05 mg/kg slow IV 5 to 7 minutes before breeding) is useful in many such cases, as an adjunct to behavior modification.

For novice breeding stallions in transition from a race career, exposure to anabolic steroids and other medications can be a factor in low libido. Depending upon the duration of treatment and the season in which treatment was discontinued, it may take several months for endocrinology, testicles, or behavior to appear normal. Libido may be more affected than fertility, and vice versa.

Management, as it affects sociosexual environment, can be a significant influence on stallion behavior, endocrinology, and sperm production. For most stallions, sexual and aggressive behavior are subdued when kept together with other stallions and away from mares as compared to being in management when the stallion is away from other stallions and exposed to mares.^3,4

Spontaneous erection and penile movements, common in all mammals, are known as “masturbation” in stallions.⁵ Episodes of spontaneous erection and penile movements occur at approximately 90-minute intervals in all stallions, regardless of age, breed, housing, exercise, sociosexual environment, breeding status, or fertility. Although the penis typically becomes fully erect and the glans penis may fully flare, ejaculation rarely occurs. While most stallions can be easily distracted momentarily from progressing with an episode, it is extremely difficult and likely beyond humane care to significantly inhibit the number of episodes. Accordingly, genital injury as well as fear of humans and suppressed sexual behavior can result when, due to simple misunderstanding of these normal periodic penile erection and movements, aggressive attempts are made to physically inhibit or punish the behavior.

In practice, a small proportion of cases of slow breeding novice or experienced stallions appears to be hormone-related, with androgens on the low side of the normal range. Some cases obviously involve trying to breed outside the natural breeding season in stallions that are especially seasonal. These stallions will likely improve with management aimed at increasing exposure to mares and reduced exposure to other stallions. This will typically increase androgen levels and general confidence, as well as sexual interest and arousal. Gonadotropin-releasing hormone (GnRH) (50 micrograms subcutaneously 2 hours and again 1 hour before breeding) can be useful in boosting libido in stallions, particularly in those with low normal levels.⁶ In rare cases when more rapid improvement is required to rescue a breeding career, treatment with testosterone can effectively “jump start” a slow novice without apparent significant adverse effects on spermatogenesis. Current recommendations are 0.1 to 0.2 mg/kg aqueous testosterone subcutaneously every other day for up to 2 weeks, with frequent assay of circulating testosterone not to exceed 4 ng/ml. As described in Chapter 1, stallion testicular function apparently will respond to artificially increased day light, but the experience has been that if long-day recrudescence is advanced with artificial lighting, so will the following seasonal decline be advanced. In other words, it appears that the stallion has a fixed length of peak seasonal reproductive function. Effects of artificially advanced photoperiod on behavior apparently have not been characterized. In practice managers report that some low libido stallions that must breed in late winter months benefit from 16 hours of light beginning in late fall. The same schedule of lighting that is used to advance seasonal cycling for mares is used.

For diminished libido in an experienced stallion, whether gradual or sudden, changes in general health, nutritional condition, housing, handling, breeding conditions, and breeding schedule should be considered as potential contributing factors. Changes in semen quality and testicular size and consistency often indicate endocrine factors.

SPECIFIC ERECTION DYSFUNCTION

Libido-independent erection dysfunction appears to be relatively rare in stallions. The majority of cases are related to traumatic damage of the corpora cavernosa, resulting in insufficient or asymmetrical tumescence (lateral or ventral deviations) that impairs insertion. In some instances, penile injury appears to impair sensory or proprioceptive feedback from the penis, delaying coupling, organized thrusting, or ejaculation. Common causes include stallion ring injuries, phenothiazine-related paralyzed penis and paraphimosis, kick injuries, and self-serve breeding dummy accidents.

A peculiar and often confusing type of erection dysfunction involves the folding back of the penis within the prepuce as it becomes erect. The behavioral hallmark of this situation is a stallion that appears aroused and ready to mount, without a visible erection. The stallion may also appear uncomfortable or intermittently distracted, pinning the ears, kicking toward the groin, or stepping gingerly on the hind legs. Close observation reveals a rounded, full appearing prepuce, with the skin stretched taut. Resolution usually requires removing the stallion from the mare until the penis detumesces. Once the penis is fully withdrawn, application of a lubricating ointment to the prepuce facilitates subsequent normal protrusion. This situation tends to repeat occasionally over time with particular stallions, particularly in stallions with either profuse smegma production or in stallions in which the penis and sheath are dry from frequent cleansing.